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Half a Century of Research on the Stroop Effect: An Integrative Review

Colin M. M a c L e o d Division o f Life Sciences

University o f Toronto, S c a r b o r o u g h C a m p u s Scarborough, Ontario, C a n a d a

The literature on interference in the Stroop Color-Word Task, covering over 50 years and some 400 studies, is organized and reviewed. In so doing, a set ofl 8 reliable empirical findings is isolated that must be captured by any successful theory of the Stroop effect. Existing theoretical positions are summarized and evaluated in view of this critical evidence and the 2 major candidate theories--rel- ative speed o f processing and automaticity of reading--are found to be wanting. It is concluded that recent theories placing the explanatory weight on parallel processing of the irrelevant and the relevant dimensions are likely to be more successful than are earlier theories attempting to locate a single bottleneck in attention.

In 1935, J. R. Stroop p u b l i s h e d his l a n d m a r k article o n atten- tion a n d interference, a n article m o r e influential now t h a n it was then. W h y has the Stroop t a s k c o n t i n u e d to fascinate us? Perhaps the t a s k is seen as t a p p i n g into the primitive o p e r a t i o n s o f cognition, offering clues to the f u n d a m e n t a l process o f atten- tion. Perhaps the r o b u s t n e s s o f the p h e n o m e n o n provides a special challenge to decipher. Together these are powerful at- tractions in a field o f c o m p l e x p h e n o m e n a where the m o s t sub- tie v a r i a t i o n m a y exert a d r a m a t i c effect.

In writing this article, I have two p r i m a r y intentions. The first is to create a t a x o n o m y o f the e m p i r i c a l work on the Stroop t a s k a n d its analogs. At least 70% o f the m o r e t h a n 700 Stroop- related articles in the literature have b e e n p u b l i s h e d since the reviews b y D y e r (1973c) a n d Jensen a n d Rohwer (1966). F u r t h e r - more, Dyer's theoretical framework now seems in need o f re- consideration, so t h a t his review no longer provides the o p t i m a l organization. T h e s e c o n d a i m , then, is to survey existing the- ories, showing the critical weaknesses in the s t a n d a r d explana- tions a n d dispelling s o m e myths t h a t have arisen. I argue for a new theoretical framework for the S t r o o p effect, i n f o r m e d b y c o n t e m p o r a r y cognitive psychology. This f r a m e w o r k is m o r e comprehensive t h a n m a n y earlier experiment-specific hypothe- ses, a n d s u g g e s t s d i r e c t i o n s in w h i c h f u t u r e r e s e a r c h m i g h t profitably proceed.

This research was supported by Natural Sciences and Engineering Research Council o f Canada Grant A7459. I am grateful to the Ontario Science Centre for providing testing facilities and to their staff for assistance. I owe a considerable debt to E J. Lootsteen, who did the bulk o f the initial work locating articles for me. I thank Jonathan Cohen, Kevin Dunbar, and Gordon Logan for their careful readings o f the theoretical section. I am especially grateful to John Flowers and an anonymous reviewer for their yeoman work as reviewers.

Correspondence concerning this article should be addressed to Colin M. MacLeod, Division of Life Sciences, University of Toronto, Scarborough Campus, Scarborough, Ontario Canada M1C IA4. Elec- tronic mail may be sent to [email protected].

Colors, Words, and Interference: Research From 1886 to 1935

The Half-Century Before Stroop

T h e roots o f Stroop's research are evident 50 years earlier in the work o f James M c K e e n Cattell (1886). In a d o c t o r a l project supervised by W i l h e l m Wundt, Cattell r e p o r t e d t h a t objects (and colors) t o o k longer to n a m e a l o u d t h a n the c o r r e s p o n d i n g words t o o k to r e a d aloud. Saying "red" to a patch o f color was slower t h a n saying "red" to the w o r d red. His explanation is strikingly m o d e r n : "This is because, in the c a s e o f words a n d letters, the association between the i d e a a n d n a m e has t a k e n place so often that the process has b e c o m e automatic, whereas in the case o f colors a n d p i c t u r e s we m u s t by a voluntary effort choose the n a m e " (1886, p. 65). Cattelrs a u t o m a t i c / v o l u n t a r y d i s t i n c t i o n strongly influenced psychologists t h e n (e.g., James, 1890, p. 559; Quantz, 1897) a n d continues to d o so (e.g., Posner & Snyder, 1975; S c h n e i d e r & Shiffrin, 1977).

M o s t early research (e.g., Bills, 1931; Brian & G o o d e n o u g h , 1929; Gails, 1922) tested the intuitive differential-practice ac- count for CatteU's observation (cf. Quantz, 1897, p. 10). I will focus only o n the few m o s t relevant to Stroop's work. Brown (1915) p r e d i c t e d t h a t ink-color n a m i n g would benefit from ex- t e n d e d practice m o r e t h a n would color w o r d reading because c o l o r n a m i n g was initially the less p r a c t i c e d skill. I n s t e a d , he found that the ratio o f the two t i m e s r e m a i n e d quite c o n s t a n t with practice. He concluded t h a t differential practice could not explain Cattell's findings.1

L u n d (1927) d i s a g r e e d with Brown, a n d held that the differ- ential practice hypothesis was quite viable. L u n d found t h a t

163

In Experiment 2, Brown printed congruent words or parts o f words on color backgrounds. Contrary to his expectation, such "assistance" was actually detrimental to color naming. On this basis, Brown argued that the two processes--reading words and naming colors--did not overlap. This is the first published instance where color and word co- occurred, and where the word interfered with naming the color. Strangely, the interference appeared in the congruent condition.

164 COLIN M. MACLEOD

c h i l d r e n younger t h a n r e a d i n g age were faster o n color n a m i n g t h a n on w o r d reading, even with careful assurance t h a t t h e y k n e w t h e s t i m u l u s words. F u r t h e r m o r e , for o n e 5 - y e a r - o l d child, 4 weeks o f alternating practice o n the two tasks suggested a c o m m o n asymptote. However, a larger scale d e v e l o p m e n t a l study c o n d u c t e d by Ligon (1932) again q u e s t i o n e d the differen- tial-practice concept. Both color-naming a n d color word-read- ing t i m e s i m p r o v e d across G r a d e s I t h r o u g h 9, yet the differ- ence between the skills r e m a i n e d unchanged. Ligon c o n t e n d e d that a " c o m m o n factor" i m p r o v e d m p e r h a p s t i m e to articulate the r e s p o n s e m b u t t h a t the tasks were otherwise i n d e p e n d e n t .

In the s a m e y e a r as his classic article, Stroop (1935a) d e m o n - s t r a t e d t h a t Ligon's d a t a d i d show differential i m p r o v e m e n t with age, perfectly in a c c o r d with the differential-practice idea. S t r o o p also faulted Ligon's c o r r e l a t i o n a l analysis o n statistical grounds. G i v e n Lund's criticism o f Brown, Stroop (1935a) advo- cated the differential-practice hypothesis, as i n d e e d d i d D y e r (1973c) four decades later.

O t h e r studies offered alternative views o f Cattelrs well-repli- c a t e d observation. Hollingworth (1912, 1915, 1923) suggested t h a t w o r d r e a d i n g r e q u i r e d o n l y a r t i c u l a t i o n , b u t t h a t c o l o r n a m i n g d e m a n d e d a r t i c u l a t i o n plus association. Brown (1915) a n d Ligon (1932) m a i n t a i n e d that b o t h tasks involved two p r o - cesses b u t with a different association element for each test. G a r r e t t a n d L e m m o n (1924) held that color n a m i n g was longer because o f a n interference factor, which they failed to specify. Perhaps they h a d in m i n d what Peterson, Lanier, a n d Walker (1925) suggested: that m a n y responses might b e c o n d i t i o n e d to a single color, b u t only one response was c o n d i t i o n e d to a single word. Telford (1930), a s t u d e n t o f Peterson's, also a d o p t e d this interpretation. A n earlier variation o n this was put forth by W o o d w o r t h a n d Wells (1911, p. 52), who argued t h a t the few c o l o r n a m e s t y p i c a l l y used in these e x p e r i m e n t s "all are equally ready a n d get in one a n o t h e r ' s way."

It is difficult to believe that no one thought to c o m b i n e colors a n d words u n t i l S t r o o p (1935b). 2 It may help to place Stroop's insight in context to realize t h a t he d e v e l o p e d his c o l o r - w o r d t a s k less from an interest in s t i m u l u s - n a m i n g t i m e - - t h e m o t i - vation for m o s t p r i o r r e s e a r c h m t h a n f r o m an interest in inter- ference between conflicting processes.

Stroop~ Classic Article

Elsewhere ( M a c L e o d , in press) I have written a biographical sketch o f John R i d l e y Stroop; I d o not recapitulate that here. However, b e f o r e d i s c u s s i n g t h e v a s t l i t e r a t u r e f o s t e r e d b y Stroop's dissertation, I will briefly outline his l e a d article in the D e c e m b e r 1935 issue o f the Journal of Experimental Psychol- ogy His basic conclusion is correctly r e p o r t e d a n d widely un- d e r s t o o d , b u t what he actually d i d has b e e n somewhat trans- f o r m e d in s e c o n d a r y sources. Stroop's contemporary, Bartlett (1932), would p r o b a b l y n o t be s u r p r i s e d by the transformation. I simply wish to refresh o u r m e m o r i e s with the o r i g i n a l study,

S t r o o p was c o n c e r n e d with how b e s t to explain interference. W i t h Peterson, his d i s s e r t a t i o n supervisor, Stroop h a d b e e n engaged in studies o f color n a m i n g versus w o r d reading, a n d hit u p o n the i d e a o f a c o m p o u n d stimulus where the w o r d was i n c o n g r u e n t with the i n k color. His two m a j o r questions were what effect each d i m e n s i o n o f the c o m p o u n d stimulus would

have o n t r y i n g to n a m e the other d i m e n s i o n , a n d what effect practice would have on the o b s e r v e d interference.

E x p e r i m e n t I e x a m i n e d the effect o f i n c o m p a t i b l e i n k colors o n r e a d i n g w o r d s a l o u d . S t r o o p u s e d five w o r d s a n d t h e i r m a t c h i n g i n k colors: red, blue, green, brown, a n d purple. F o r the e x p e r i m e n t a l condition, each i n k color a p p e a r e d twice in each row a n d c o l u m n o f a 10 × 10 stimulus card. Each w o r d a p p e a r e d twice on every line, equally often in each o f the o t h e r four i n k colors. T h e stimuli a p p e a r e d in reverse o r d e r o n a second card. T h e c o n t r o l - c o n d i t i o n c a r d s were each identical to one o f the e x p e r i m e n t a l c a r d s but in b l a c k i n k only. A f t e r a 10-item practice c a r d , all 70 subjects were tested o n all four cards, h a l f in the o r d e r o f Control 1, E x p e r i m e n t a l 2, Experi- m e n t a l 1, Control 2, a n d h a l f in the reverse order. Subjects were to r e a d the words a l o u d as quickly as possible, leaving no errors uncorrected.

T h e t o p row o f Table 1 presents the d a t a f r o m E x p e r i m e n t 1. Although subjects averaged 2.3 s longer to r e a d the 100 words on the e x p e r i m e n t a l cards, this 5.6% increase was far from sig- nificant. I recently replicated this experiment: My d a t a for 50 subjects a p p e a r in the b o t t o m row o f Table 1, a n d are r e m a r k - ably consistent with Stroop's data. T h e r e was no interference f r o m i n c o n g r u e n t c o l o r s in r e a d i n g words, F ( I , 49) = 1.86, MSe = 2.45, p = .17.

In E x p e r i m e n t 2, the t a s k was switched to n a m i n g the colors aloud. T h e control cards h a d the i n k colors in the s a m e o r d e r as the e x p e r i m e n t a l cards; solid color squares were substituted for w o r d s ) The e x p e r i m e n t a l cards a n d the p r o c e d u r e were identi- cal to E x p e r i m e n t 1, except that twice the average response t i m e p e r item was a d d e d to a subject's t o t a l for every u n c o r r e c t e d error, a p r o c e d u r e Stroop a d m i t t e d was"arbitrary." T h e d a t a o f the 100 subjects in this e x p e r i m e n t are presented in the t o p row o f Table 2.

Subjects averaged 47 s longer to n a m e i n k colors o f incon- gruent words t h a n solid-color squares. Stroop d e s c r i b e d this 74% increase as a " m a r k e d interference effect"; 99% o f the t i m e s o n e x p e r i m e n t a l c a r d s exceeded those on control cards. I also replicated this e x p e r i m e n t using 40 subjects. My data, in the b o t t o m row o f Table 2, again duplicate those o f Stroop; m y slightly lower values are p e r h a p s due to my not using the arbi- t r a r y c o r r e c t i o n for errors. T h e r e was highly significant interfer- e n c e f r o m i n c o n g r u e n t w o r d s in n a m i n g colors, F ( I , 39) = 363.65, MS~ = 99.36, p < .001.

S t r o o p ' s l i t t l e - k n o w n t h i r d e x p e r i m e n t was m o r e c o m p l i - cated. Thirty-two subjects n a m e d the i n k colors o f i n c o m p a t i - ble words for 8 days. O v e r days, color-naming t i m e s decreased 16.8 s f r o m 49.6 s to 32.8 s (for a c a r d o f 50 words). Thus, interference from i n c o m p a t i b l e words a p p e a r e d to decrease

2 According to Jensen and Rohwer (1966), Jaensch (1929) was the first to combine the word and color stimuli in a potentially conflicting situation. Perhaps, then, Jaensch should receive at least partial credit, but the "Jaensch-Stroop effect" is an extremely unwieldy label.

3 Stroop changed the color-naming control-condition cards from Ex- periment 2 to Experiment 3. The solid-color squares were replaced by a nonalphanumeric character, which, in Stroop's view, was more like a letter in appearance and more easily colored to match the shades of the letters on the experimental cards. The stimulus Stroop chose was the swastika.

THE STROOP EFFECT 165

Table 1 Experiment 1: Mean Times (in Seconds With Standard Deviations [SD] ) for Reading Color Words in the Experimental Condition (Incompatible Colored Inks) and in the Control Condition (Black Ink Only)

Experimental: Control: words in color words in black

Experiment Sample size M SD M SD

Stroop (1935b) 70 43.30 6 . 1 5 41.00 4.84 MacLeod (1986) 50 41.58 6 . 9 8 41.16 7.12

with practice. However, there m a y be a general practice or learn- ing-to-learn effect here: Ink color-naming baselines were not collected each day.

Experiment 3 also explored the impact o f practicing color n a m i n g on the development o f interference in word reading. Comparison o f a pretest and a posttest where subjects read words in incongruent colors showed that the intervening 8 days o f practice introduced interference into word reading (from 19.4 s before to 34.8 s after), but that this newly developed inter- ference quickly disappeared in a second posttest (22.0 s). This was the first report o f what is now called the "reverse Stroop effect"

Overall, Stroop took his experiments to show that differential practice did offer a reasonable account o f the asymmetrical interference pattern he obtained, and accepted the Peterson et al. (1925) explanation that words evoked a single reading re- sponse, whereas colors evoked multiple responses, thereby m a k i n g naming colors slower than reading words. Indeed, in his 1938 article, Stroop directly tested this idea and reported support for it.

The misconceptions about Stroop's work are subtle but note- worthy. People often talk as if there was a single experiment contrasting four conditions; in fact, he performed two separate experiments. Frequently, too, his word-reading data (Experi- ment 1) go unmentioned. I have even heard Stroop credited with a compatible c o n d i t i o n - - c o l o r words in congruent ink c o l o r s - - w h i c h he never used. He also never timed single stim- uli; his experiments used only lists o f stimuli. O n the other hand, he did observe reverse Stroop interference first. Although his theoretical account did not mention automaticity, it did rely o n a primitive t y p e o f response competition.

In summary, Stroop's article was a careful treatment o f an interference p h e n o m e n o n . His scholarly acknowledgment o f his intellectual precursors, his succinct data analysis, and his clear writing all contributed to the article's influence. The reli- ability, size, and apparent simplicity o f the effect also no doubt captured the interest o f later investigators. I now consider what these investigators have found out about the Stroop effect since his dissertation.

I n v e s t i g a t i n g t h e S t r o o p Effect: R e s e a r c h F r o m 1935 t o 1989

The set o f articles on the Stroop effect numbers in excess o f 700. At least 300 o f these feature Stroop interference as a mea-

sure o f attention in studying individual differences, drug ef- fects, and the like. Some o f these may assist theory building in other domains, but they are not particularly relevant to a theo- retical account o f the Stroop effect. I shall omit these "applied" studies, leaving t h e m for a n o t h e r reviewer. Even with such pruning, however, it would still not be reasonable to discuss all o f the remaining articles in equal detail. Thus, I will describe pivotal studies on a topic in detail and then provide reference pointers to other pertinent work as appropriate. The review is selective, but the bibliography is comprehensive.

The studies are divided into five sections. The first section examines the various versions o f the Stroop task, now actually a family o f tasks. Next are two sections o n stimulus manipula- tions: first those conducted o n individual trials and then those involving experiment-wide variables. The fourth section sum- marizes manipulations pertaining to the subject's response, and the fifth section surveys subject-related variables. Each sec- tion is in t u r n subdivided in terms o f the sorts o f manipulations involved to allow a particular facet o f the work to be located easily. Appendix A outlines this organization and serves as an index.

Variations on the Stroop Procedure

T h e Stroop Color-Word Interference Test

T h e standard Stroop Color-Word Test, sometimes called the Serial Color-Word Test, is the procedure Stroop (1935b) used in his Experiment 2. Typically, subjects are tested only o n naming colors o f incompatible words and o f control patches. Interfer- ence is expressed as the difference between the times o n these two types o f cards. Total time per card divided by n u m b e r o f stimuli on the card occasionally is used to estimate time per stimulus. Jensen and Rohwer (1966) detailed m a n y o f the subtle shadings o f this standard procedure, focusing particularly o n the Thurstone (1944) version.

It is surprising h o w few studies investigating interference have been concerned at all with Stroop's Experiment 1: reading words in black versus incongruent colors. I agree with Smekal a n d Dvoracek (1977) that more attention should be given to this second measure if only to reduce misinterpretation o f the stan- dard interference measure. This may be especially crucial in clinical settings and individual-differences studies, where more convergence would be helpful.

Table 2 Experiment 2: Mean Times (in Seconds with Standard Deviations [SD]) for Naming Ink Colors in the Experimental Condition (Incompatible Color Words) and in the Control Condition (Solid Color Squares)

Control: Experimental: squares in words in color color

Experiment Sample size M SD M SD

Stroop (1935b) 100 110.3 18.8 63.3 10.8 MacLeod (1986) 40 102.27 1 8 . 0 6 59.76 8.09

166 COLIN M. MACLEOD

How should the standard color-word test be scored? Jensen and Rohwer (1966, pp. 4 5 - 4 8 ) discussed many approaches, but the most prevalent is to calculate a difference score (interfer- ence c a r d time - pure color c a r d time) following S t r o o p (1935b). 4 Various more complicated scoring systems have been proposed (e.g., Smith, 1959; Smith & Borg, 1964; Smith & Klein, 1953), but their use is nowhere n e a r as widespread as the straightforward difference score. The same applies to attempts to deal with errors o n the task (e.g., Gardner, Holzman, Klein, Linton, & Spence, 1959; Rand, Wapner, Werner, & McFarland, 1963).

Although Sj6berg (1969,1974) was pessimistic about the reli- ability and validity o f the Stroop Color-Word Test based on his regression analyses o f various clinical populations, others have been more optimistic. Smith and N y m a n (1974) and Sehubo and Hentschel (1977, 1978) argued that reliability was actually quite good. Santos and Montgomery (1962) directly examined reliability, finding it to be g o o d and uninfluenced by events interpolated between test and retest. Uechi (1972) also reported high reliability. In the m o s t comprehensive study, Jensen (1965) concluded that, with multiple administrations, the Stroop test was probably more reliable than any other psychometric test.

O f course, m a n y studies have centered on methodological issues in the Stroop task. Zajano, Hoyceanyls, and Ouellette (1981 ) pointed out that ink color and shape change repeatedly on the standard interference card, but only ink color changes on the standard control card. Changing shapes on the control card to correct this confound did not alter the basic effect, however. Sichel and Chandler (1969) argued that variation over stimuli was too great in the standard test; their procedure using pairs o f stimuli and tighter controls on hue, brightness, and so forth also left the basic pattern intact.

Modifications only modestly affect its magnitude, not its qual- itative form. McCown and Arnoult (1981) printed the stimuli vertically versus horizontally using either the entire color word or just its first three letters, and reported equivalent interfer- ence in all cases. Regan (1978) found interference even using only the first letter o f a conflicting color word (e.g., the letter B in red ink). N o r do instructional manipulations have m u c h im- pact. For example, Peretti (1971 ) found that varying the degree o f competitiveness in instructions to subjects affected their per- formance, but substantial interference appeared in b o t h in- structional conditions.

G r o u p versions o f the color-word test originated with Kipnis and Glickman (1958, 1959a, 1959b, 1962). They had subject groups view cards containing 150 stimuli and then write down the first letter o f each ink color, scoring the n u m b e r o f items correctly identified in a 3-min interval. Similar schemes were developed by U h l m a n n (1962) a n d by Podell (1963). Unfortu- nately, as Kipnis and Glickman (1962) noted, these tests actu- ally measured clerical speed more than anything else.

Golden (1975) had subjects in a group silently read as many items as they could o n both experimental and control cards in 45 s. (Although he claimed that performance was equivalent to the same task done aloud, I do not r e c o m m e n d switching from overt to covert naming.) Work by Harbeson and her colleagues (Harbeson, Kennedy, & Bittner, 1981; Harbeson, Krause, Ken- nedy, & Bittner, 1982) including a group Stroop test found indi- vidual card times to be reasonably reliable, but not the derived

difference scores. Thus far, then, use o f group tests has been infrequent and the results not very compelling.

O n the whole, the Stroop Color-Word Test as a psychometric tool a p p e a r s t o have reasonable reliability a n d validity (al- though the appropriateness o f any given criterion o f validity could be questioned). Coupled with its ease o f administration, this has made the test a widely used diagnostic instrument in its standard form. Not surprisingly, then, many variations on the test have been developed and tested.

The Individual Stimulus Version o f the Color-Word Task

Researchers interested in s t u d y i n g interference w a n t e d a more analytic methodology whereby individual stimuli could be presented a n d timed. A l t h o u g h D y e r (1973c) credited Dalrymple-Alford and Budayr (1966) with first using this pro- cedure, Tecce and D i m a r t i n o (1965) used it a year earlier to study shock effects a n d response competition. However, Dalrymple-Alford and Budayr's (1966) concern was with the procedural modification itself, and they found it acceptable. They did note, though, that the standard color-word test intro- duced list-structure effects in addition to item-specific interfer- ence.

By the time o f Dyer's (1973c) review, quite a few studies had used this procedure; it has certainly b e c o m e predominant in laboratories since then (e.g., D u n b a r & MacLeod, 1984; Neill, 1977). Indeed, it was not enough to isolate a single Stroop stimu- lus; investigators then began to modify the format o f that stimu- lus. For example, K a m l e t a n d Egeth (1969) embossed color words in white letters o n colored plastic tape and had subjects name the tape color. They observed interference virtually iden- tical to that in the integrated, word-in-color standard proce- dure.

To examine laterality and stimulus-integrality effects, Dyer (1973a) separated word from color, placing one on each side o f the fixation point. In like manner, K a h n e m a n and Chajczyk (1983) placed a word above or below a color patch in their studies o f automaticity. Both studies obtained robust interfer- ence. Dyer and Severance (1973) also separated word and color, presenting an incompatible color word 0, 25, 50 or 100 ms be- fore a color patch. Although they observed interference in every case, it was about half that o f the n o r m a l integrated stimulus. Their procedure has now become conventional for the investi- gation o f stimulus onset asynchrony (SOA) (see later discus- sion). Indeed, all o f these innovations are n o w firmly en- sconced, a n d the single-trial (or single-response) p r o c e d u r e dominates the field.

Sorting and Matching Versions o f the Color-Word Task

Rather than naming or reading stimuli aloud, subjects can be asked to sort stimuli into categories. The originators o f this procedure, Tecce and H a p p (1964), showed that sorting color- only cards was much faster than sorting incongruent color-

4 Saunders (1980) suggested the use o f the logarithm of this differ- ence score on the curious grounds that willingness to take part in psychological experiments, especially Stroop studies, seems to corre- late with this measure.

THE STROOP EFFECT 167

word cards into categories identified by color patches. Dyer (1973c) wondered what would have happened had the piles been labeled with color words instead. The answer came when Chmiel (1984) labeled the bins both ways (in separate condi- tions), and observed both normal Stroop interference and re- verse Stroop interference. Martin (1981) also obtained a reverse Stroop effect using the card-sorting procedure.

Treisman and Fearnley (1969) had subjects sort cards con- taining two words, two colors, or a word and a color into one of two piles identified by whether the two stimuli on a given card were the same or different. When both stimuli were words or both were colors, sorting was rapid. When the stimuli were a word and a color, sorting was slower, especially when one o f the two was an incongruent stimulus.

The sorting task continues to appear periodically in the litera- ture (Flowers & Blair, 1976; Flowers & Dutch, 1976; Naish, 1980; Taylor & Clive, 1983; Virzi & Egeth, 1985), sometimes using materials others than colors and words (e.g., Morton, 1969). Because some o f these findings suggest that the sorting task may not be perfectly analogous to the standard Stroop task, it will be necessary to compare results from the different procedures directly (as already begun by Taylor & Clive, 1983).

Although a common cognitive task, most notably in the work o f Posner (1978), matching has seen limited use in the Stroop situation. Flowers (1975) first used the technique to examine time to match sequentially presented color words to color patches. If printed in an incongruent ink color, time to match the word with the color patch was slowed (relative to the con- gruent or control cases). Compton and Flowers (1977) repli- cated and extended this observation. Others (e.g., Alivisatos & Wilding, 1982) have used matching to study interference in Stroop analogs, but only rarely. There appears to be room for more research using matching, particularly in attempting to determine the role o f response modality in interference (see Response Modality: Oral Versus Manual).

The Picture-Word Interference Task

Cattell (1886) emphasized the naming o f colors and pictures being slower than the reading aloud of corresponding words (see also Fraisse, 1969). So it is surprising that the picture-word interference task did not appear until after Dyer's (1973c) re- view. Hentschel (1973) embedded words inside line drawings and required subjects to name the pictures. Although uncertain whether the two tests measured precisely the same phenome- non, he validated the resulting interference against the stan- dard Stroop test.

Hentschel used the multiple-stimulus form; more prevalent today is the individual-stimulus version, originating with Las- sen (1975) in an unpublished dissertation. He observed the larg- est interference when the word preceded the picture by 100 ms. (Note that his parallel study o f word reading with incongruent pictures also demonstrated some interference, although this was unaffected by SOA.)

The task quickly caught on. Rosinski, Golinkoff, and Kukish (1975) demonstrated that incongruent words printed inside pictures interfered substantially with picture naming, but that incongruent pictures had only small effects on word reading. As in the color-word task, this asymmetry has come to be the

expected pattern (e.g., Glaser & DiJngelhoff, 1984; Smith & Ma- gee, 1980). Golinkoffand Rosinski (1976) showed that picture- word interference was unaffected by reading-comprehension ability. Rosinski (1977) reported that same-category words in- terfered more with picture naming than did unrelated words or nonsense trigram control items. These studies set the stage for subsequent work on picture-word interference.

The major advantage o f the picture-word task is its greater flexibility in allowing many manipulations not possible with the restricted set of colors in the color-word task. In compari- son to pictures-only, rows o f Xs, or other controls (e.g., *#@?%), incongruent words consistently interfere with picture naming. Also congruent words speed responding (Posnansky & Rayner, 1977; Rayner & Posnansky, 1978; Underwood, 1976). Non- words cause less interference than words (e.g., Lupker, 1982), and unpronounceable nonwords cause less interference than pronounceable nonwords (e.g., Guttentag & Haith, 1978).

More intriguing, perhaps, are the findings unique to the pic- ture-word task. Consider first the semantic domain. Given the numerous demonstrations of semantic facilitation as a result o f association (e.g., Meyer & Schvaneveldt, 1976), it is puzzling that an associated word printed on a picture (e.g., the word cheese on a picture of a mouse) does not produce more interfer- ence in picture naming than does an unassociated word (Lupker, 1979). Yet a member of the same semantic category (e.g., the word ankle on a picture o f a hand) does produce more interference than an unrelated word (e.g., Ehri, 1976; Golinkoff & Rosinski, 1976), regardless o f exposure duration (Rayner & Springer, 1986). Furthermore, although category typicality has no influence on interference (Lupker, 1979), category centrality apparently does (Job & Rumiati, 1984). Obviously, this is a com- plicated issue.

There is now a large data base of studies manipulating se- mantic relations between word and picture (e.g., Babbitt, 1982; Bryson, 1983; Dunbar, 1986; Irwin & Lupker, 1983; Lupker & Katz, 198 l, 1982; Lupker & Sanders, 1982; Magee, 1982; Rayner & Springer, 1986; Reiner & Morrison, 1983; Smith & Kirsner, 1982; Smith & Magee, 1980; Toma & Tsao, 1985). Many o f these are discussed later in more detail. Although a quick glance might suggest that increasing the semantic similarity between the word and the picture increases the interference in picture naming, this comforting summary is flawed. For example, it does not capture the difference just noted between categori- cally and associatively related words. A complete account awaits further research.

A smaller subset o f picture-word studies has been concerned with nonsemantic relations. Ehri (1977) demonstrated that pic- ture-naming latencies showed less interference as the words changed from nouns to adjectives to functors, and that functors did not differ from control nonsense syllables. Thus, syntactic function (or meaningfulness, or both) plays a role roughly in line with intuition. However, most variations have centered on the phonemic, graphemic, or orthographic characteristics of words. The basic message is that these variations influence in- terference, but not as much as semantic manipulations (e.g., Briggs & Underwood, 1982; Posnansky & Rayner, 1977). As one example o f a nonsemantic effect, Rayner and Posnansky (1978) found that nonwords that resembled words graphemi- cally (e.g., leuf for leaf) assisted naming of the corresponding

168 COLIN M. MACLEOD

picture when presented very briefly (35 ms). With increased exposure duration, this facilitation gradually became interfer- ence, although interference was never as great for these look- alikes as for dissimilar nonwords. To complicate matters, they observed that phonemically related nonwords (e.g., lefe for leaf) led to increased facilitation with exposure duration, in opposi- tion to the graphemic effect.

Posnansky and Rayner (1978) established that first-letter co- incidence of the word and the picture exerted its effect through the graphemic rather than the phonemic route. Like Rayner and Posnansky (1978), Lupker (1982) and Underwood and Briggs (1984) showed that the graphemic similarity effect was due to letters appearing in the correct location, and that they did not have to overlap in the first position (e.g., the word bomb on a picture of a comb produced the effect). (Note that this observation is consistent with the predictions of McClelland and Rumelhart's, 1981, activation model.)

What do these studies tell us? Lupker (1982; Lupker & Sanders, 1982) suggests that graphemic effects are a conse- quence o f easier word retrieval, and are unrelated to the re- sponse-competition factors underlying the basic interference effect. Rayner and Springer (1986) showed that graphemic simi- larity between the picture and the word actually reduced the size o f the semantic-category effect. They maintained that the bulk o f the interference in the picture-word task stems from semantic evaluation, not response competition. Investigators seem to agree that there is more than one factor involved in picture-word interference, but what the factors are and how fundamental each is remain to be worked out.

A critical question is how directly the picture-word and color-word findings relate to each other. Given the quite similar patterns outlined, I will discuss them together in the rest of this article. However, the essential work o f ascertaining empirically just how similar the two tasks are remains to be performed. This is not a straightforward problem given the different sorts of questions being addressed in the two domains. One way to make some inroads would be through an individual-differences study examining the correlation between interference in the two tasks. The greater the comparability o f results in the two tasks over the same individuals, the more confident we will be that they measure the same cognitive processes.

Auditory Analogs o f the Stroop Task

If people could selectively filter by modality (as Broadbent, 1958, suggested), then there ought not be much cross-modality interference. A number o f studies called this prediction into question (e.g., Greenwald, 1970), so creation o f an auditory Stroop task was only a matter o f time. Credit goes to Hamers (1973) in her dissertation (see Hamers & Lambert, 1972). Her compatible condition required subjects to say "low" to the word low presented at a low pitch (110 Hz) and "high" to the word high presented at a high pitch (175 Hz). In the incompatible condition, the word at 110 Hz was high and the word at 175 Hz was low. Examining only her monolingual condition, she re- ported strong interference: The incompatible case (1,169 ms) took 138 ms longer than the compatible case (1,031 ms) on average.

Unfortunately, as Dyer (1973c) pointed out, we cannot be sure

how much o f this difference truly is interference caused by in- congruent words and how much is facilitation resulting from congruent words because there was no neutral-word control condition. This omission is all too frequent in studies of interfer- ence and facilitation, complicating interpretation. Without such a control, all that can be examined is the difference be- tween the congruent condition and the incongruent condition - - t h e sum of facilitation and interference, each in unknown amounts. A second problem is closely related: What is the ap- propriate neutral control?. I return to this thorny issue later (cf. Jonides & Mack, 1984); for now, let us continue examining audi- tory Stroop analogs.

Dyer (1973c) described an unpublished experiment by Dyer and Mosko wherein subjects had to report the name o f the speaker. Some words spoken were neutral, some were compati- ble (the speaker's name), and some were incompatible (the other speaker's name). They found no interference or facilitation, ap- parently in conflict with Hamers (1973). However, the Hamers result has been replicated (Cohen & Martin, 1975; Shor, 1975). In fact, Shor obtained interference in both directions, although the word interfered more with identifying the pitch than vice versa. Bidirectional interference is rarely reported (partly be- cause subjects are rarely required to name both dimensions o f the stimulus), yet it has serious ramifications for interpreting the Stroop effect.

In an extension of Hamers's task, McClain (1983c) contrasted time to identify the pitch with time to identify the word using three response modes: verbal, buttonpress, or pitched hum. Interference occurred only when the to-be-reported dimension and the reporting mode differed (pitch-verbal, pitch-button, word-hum), and not when they were compatible (word-verbal, word-button, pitch-hum). She implicated response modality as crucial in interference, an issue discussed under Response Mo- dality: Oral Versus Manual.

Other auditory procedures have been reported as well. Pieters (1981) used the auditory words left or right to the left or right of the subject, and obtained interference in reporting the spatial location o f a conflicting word. Green and Barber (1981, 1983) obtained an auditory Stroop effect when subjects had to judge a speaker's gender in conflict versus no-conflict situa- tions. Hatta and Kirsner (1983) reported a similar result, which also held with a buttonpress rather than a vocal response. Ap- parently, the Dyer and Mosko finding is the outlier.

Most recently, Zakay and Glicksohn (1985) used musical notes or the names o f those notes printed at congruent or incon- gruent locations on the staff. Subjects responded either orally or on a piano keyboard. Zakay and Glicksohn implicated both stimulus-response compatibility and the relation between the dimensions as important, suggesting in fact that these effects were additive.

All o f these authors explicitly or implicitly argue for a close analogy between their task and the Stroop task. As with pic- ture-word interference, though, this connection has not been firmly established. Indeed, it may be somewhat less clear-cut here. Still the effects are generally qualitatively similar, suggest- ing generalizability of the interference phenomenon.

Other Analogs o f the Stroop Task These variants only hint at the ingenuity of investigators in

creating interference tasks. Many of the others involve two (or

THE STROOP EFFECT 169

more) verbal stimuli on the same trial (or on successive trials), where one is the target and the other is the distractor. These tend to be thought o f in the literature more as priming studies, outside the domain o f this review. As one illustration, though, a widely used procedure is the "flanker" task (e.g., Eriksen & Eriksen, 1974; Eriksen & Hoffman, 1972; Flowers & Wilcox, 1982) in which irrelevant letters or words are presented adjacent to the target.

Eriksen and Eriksen (1974) showed that, for two response sets o f letters (H and K vs. S and C), irrelevant flankers from the wrong set interfered with making the response. This seems to be an attentional-selection problem, because presenting the flankers in a different color reduced the interference (Harms & Bundesen, 1983). Hatta, Hatae, and Kirsner (1984) contrasted English monolinguals to English-Japanese bilinguals in identi- fying English letters, and found that flanker characters from either language interfered for the bilinguals, but not for the monolinguals.

Other investigators adapted this procedure for studying word rather than letter processing. Dallas and Merikle (1976) showed that presence o f a distractor word influenced time to name the target word, with slower naming when the distractor was unas- sociated. Shaffer and LaBerge (1979) replicated the Eriksen and Eriksen (1974) pattern using categorized words instead of let- ters. Guttentag, Haith, Goodman, and Hauch (1984) used the technique with bilinguals to show that flanker words interfered even when they were not in the same language as the target, contrasting somewhat with the Hatta et al. 0984) result for letters, perhaps because o f the semantic element. The bilingual Stroop situation is treated in more depth under Language Dif- ferences: The Case o f the Bilingual.

The growing literature on the flanker task has been extended to the color-word and picture-word situations by Kahneman and Chajczyk (1983) and Glaser and Glaser (1989). Several stud- ies also have investigated aspects o f meaning using a priming procedure in a Stroop-like task (e.g., Conrad, 1974; Henik, Friedrich, & Kellogg, 1983; Oden & Spira, 1983; Regan, 1977, 1981; Warren, 1972, 1974), but these are all saved for the more extensive discussion o f semantic variation later.

Most relevant interference experiments have used words as one dimension o f the two-dimensional stimulus, such as when Warren and Lasher (1974) had subjects identify the type fonts o f words that were in either the congruent or the incongruent font (e.g., the word bold typed in modern, nonbold font), obtaining the expected interference pattern. However, shapes have been used occasionally in an effort to eliminate words altogether (e.g., Irwin, 1978; Redding & Gerjets, 1977; Shor, 197 l). Thus, Hents- chel (1973) developed a figure-word interference test in the same article in which he introduced the picture-word task. When geometrical shapes surrounded incongruent words (e.g., a square around the word circle), interference occurred for nam- ing the shape.

Compton and Flowers (1977) had subjects match words to geometric shapes presented shortly after the words. Matching speed was slowed by incongruent shapes being present at the time o f word presentation, but this was primarily true at short SOAs (100 ms or less). They took this interference to be visual, unlike the verbal interference in the standard Stroop task. MacLe~d and Dunbar (1988) assigned color names to random

polygons, and trained subjects in giving the new color name to each shape, monitoring the development o f interference with practice. The advantage o f studies using shapes is that they step free o f words and reading, which dominate this literature.

Few studies have used naturally colored objects, although Cramer (1967) and Arochova (1971) did develop an object- based test for use with children. When Mtnard-Buteau and Cavanagh (1984) had subjects name the color o f an incongru- ously colored object, such as a blue banana, as opposed to a neutral object, such as a blue book, they found interference. However, practice in the word version o f the same task did not transfer to the object version, so they took these to be indepen- dent types o f interference, the object-related one being due to a more perceptual process. This also relates to the semantic ma- nipulations (discussed under Semantic Variation) and to the re- lation between the picture-word and color-word tasks.

Arrows have been moderately popular stimuli. In the first such study, Shot (1970,1971) embedded the words left, right, up, and down in arrows pointing in directions other than the one named by the word. The control condition omitted the words. Shor observed interference in naming the direction o f the arrow, but smaller in magnitude than common in the color- word task. This interference decreased as the semantic overlap between the embedded word and the arrow declined (Fox, Shor, & Steinman, 1971). Shot, Hatch, Hudson, Landrigan, and Shaffer (1972) demonstrated that extensive training (30-50 days) in naming such incongruent stimuli did not eliminate the interference observed.

Shor's task is analogous to the picture-word task in that the two dimensions are not integrated, unlike the standard Stroop task. To integrate the two dimensions more, Dyer (1972) used a moving display in which the same four directional words (or the control stimulus XXXX) could be moved either in the named direction (congruent) or in another direction (incongruent). He observed both interference and facilitation for naming direc- tion, but the interference was again small relative to the stan- dard Stroop task. Two other similar studies produced the same results. Clark and Brownell (1975) had subjects judge whether an arrow was pointing up or down inside a rectangle; White (1969) used the words north, south, east, and west inside a rect- angle, and had subjects name the word's position. Both studies reported small but reliable interference.

A related series o f studies began with the work o f Seymour (1973, 1974). He presented the words above, below, left, and right, and required subjects to name the location o f the word relative to a dot. Subjects showed both facilitation and interfer- ence. Palefand Olson (1975) followed up this work, presenting the words above and below either above or below the fixation point. When spatial location was processed faster, responses to word meaning showed interference from incongruent loca- tions; when word meaning was processed faster, responses to spatial location showed interference from incongruent words. This has become a hallmark finding in support o f the idea that interference is a direct consequence o f the speed o f processing each dimension. Palef (1978) then complicated the stimulus, adding an asterisk so that subjects could respond to relative or absolute position as well as to the word itself. She argued that the effects o f the two potentially interfering dimensions were additive. Logan and Zbrodoff(1979; see also Logan, 1980) ex-

170 COLIN M. MACLEOD

tended this work by manipulating the frequency o f trial types (see Probability of Various Trial Types). More recently, Harvey (1984) examined SOA effects using the words hi and lo in com- patible versus incompatible positions.

Another perennial favorite is numerosity. Windes (1968) first showed that c o u n t i n g is interfered with if the stimuli being counted are incompatible numerals. Shor (1971) and Fox et al. (1971) reported similar interference effects with numbers. Mor- ton (1969) also obtained reliable interference from the presence o f irrelevant digits in counting small sets o f stimuli. In a more complicated study, Flowers, Warner, and Polansky (1979) dem- onstrated that there was also a major contribution o f response modality to the likelihood o f observing interference in a numer- osity task.

Since Navon's (1977) and Stifling and Coltheart's (1977) arti- cles on global versus local features in perception, this task has b e c o m e popular as well. Here subjects saw a large letter com- posed o f small letters (e.g., the letter H formed from small Hs vs. small Ss). W h e n required to respond to the large letter, the small ones had little impact. W h e n required to respond to the small letters, the large one interfered. A similar effect was ob- tained using shapes: The global feature again overrode the local feature. Later studies (e.g., Kinchla, Solis-Macias, & Hoffman, 1983) pursued the role o f attention in this remarkably good Stroop analog. M a r t i n (1979) suggested that global features dominate for a multiple-element display, whereas local features take over for a few-element display.

Egeth, Blecker, a n d K a m l e t (1969) f o u n d n o interference when subjects indicated whether the word and its ink color matched using the responses "same" and "different" but reli- able interference when the words were switched to the stimuli same and d/ft. Thus, words can get through even in a compari- son task, but they must relate to the response. Francolini and Egeth (1980) found Stroop-like interference only when manipu- lations o f the relevant, to-be-named stimuli were undertaken in a color-numerosity task. Virzi and Egeth (1984) used a version o f the Stroop task in the perceptual d o m a i n to examine the p h e n o m e n o n o f illusory conjunctions, finding that subjects have difficulty discriminating whether information came from a color patch or a word.

Daniel (1968, 1969, 1970a, 1970b) tried several variations to heighten interference in the s t a n d a r d c o l o r - w o r d task. He printed the color words in color and added a differently colored background, or he had a tape recorder spewing out color words while subjects attempted to name the ink colors o f incongruent words. Interference increased, but overall response rate also slowed down. Others also altered the standard color-word task (e.g., Dyer, 1973b; Friedman & Derks, 1973; Hall & Swane, 1973; U l e m a n & Reeves, 1971). Sometimes more than one o f the dimensions have been combined, as in the case o f the color- digit interference task devised by Wolitzky, Hofer, and Shapiro (1972).

The recent past has seen extensions o f the Stroop effect even farther afield to d o m a i n s such as affect (Gardner, 1985) and even arachnophobia (Watts, McKenna, Sharrock, & Trezise, 1986). How equivalent are all o f these tasks that superficially resemble the Stroop task? Even for the very prevalent alterna- tives, such as the picture-word task, we do not know. Correla- tional studies are one way to find out, but few have been per-

formed. Obviously, though, it is o f theoretical importance to know whether similar processes are invoked in these m a n y vari- ations, but we have insufficient evidence at present.

From here on, the sections end with empirical generaliza- tions where warranted. These are collected in Appendix B to form the basic findings that must be a c c o m m o d a t e d by any viable theory o f the Stroop effect. Then, under Theoretical Ac- counts o f the Stroop Effect, the existing theories are considered in view o f this fundamental evidence. Here, then, is the first such s u m m a r y statement: The Stroop effect is observed with lists o f stimuli, with single stimuli, and with many variations on the response required. Similar data patterns are evident in numerous Stroop analogs, such as the picture-word task. I now consider what has been learned about the causes o f Stroop interference from the hundreds o f studies over the past five decades.

Manipulations o f Information on Critical Trials

From here on, empirical studies are grouped as to whether their manipulations pertain to individual trials, the overall ap- proach to the whole task, the response demands, or qualities o f the individual subject. Such divisions are c o m m o n in cognitive psychology and serve a useful organizational function here.

H u e Variation

Most manipulations in the Stroop task have centered on the to-be-ignored verbal dimension. Only a few have examined the hues o f the to-be-named inks (e.g., Sichel & Chandler, 1969). D y e r (197 lb) showed that subjects n a m e d a c h r o m a t i c color patches slightly faster t h a n c h r o m a t i c patches. W h e n the patches were replaced with incongruent color words, interfer- ence was greater for the chromatic than for the achromatic ver- sion. Dyer argued that interference enhanced basic processing- time differences.

The only other approach to varying hue was that o f Flowers and his colleagues (Flowers & Blair, 1976; Flowers & Dutch, 1976). Flowers and Blair had subjects classify six colors into two categories, and found that a "natural" split o f adjacent hues (red, orange, and yellow vs. green, blue, and purple) caused interference only if blocks o f nonadjacent hues were included (red, yellow, and blue vs. orange, green, and purple). However, interference was always present when using the nonadjacent classification. Flowers and Dutch replicated this, and argued that nonadjacent categorization required access to names, not just to perceptual qualities o f the colors, perhaps because the nonnatural set members had to be constantly rehearsed.

Most Stroop studies use from two to six hues, usually highly differentiable, prototypical colors. Yet variations in number, saturation, similarity, or other features o f the color dimension could be used to manipulate processing on that dimension, and would be relevant to the explanations discussed under Theoreti- cal Accounts o f the Stroop Effect. With so few relevant experi- ments, however, it is premature to offer an empirical generaliza- tion now.

Acoustic Variation

A c o m m o n introspection attributes interference to the artic- ulatory-phonemic-acoustic dimension. Before it can be pre-

THE STROOP EFFECT 171

vented, the wrong response is begun, and its sound conflicts with producing the right response. Hollingworth (1915) sug- gested a version o f this idea, which is quite consistent with the hypothesis that Stroop interference occurs at a response stage late in processing (e.g., Posner & Snyder, 1975) after at least part o f the word has been identified.

Bakan and Alperson (1967; see also Langer & Rosenberg, 1966) f o u n d increasing interference with increasing pro- n o u n c e a b i l i t y o f n o n s e n s e syllables in the c o l o r - w o r d task. However, p r o n o u n c e a b i l i t y was n o t the whole story: Words equated with nonsense syllables for pronounceability produced still m o r e interference. Thus, m e a n i n g also matters, as dis- cussed u n d e r Semantic Variation. Still their study suggested some role for nonsemantic factors.

Shortly afterward, Dalrymple-Alford (1972a, 1972b) pursued pronunciation effects. In addition to color words, he used three types o f noncolor words: same initial sound (e.g., run for redand grown for green), same terminal sound (e.g., Jed for red and clean for green), or unrelated acoustically (e.g., cat for red and flown for green, an unfortunate choice given the c o m m o n final pho- neme). Color words caused the most interference (177 ms), but both types o f acoustic relation--initial sound (62 ms) and termi- nal sound (49 ms)--also caused interference. Dalrymple-Alford proposed that a competing color n a m e could be primed seman- tically (1972a) o r acoustically (1972b).

Singer, Lappin, and Moore 0 9 7 5 ) contrasted the effect on ink-color naming o f the first, middle, or last two letters o f a color word versus the full color word. First letters interfered more than later letters, but not as much as the whole word. Adding unrelated letters to the first letters did not alter interfer- ence, leading Singer et al. to speculate that the interference s t e m m e d from activation o f a m o t o r program to articulate the beginning o f a word. Regan (1978) confirmed this initial-letter effect, also showing that the first letter could produce facilita- tion in the congruent case. She took this as evidence for auto- matic processing o f letters and words.

Underwood, Briggs, and Underwood (1984) manipulated the p h o n e m i c or graphemic relation o f a word or a nonword to the ink-color name. For b o t h g o o d a n d p o o r 9- to I l-year-old readers, c o n g r u e n t items generated facilitation a n d i n c o n - gruent items generated interference. So far all o f these studies show at least an orthographic/graphemic effect: C o m m o n let- ters, especially beginning a word, have an influence. However, it is harder to tell whether there was any acoustic/phonemic effect (with the possible exception o f the Bakan and Alperson study).

A study by Dennis and Newstead (1981) demonstrated that acoustic/phonemic effects are indeed present. They c o m p a r e d color words with p s e u d o h o m o p h o n e s (e.g., blue vs, bloo) and found similar interference. Furthermore, this was not due just to their c o m m o n initial letters because the baseline was a non- word matched for visual similarity and sharing the initial letters (e.g., blir). Interestingly, though, real color words led to facilita- tion o n congruent trials but pseudohomophones did not. This is just one o f m a n y situations where facilitation and interfer- ence c o m e decoupled.

In presenting a prime word before a Stroop trial, Tanenhaus, Flanigan, and Seidenberg 0 9 8 0 ) found that if the prime was related orthographically o r phonologically, o r both, to the irrele-

vant word o n the Stroop trial, interference with color naming was greater than i f the prime and irrelevant words were unre- lated. This occurred for both auditorily and visually presented primes, generalizing the phonological effect in Stroop interfer- ence.

W h a t about facilitation? Effler (1978b; see also 1977a) re- p o r t e d r e d u c e d interference when the irrelevant w o r d was acoustically similar to its to-be-named ink color. The effect also obtained when the relation was between the ink color o f item n and the word in item n + 1, but not vice versa. Apparently, having b o t h d i m e n s i o n s converge o n a c o m m o n - s o u n d i n g name speeds responding. Sequential effects like these are dis- cussed more extensively under Trial Sequence.

Four studies have tried to tie up the articulatory system to determine what consequences this would have in the Stroop task. All used the card-sorting paradigm. Martin (1978) re- ported reduced interference in sorting by color when subjects had to say "blab, blah" concurrently. However, Besner, Davies, and Daniels (1981) reported no reduction in the same task. Martin (1981) found that simuRaneous irrelevant articulation had no effect on the reverse Stroop interference she observed. Chmiel (1984) confirmed all three o f these results, implicating the labeling o f the sorting bins as critical. W h e n labeled with color patches, interference was r e d u c e d d u r i n g a r t i c u l a t o r y suppression; when labeled with words, there was no reduction.

Using the "large letter c o m p o s e d o f little letters" task, Stifling and Coltheart (1977) argued against an acoustic effect. Making the small letters easier to confuse visually with the large letter increased interference, b u t m a k i n g t h e m easier to c o n f u s e acoustically had no impact. Possibly, this result reflects differ- ent processes in the two tasks. For example, letter-shape analy- sis seems to be more critical in this task than in the Stroop task. Investigation o f response modality (e.g., vocal vs. buttonpress) might help elucidate such task differences.

In the p i c t u r e - w o r d task, Ehri (1977) d e m o n s t r a t e d t h a t nouns interfered more than adjectives or functors. Ehri took this to mean that interference was not phonologically based, or all word types should have caused similar interference. How- ever, using nonsense syllables as a baseline does present prob- lems (cf. Bakan & Alperson, 1967), as Ehri acknowledged. Lupker 0 9 8 2 ) obtained results more like Effler's (1978b): Or- thographic and phonemic similarity between the word and the picture's name led to facilitation relative to a nonword (but not to a picture alone). Lupker attributed this benefit to decreased name-retrieval time for the picture, an effect he saw as separa- ble from the basic response-competition source o f interference.

At least for the standard color-word task, then, the visual or acoustic characteristics, or both, o f the irrelevant word a p p e a r to exert an influence. Much o f this hinges on the compatibility o f the ink name and the first letter or two o f the word, although later parts o f the word can also have some effect. To summarize, both orthographic and particularly acoustic/articulatory relations between the irrelevant word (or part o f the word) and the to-be- named ink color contribute to the interference.

As a final note, one useful study would combine the articula- tory-suppression t e c h n i q u e o f M a r t i n (1978, 1981), C h m i e l (1984), and Besner et al. (1981) with the letter manipulations o f Dalrymple-Alford (1972b) and Singer et al. (1975). Would in- compatible letters interfere as m u c h if they could not be articu-

172 COLIN M. MACLEOD

lated? Regardless, nonsemantic features o f the words d o contrib- ute to interference, supporting the i n t u i t i o n s - - a n d the articula- tory s t u m b l e s - - o f subjects in Stroop experiments.

S e m a n t i c Variation

Since Klein's (1964) b e n c h m a r k study, how word meaning influences color n a m i n g has been attacked from a host o f dif- ferent angles. Indeed, the enterprise has come full circle: The Stroop task has b e c o m e a c o m m o n tool in understanding how we draw meaning from words. I n this section, I begin with studies manipulating the meaning o f a single irrelevant word, a n d t h e n t u r n t o studies involving p r i m i n g with words o r phrases.

Semantic variation a n d the irrelevant word. The classic study is that o f Klein (1964, Experiment 1). His goal was to under- stand the sources o f the word's interfering effect in color nam- ing and the processes involved in that interference. To d o this, he manipulated the relation o f four words to four ink colors (red, green, yellow, and blue). All subjects performed the con- trol, "colors-alone" card, averaging about 44 s for the card, and then performed one o f six interference cards. As always, the standard incongruent condition (where red, green, yellow, and blue are the words) showed large interference: an increase o f about 37.5 s. W h e n four other color words were used (tan, pur- ple, gray, and black), interference was cut to 18 s. Interference decreased further to 15.5 s for color-related words (fire, grass, lemon, and sky). Substituting c o m m o n , unassociated words (put, heart, take, and friend) or rare words (sol, helot, eft, and abjure) d r o p p e d interference to 12 s and 7.5 s, respectively. Fi- nally, unpronounceable, nonsense syllables (hjh, evgjc, bhdr, and gsxrq) produced only 5 s o f interference.

Although some o f Klein's items were not ideal (e.g., bhdr, gsxrq, and especially grass, which share initial letters with target color names), his basic point was sound: The more meaningful the irrelevant word, the more interference it caused. This was particularly true for color-related words, b u t even n o n c o l o r words produced some interference. Klein's experiment opened the floodgates for numerous studies o f how word meaning af- fects color naming.

D a l r y m p l e - A l f o r d (1968, 1972a; D a l r y m p l e - A l f o r d & Az- koul, 1972; see also Harrison & Boese, 1976) extended Klein's procedure. Dalrymple-Alford (1972a) found that, c o m p a r e d with unrelated control words, color words interfered more than color-related words (red, 131 m s vs. blood, 43 ms), but both caused reliable interference, replicating Klein. More novel, both also caused reliable facilitation on congruent trials (red, 63 ms, and blood, 42 ms). Dalrymple-Alford discounted a purely articulatory explanation because words like s k y and snow (for blue and white) caused just as large effects as words like grass (for green) even without shared letters.

As Stroop knew, choosing the appropriate control stimulus is not trivial and warrants an aside here. Some studies use an unrelated word, some a nonword, some a row o f Xs or other r e d u n d a n t character, a n d some a color patch. For example, Dalrymple-Alford's (1972a) study also included a X X X X con- trol. I f X X X X rather than the unrelated word is used as the baseline, interference estimates for the incongruent cases in- crease by 65 ms, and there is no facilitation for the congruent

cases. A n o n w o r d control might have p r o d u c e d yet a n o t h e r baseline value.

I n general, using any characters in the control c o n d i t i o n stands to be distracting and to increase the time to respond to the color, but substantial changes in interference typically re- quire some connection to the concept o f color (or some kind o f priming). Thus, although H i n t z m a n et al. (1972) obtained reli- able interference and facilitation for color words relative to a nonword control, words unrelated to color did not differ from the control. A similar result was reported by Redding and Get- jets (1977), who also found that scrambled color words were equivalent to noncolor words. Using unrelated words versus nonwords, then, makes little difference, hut the switch to color patches o r X X X X - t y p e controls m a y have more effect.

Scheibe, Shaver, and Carrier (1967) found that a m o u n t o f interference in color n a m i n g was a direct function o f a m o u n t o f color association for the irrelevant words, and argued that word frequency was critical. 5 Langlois (1974) reported that a direct manipulation o f word frequency influenced a m o u n t o f interfer- ence, hut he failed to separate response-set membership from the frequency manipulation, a c o m m o n problem in Stroop re- search. Effier (1977a, 1980, 1981) has also reported frequency effects.

Proctor (1978) pointed out that strength o f association and frequency were typically confounded with response-set m e m - bership. He broke this confounding and showed that response- set members still interfered more than nonmembers. For color words not in the response set, interference was a direct function o f the strength o f association o f the words to the concept o f color, a conclusion in accord with that o f Fox et al. (1971).

Alperson (1967) trained subjects on paired associates where the response terms were either connected o r unconnected to the color-naming task. W h e n the stimulus terms were made the irrelevant materials in the color-naming task, semantic related- ness o f the response terms had a considerable effect, even with these newly learned pairs. To bolster the conclusion that mean- ing was the key, Bakan and Alperson (1967) demonstrated that words interfered more than did nonsense syllables equated for pronounceability.

Using the same approach, Pritchatt (1968) had subjects asso- ciate nonsense syllables with colors, and observed interference when the nonsense syllables appeared later as the irrelevant dimension in a color-naming task. Analogously, Stirling (1979) required subjects to learn to respond to color patches with letter names. W h e n these letters became the incongruent dimension in color naming, interference resulted. Similar procedures have been used elsewhere (e.g., Glaser & Dolt, 1977; MacLeod & Dunbar, 1988). O f course, whether these studies speak to the role o f meaning in interference depends u p o n the representa- tion underlying such paired-associate learning.

More in keeping with Klein's procedure, White (1969) had subjects identify color versus direction attributes in the pres- ence o f incongruent attribute names. The more semantically similar the incongruent attribute name, the more interference

5 A set of norms for colors and color words has since been con- structed and could be used in a detailed investigation of association (Solso, 1971).

THE STROOP EFFECT 173

was obtained, particularly for the color dimension. Fox et al. (1971) showed semantic gradients o f interference o n several Stroop analogs, including spatial direction a n d numerosity, and strongly supported Klein's arguments.

Murray, M a s t r o n a r d i , a n d D u n c a n (1972) also s u p p o r t e d Klein in showing that subjects n a m e d the colors o f color words more slowly than those o f animal words. However, when sub- jects were required to sort items o n the basis o f whether the words o r the ink colors were the same o r different, colors were sorted faster than words. T h e nature o f the task can completely determine the form o f the interference observed. Similar points have been made in the picture-word work (Glaser & Glaser, 1989; Smith & Magee, 1980) and in the priming work (Henik et al, 1983).

I n an often-cited study, Keele 0 9 7 2 ) argued that interference must occur after the memory-retrieval stage. W h e n subjects pressed buttons to indicate their responses (rather than speak- ing them), only c o l o r words interfered; n o n c o l o r words a n d scrambled color words did not differ from his control (letter- like G i b s o n forms). The scrambled-condition result is surpris- ing, though; every stimulus began with the same letter as its corresponding word (rde, gnere, ywoloe, and belu), which ordi- narily causes interference (see Acoustic Variation). 6 Possibly Keele's use o f manual responses, known to decrease interfer- ence overall, is responsible (see Response Modality: Oral Versus Manual).

Many investigators have manipulated the meaning o f the sin- gle, irrelevant word. For instance, Etiler (1978a) practiced sub- jects o n color-related versus color-unrelated words a n d re- p o r t e d a decreased difference in interference with practice. Also Ridley, Johnson, a n d Braisted (1978; see also Ridley, 1980) presented evidence that semantic-differential ratings o f non- color words predicted color-naming times. In sum, the many manipulations o f meaning o f the irrelevant word on a single Stroop trial all converge on this empirical generalization: Com- pared with naming the ink color alone, irrelevant verbal stimuli unrelated to the concept of color inteoCere only minimally with color naming. However, as the words semantic association to the concept of color increases, so does its potential to interfere.

Semantic variation induced by priming. Since the early 1970s, the semantic-priming technique has had increasing con- tact with the Stroop literature. Probably the two most widely k n o w n m e m b e r s o f this set are the dissertations o f Warren 0 9 7 2 ; see also Warren 1974) and C o n r a d (1974). Discussing these in detail will both introduce the m e t h o d and set the stage for the variations that have succeeded them.

Warrens (1972) basic idea was that the irrelevant word o n a Stroop trial should interfere more if it had just been activated by a preceding event. Preceding each critical color-word trial, he placed a priming event. O n control trials, this event con- sisted o f the phrase " N o list this trial:' O n priming trials, the event consisted o f a list o f three words from a single semantic category: aunt, uncle, cousin or dog, cat, horse, for example. Then the critical color-naming trial occurred, with either the category name (relatives), a repeated category m e m b e r (aunt), an unrelated word (doctor), or a row o f Xs as the "word y

W h a t Warren observed was increased interference in naming the ink color when the irrelevant word was primed. For the words aunt or relatives in red ink, subjects were slower to say

"red" if they had just seen relevant primes; the effect was some- what larger for aunt. In later experiments, Warren established that this activation persisted over at least five intervening items when subjects had to remember earlier words for a final test, but only a bit over one intervening item (30 s) when this require- ment was dropped. This was true whether the critical irrelevant word was the category n a m e or a repeated category member. He did not test nonstudied category members.

In his 1974 follow-up study, Warren varied the associative strength between an auditory prime word and the irrelevant visual w o r d o n the critical c o l o r - n a m i n g trial. T h e relation could be high (king-queen), m e d i u m (bath-clean), or low (wish- dream). Subtracting the control times for unrelated auditory primes (e.g., food, law, or hand), there was significant interfer- ence for high (95 ms) and m e d i u m (50 ms) but not for low (20 ms) items. Thus, interference declined systematically with asso- ciative strength. Warren then demonstrated that this occurred only for forward associates, not backward associates, strongly suggesting that the prime activates the irrelevant word (rather than the reverse) on the critical trial.

Conrad's (1974) thesis used Warrens as a springboard, ex- tending the priming d o m a i n from words to sentences. W h e n the sentence context c a m e before the a m b i g u o u s w o r d (e.g., "The toy costs a nickel"), there was considerable interference in color n a m i n g for the ambiguous word itself(nickel, 100 ms), the appropriate category (money, 106 ms), and even the inappropri- ate category (metal, 64 ms), relative to a control sentence (e.g., "The craftsman made the chair"). Apparently, both senses o f nickel were activated by the sentence despite its clear meaning.

Interestingly, when the sentence context came after the ambig- uous word (e.g., "That nickel is from her coin collection'), inter- ference in the three conditions virtually disappeared (19, 2, and 35 ms, respectively). C o n r a d took this to indicate either decay o f activation or interference from succeeding words. The critical result, though, was the context-before condition. She went o n to show that the interfering capability o f an ambiguous word was no greater in an ambiguous sentence: Both senses o f the word were activated regardless o f how it was used in the sentence. Oden and Spira (1983) confirmed that both senses o f an ambigu- ous word were indeed activated, but that their extent o f activa- tion was affected by context. All candidates were activated, and then further processing narrowed the possibilities.

M a n y studies sprang f r o m those o f Warren a n d C o n r a d . M c C l a i n (1983a) a t t e m p t e d t o link their logic with Klein's (1964). She manipulated both the prime word and the irrele- vant word on the color-naming trial using incongruent combi- nations only. There were four critical relations: (a) prime-red, irrelevant-blood, (b) prime-apple, irrelevant-fire, (c) p r i m e - a p - ple, irrelevant-apple, and (d) prime-nail, irrelevant-nail. Be- cause Condition (a) explicitly activates color and Condition (b) repeats the irrelevant but color-related word, she predicted that these two conditions would produce more interference, which they did.

Merrill, Sperber, and McCauley (1981) pursued the influence

6 Furthermore, Michael Masson and I have data showing that scram- bled color words with no letters in the correct position still cause inter- ference in an oral response.

174 COLIN M. MACLEOD

o f context on semantic interpretation. O n a critical trial, a tar- get word such as claw appeared in a color to be named. Before the critical trial, either an appropriate ("The girl fought the cat"), an inappropriate ("The girl touched the cat"), or a neutral ("The m a n fixed the car") sentence or word (cat vs. car) was displayed. Poor and good comprehenders showed equivalent interference when the prime was an appropriate or inappropri- ate word. However, when the prime was a sentence, poor com- prehenders again showed interference for both types o f primes, but good comprehenders showed interference only for appro- priate primes. Merrill et al. c o n c l u d e d that g o o d c o m p r e - henders encode words with reference to an entire sentence's meaning, whereas poor comprehenders encode the words as distinct units.

Dosher and Corbett (1982) examined whether sentences like " T h e m a n swept the floor" activated relevant instruments (e.g., broom). I f so, then broom should interfere with naming its color o f print. There was no evidence for such activation, relative to an unrelated prior sentence, except if subjects were explicitly told to think o f the instrument. Interestingly, such an instruc- tion led to facilitation, not interference, contrary to the other studies in this section (but see Keenan, Potts, Jennings, & Gold- ing, 1988, and Whitney, 1986, for similar findings). Perhaps i f a p r i m e d irrelevant w o r d is processed especially quickly, it is dealt with before it can exert a negative effect. This idea merits further exploration; it may represent a potentially useful tool in psycholinguistic research.

In a series o f studies, Whitney and his colleagues (Whitney, 1986; W h i t n e y & Kellas, 1984; Whitney, McKay, Kellas, & Emerson, 1985) examined lexical access using the Stroop task. Initially, Whitney and KeUas (1984) presented sentences that biased a typical m e m b e r o f a category (e.g., "The guest saw the bird that landed o n the branch"--robin) or an atypical member (e.g., " T h e guest saw the bird that r o a s t e d o n the g r i l l " - - chicken), or that were neutral (e.g., "The guest saw the towels that hung in the bathroom"). Regardless o f the prime sentence, the typical word always caused interference, whereas the atypi- cal word did not. Yet typical words were better retrieval cues for typical sentences, and atypical words were better retrieval cues for atypical sentences. They concluded that category referents are not necessarily instantiated for single sentences.

O n the basis o f manipulations o f item d o m i n a n c e a n d delay before testing, Whitney et al. (1985) confirmed the findings o f Conrad (1974) and o f Oden and Spira (1983) that initial seman- tic access is independent o f context both for ambiguous and unambiguous nouns. Whitney's (1986) dissertation extended this work to anaphoric reference. Priming with atypically bias- ing sentences led to interference in color naming for atypical words, whereas priming with typically biasing sentences led to facilitation for typical words, the latter reminiscent o f Dosher and Corbett's (1982) finding. Clearly, word meaning matters; even noncolor words can produce (or increase) interference--or even facilitation--under suitable priming conditions.

Also important is the subject's strategy. Henik et al. (1983) d e m o n s t r a t e d that when subjects n a m e d p r i m e s before the color-naming trial, n o r m a l interference was observed. How- ever, when subjects examined the primes for specified target letters, very little interference was observed. A corresponding p a t t e r n o c c u r r e d for facilitation in lexical decision. Again

meaning matters: Diverting the subject from thinking o f the meaning o f the prime word virtually eliminated the usual ef- fects.

Much o f the research o n word meaning has been in the pie- ture-word task, where the meaning o f both the picture and the word---and their r e l a t i o n - - c a n be varied. Probably the best known work using this task was by Lupker (1979; Lupker & Katz, 1981, 1982). In his 1979 paper, he confirmed Rosinski's (1977) finding that same-category words magnified interfer- ence relative to either unrelated words or pronounceable non- words, which caused equivalent interference. Categorical rela- tions have a similar effect in the "flanker" task where both stimuli are words (Shaffer & LaBerge, 1979).

Lupker went on to show that association between the picture and the word did not increase interference, nor did instance typicality modulate the extra interference as a result o f shared category. Thus, some semantic variables are not potent. Word imageability was a critical determinant in Lupker's study, which led h i m to suggest that the word's relevance to response de- mands was crucial, as in the color-word task. Lupker (1979, p. 494) went so far as to maintain that the two tasks were "nothing more than two sides o f the same c o i n : '

Smith and Magee (1980; see also Magee, 1982) demonstrated that although incongruent words interfere with picture naming, incongruent pictures interfere with word categorization. They argued that pictures gain access to semantic information more readily than to name information, but the reverse is true for words. Glaser and Glaser 0989) took this argument consider- ably farther, building a model for n a m i n g versus categorizing. These studies, like that o f Henik et al. 0 9 8 3 ) with the color- word task, highlight the point that the task required o f the sub- ject when faced with a multidimensional stimulus is a crucial determinant o f the pattern o f interference observed.

T h e e m p i r i c a l generalization is straightforward: A color- unrelated word can be made to cause greater interference (or facili- tation, or both) with color naming i f its meaning is activated by a related word or phrase shortly before the color-naming trial. Re- lated words and primed words are hard to ignore. Certainly, questions remain to be addressed here (e.g., the time course o f such priming effects), but the basic result is well established.

C o n g r u e n c y Effects

So far this review has focused on Stroop interference. How- ever, there is another side to the coin: Stroop facilitation. I f the wrong word can slow ink-naming performance, then intuitively the right word ought to speed performance. Curiously, the con- gruent case (e.g., red in red ink, respond "red") was not investi- gated until the advent o f the individual-trial version o f the Stroop task. Although Langer and Rosenberg (1966) had a con- gruent condition o f sorts in their study, the first actual use o f color-word congruent trials was by Dalrymple-Alford and Bu- dayr (1966). Their inclusion o f congruent items a m o n g incon- gruent ones on Stroop cards made little difference in overall response time.

Shortly thereafter, Sichel and Chandler (1969) used the indi- vidual-item procedure, and reported that color naming was in fact faster for congruent items (645 ms) than for incongruent items (777 ms), but that both showed interference relative to a

THE STROOP EFFECT 175

X X X X - t y p e control (541 ms). Thus, it is more reasonable to say that there was reduced interference rather than true facilitation for the congruent condition. Perhaps the extent o f apparent facilitation depends o n the choice o f control condition; recall that Dalrymple-Alford (1972a) observed no facilitation relative to a X X X X control but reliable facilitation relative to an unre- lated-word control.

Although there were reports o f interference in the congruent condition (e.g., Nealis, 1973; Schulz, 1979), more often some degree o f facilitation was reported (e.g., Kalkofen, 1969). One complication was whether congruent and incongruent items were mixed or separated (blocked), as indicated in the exchange between Smith (1970) a n d Kalkofen (1970). Indeed, using a spatial analog, Logan and Zbrodoff(1979) showed that the pro- portion o f congruent trials over the entire task affects process- ing time (see Probability of Various Trial Types). A further c o m - plication is that congruence o f word and color across successive trials can also have an impact (Ettler & Rabenstein, 1979).

Dyer (1973a) presented word and color o n different sides o f the fixation point and observed 26 ms o f facilitation for the congruent case (compared with a X X X X control). Interference was 48 m s in the incongruent case. In a same-different analog also using the X X X X control, Dyer (1973b) found significant interference (179 ms) a n d facilitation (55 ms). Recalling Dalrymple-Alford's (1972a) findings o f 131 ms o f interference and 63 m s o f facilitation, it is clear that there is usually much less facilitation than interference (see also Dyer, 1974).

H i n t z m a n et al. (1972) reported 42 m s o f facilitation relative to a scrambled color-word control or a "neutral" control, both retaining the first letters o f color words. My concerns are that (a) such controls would themselves be expected to interfere, so the facilitation might disappear with a more standard control, and (b) the claim that neutral words do not interfere in the individ- ual-trial version o f the task (contrary to Klein's finding that they do in the list version) is suspect given their choice o f con- trol condition. Strengthening my suspicions, Redding and Ger- jets (1977) found facilitation in the congruent condition relative to scrambled color words a n d neutral words, but not relative to nonword controls.

In her dissertation, Regan (1979) obtained facilitation for congruent words and interference for neutral words; she also obtained facilitation using just the first letter o f the correct color name (see also Regan, 1978). Yet turning the words upside d o w n and backward to slow reading, D u n b a r and MacLeod (1984) obtained little facilitation in the congruent condition despite huge interference in the incongruent condition. In fact, the congruent condition was almost identical to the X X X X control condition across o u r experiments.

Glaser and Glaser (1982) obtained 16 ms o f facilitation and 72 ms o f interference, relative to a X X X X - t y p e control, when the color and word were simultaneous and the task was color naming. Thomas's (1977) dissertation presents similar data. This a s y m m e t r y is by now the expected pattern: Facilitation is not a necessary concomitant o f interference, although this may have more to do with measurement than with a true theoretical decoupling.

With a neutral word as the control condition, Duncan-John- son and Kopell (1980, 1981) found reliable facilitation in the c o n g r u e n t c o n d i t i o n (and s t r o n g interference in the i n c o n -

gruent condition). However, they observed no change in P300 latency, an event-related brain potential assumed to be a "pure" measure o f stimulus processing. O n this basis, they favored a r e s p o n s e - c o m p e t i t i o n a c c o u n t o f the S t r o o p effect, as did Warren and Marsh (1979) in a similar study.

Rayner and Posnansky (1978) obtained facilitation for pseu- d o h o m o p h o n e s (e.g., bloo) in the picture-word task, but their use o f blocked trial types allowed a strategy o f responding by reading the words. Mixing trial types, Dennis and Newstead (1981) obtained facilitation for color words but not pseudoho- mophones, although both produced interference. This suggests that the slow-to-read p s e u d o h o m o p h o n e s could not be pro- cessed sufficiently quickly to assist color naming in accord with a relative speed-of-processing account.

There have also been demonstrations o f facilitation in Stroop analogs. Certainly, this is evident in the picture-word task (e.g, Ehri, 1976; Golinkoff& Rosinski, 1976; Underwood, 1976). For example, Ehri (1976) reported facilitation in picture naming for c o n g r u e n t pictures a n d words using a picture-only control. More recently, Glaser and Diingelhoff (1984) showed reliable facilitation for the c o r r e c t n a m e o f a p i c t u r e relative to a X X X X control, although the facilitation o f 44 ms was substan- tially less than the interference o f 131 m s caused by a different m e m b e r o f the same category.

Using color naming o f season-appropriate or inappropriate words, Seymour (1977) obtained facilitation plus interference. Using the arrows task, Clark and Brownell (1975) found sub- jects to be faster to identify which way an arrow was pointing (up or down) if it was located in the appropriate place in space (top or bottom, respectively). For letter identification in the pres- ence o f irrelevant letters, Taylor (1977) observed reliable facilita- tion as well. O f course, facilitation is very well known in the word-priming domain.

To summarize, here is the general conclusion: Congruence between the irrelevant word and the to-be-named ink color often produces facilitation. However, this facilitation is much less than the corresponding interference in the incongruent condition, and the choice of control condition may be crucial Partly, this is a c o n s e q u e n c e o f the relatively smaller a m o u n t o f facilitation usually obtained, itself no doubt resulting partly from the prob- lem o f trying to speed up an already rapid naming response.

It would be interesting to perform a color-word study along the lines o f the recent work by La Heij and Vermeij (1987), which showed that, whether reading words or naming pictures, interference decreases and facilitation increases as a function o f increasing target set size. The small facilitation we have come to expect may be at least partly a consequence o f the small set size usually used in Stroop experiments. 7

Integration o f the Two Dimensions

So far in this section, I have considered studies focusing on the relevant dimension (color) and on the irrelevant dimension (word). The only aspect o f the single trial that remains to be

7 Note, however, that Marina Vanayan and I have data from the pic- ture-word task showing an increase in interference with increasing set size, so this matter is not resolved as yet.

176 COLIN M. MACLEOD

dealt with is the relation between the two dimensions. O n a given trial, the two dimensions o f the stimulus can be more or less perceptually integrated. We know that stimulus integration can be a powerful determiner o f performance in many tasks (see Garner, 1974); how m u c h does it influence the Stroop ef- fect?

Despite relatively little published work, this issue m u s t be addressed if we are to c o m p a r e various versions o f the Stroop task meaningfully. For instance, the word is not integrated into the picture in the picture-word task to the same extent that it is integrated into the color in the standard color-word task. Fur- thermore, if color/picture and word are separated, as in SOA studies (e$., Glaser & Glaser, 1982; Glaser & Dfingelhoff, 1984) or in the bar-above-word procedure ( K a h n e m a n & Chajczyk, 1983), d o results f r o m these modifications translate readily back to the original task?

There were some hints as to the importance o f integration even 30 years ago (e.g., G a r d n e r et al., 1959; G a r d n e r & Long, 1962). More recently, though, Flowers and Stoup (1977) ob- served that interference persisted virtually unchanged over four practice sessions in a card-sorting task where word and color were integrated (i.e., the word red printed in green ink), but vanished with the same a m o u n t o f practice when they were not integrated (i.e., the word red inside a green-outlined rectangle). Moreover, interference was greater in the integrated case.

Although there is no doubt that nonintegrated Stroop stimuli can show interference and facilitation (Dyer, 1973a; Gatti & Egeth, 1978; Glaser & Glaser, 1982; K a h n e m a n & Chajczyk, 1983; Merikle & Gorewich, 1979), this does not m e a n that (all o f ) the same m e c h a n i s m s are involved. As K a h n e m a n a n d Henik (1981) demonstrated, if the incompatible word and color patch appear in the same attended location, there is more inter- ference (202 ms) than if they appear in adjacent locations, only one o f which is the focus o f attention (50 ms). K a h n e m a n and Henik also observed that compatible words facilitated color naming if the words were in the attended location (48 ms), but interfered if they were nonattended (38 ms).

Bradlyn and Rollins (1980) obtained interference in the inte- grated version but not in a nonintegrated one (the word in black ink above a color patch); however, this result appears not to generalize. Thus, Gatti a n d Egeth (1978) observed a decrease in interference from 90 to 40 m s as the word moved farther from the to-be-named color patch (from one to five degrees), but all cases produced reliable interference. T h e integrated version o f the Stroop task,is harder, but both versions produce interfer- ence.

Although it seems that nonintegrated stimuli do produce in- terference fairly reliably, the a m o u n t is relatively small, appar- ently falling offas separation increases. Using the picture-word task, U n d e r w o o d (1976) a r g u e d t h a t locational u n c e r t a i n t y with respect to the two dimensions o f the task was very influen- tial. W h e n subjects knew where to expect the relevant picture versus the irrelevant word, they showed a small a m o u n t o f inter- ference from the competing word. W h e n they did not know where to expect picture versus word, interference was consider- ably greater. Apparently, not being able to prepare for where to direct attention makes the separated case more analogous to the integrated case.

W h a t is needed is a systematic parametric comparison o f the

two versions o f the task, perhaps manipulating practice as in the Flowers and Stoup (1977) study. Until such time, the extent o f importance o f dimensional integration is difficult to ascer- tain. Certainly, though, it seems to matter, so here is my some- what tentative general conclusion: I f the to-be-named color and the to-be-ignored word are presented in separate spatial locations, interference will be reduced (but not eliminated) relative to the standard, integrated version o f the task. Locational uncertainty makes an important contribution in nonintegrated situations.

We have seen what happens when the dimensions o f a single Stroop trial are varied; let us now examine what happens with manipulations intended to affect the entire task.

E x p e r i m e n t - W i d e Manipulations o f Information

This section focuses on variables affecting more than one trial in a Stroop experiment. Some o f these variables--such as the presence or probability o f certain trial t y p e s - - o n l y exist with reference to more than one trial. O t h e r s - - s u c h as S O A - - occur at the level o f a single trial, but have broader implications. These variables all may be thought o f as affecting how the sub- ject approaches the task as a whole.

Probability o f Various Trial Types

Probably the most straightforward way to set up a Stroop experiment is Stroop's (1935b). He used four conditions, each in an independent block o f trials: incongruent color naming, con- trol color naming, incongruent word reading, and control word reading. At first glance, this might also a p p e a r to be the "safest" way, reducing the likelihood o f subjects adopting special strate- gies to deal with a mixture o f trial types. Does blocking condi- tions minimize strategy use or does it promote creating a spe- cial strategy for this special case? The m o s t reassuring results are those that hold up under both mixed and blocked condi- tions. No doubt, too, the composition o f the entire set o f trials influences the subject's strategy. In the limit, if all trials in a block were congruent and the task was to name the ink colors, clever subjects could decrease their response time by reading the words instead. Such a strategy would not work if all o f the trials were incongruent. The question, then, is what happens at intermediate values.

I have already mentioned that Dalrymple-Alford and Budayr (1966) found n o change in interference when some congruent trials were added to a card made up largely o f incongruent trials, but they did not systematically manipulate the frequency o f trial types. Zajano and G o r m a n (1986) showed that total color-naming time per card decreased as more congruent trials were added to the card. Shor (1975) used Hamers's (1973) high- low auditory analog and found more interference when half o f the trials were congruent and half incongruent than when all o f the trials were incongruent. This counterintuitive result might be a consequence o f subjects listening to the word and saying its opposite in the incongruent case, a strategy that could be elimi- nated by using more than two responses. My guess is that Shor's result would reverse itself with such a modification.

Logan and Zbrodoff(1979) used the above/below spatial an- alog o f the Stroop task where subjects are to read the word and ignore its spatial location. (Note that in this task interference

THE STROOP EFFECT 177

occurs in reading the word, unlike in the Stroop task.) As the frequency o f incongruent trials increased from 10% to 90% over experiments, response times shifted from congruent trials be- ing faster to incongruent trials being faster. Logan and Zbro- d o f f were careful to point out that low error rates indicated subjects were not simply "cheating" by attending to the suppos- edly u n a t t e n d e d d i m e n s i o n . Apparently, subjects a d o p t e d a strategy o f dividing attention over the two dimensions.

Still it would be better to use a larger set o f items to preclude the strategies possible with only two stimuli. Lowe and Mitterer (1982) used three ink colors (red, blue, and green), the three corresponding color words, a n d three neutral words (far, most, and slant). O n each trial, two words were presented. The target was always a neutral word in color, and the subject was to name the ink color. T h e other word was always printed in black ink, but could be a color word o r neutral word. W h e n this irrelevant word was a color word, it could either be congruent o r incon- gruent with the target ink color.

Lowe and Mitterer manipulated the proportion o f congruent stimuli in steps o f .25 from 0 to .75. Relative to the neutral control, interference increased from 35 ms with 0% congruent trials to 66 ms with 75% congruent trials; however, facilitation hardly changed (12 to 17 ms). This was taken as evidence o f automatic word encoding in both conditions, but with a further conflict-resolution process in the incongruent case following Taylor's (1977) arguments. Note, too, the different effects o n facilitation and interference.

Although there are limited data, it seems that the composi- tion o f the entire set o f trials does matter. As the proportion o f congruent trials increases, the "irrelevant" word becomes pro- gressively more relevant. Here, then, is the general conclusion: The presence o f congruent trials among the incongruent and con- trol trials will tend to invoke the tactic o f splitting attention over the two dimensions, thereby increasing interference on incongruent trials. For this reason, it is probably best to conduct novel exper- iments initially without a congruent condition, and then to add that condition and observe whether its presence changes inter- ference fundamentally. T h u s far very few experiments have taken this tack. We should also determine the extent to which this attention-splitting strategy is under subject control, per- haps through a manipulation o f instructions o r payoffs.

S t i m u l u s S e t S i z e

There is typically quite a small set o f stimuli in the color- word task and quite a large set in the picture-word task. Proba- bly this is because creating large, differentiable sets o f items is easier with pictures than with colors. Indeed, the bulk o f the color-word studies use from two to five stimuli. We know that increases in stimulus set size increase time to name but have little i m p a c t o n t i m e t o read (cf. Fraisse, 1969; G h o l s o n & Hohle, 1968b). Consequently, some investigators adopted set- size manipulations as one way to attack interference.

A n illustration o f how few colors are typically used in the Stroop task comes from a study o f set-size effects by Golden (1974a); he found that using three, four, or five different colors made little difference in interference. Other research supports his conclusion. Gholson and Hohle (1968a) found that, in a choice reaction-time version o f the Stroop task, increasing set

size from two to four increased overall response time but not interference. Essentially the same result was obtained for set sizes three to five by Ray (1974) and for set sizes two to five by McClain (1983b). Although this is the usual range, I a m reluc- t a m to infer from such small variations that set size is not a factor.

Until recently, the only discordant Stroop study was that o f Williams (1977), who used the largest variation: from 2 to 8. She found that both interference and facilitation increased as set size increased. The picture becomes even more complicated as soon as we step beyond the color-word task. In the increas- ingly c o m m o n two-word analog, several studies have consis- tently found decreases in interference coupled with increases in facilitation as set size increased. Taylor (1977) obtained this result in moving from 2 to 4 alternatives; La Heij, Van der Heijden, and Schreuder (1985) obtained it in moving from 2 to 12 alternatives. How are we to bring these disparate findings together?

La Heij and Vermeij (1987) recently tried to answer this ques- tion. Varying set size from two to eight in both a letter-reading a n d a p i c t u r e - n a m i n g task (in separate experiments), they found that interference decreased and facilitation increased for both types o f materials relative to a X X X X - t y p e control. Al- though they claim (p. 359) that it would be difficult to replicate this experiment using the color-word task, I a m not convinced: Their pattern is evident even between set sizes two and four. Partly because their findings conflict with those just reviewed, a replication would be useful. Results with the color-word task that corresponded to those o f L a Heij and Vermeij (1987) would further strengthen the argument that all o f these superficially different interference tasks in fact tap a c o m m o n element.

A general conclusion here, given the conflict in the data base, is risky Furthermore, there is the issue o f response set size yet to be discussed (see Response Set Size and Composition). So I sim- ply note that stimulus set size may influence the Stroop effect. Part o f my caution stems from some data that Marina Vanayan and I have collected: Increasing set size in the picture-word task from 4 to 12 increased interference in our study This disagrees with La Heij a n d Vermeij, and we are pursuing possible reasons for the difference. Until such discrepancies are resolved, it would be premature to offer a firm statement about stimulus set-size effects.

Trial S e q u e n c e

People often feel particularly tongue tied when the to-be-ig- nored word on one trial turns out to be the to-be-named color on the next trial (e.g., green in red ink precedes blue in green ink). H a v i n g just suppressed the response "green" seems to make it harder to say it on the next trial: There are sequential effects in the Stroop task. Dalrymple-Alford and Budayr (1966) first pointed out that interference in the list version o f the Stroop task was greatest if each ink color was that n a m e d by the immediately preceding word.

The most detailed investigations in this area are those o f Neill (1977, 1978; NeiU & Westberry, 1987) and Etiler (1977a, 1977b, 1978b, 1980; Effier & Rabenstein, 1979). Effier (1977a) reported a "serial interference": W h e n the irrelevant word on trial n was acoustically similar to the ink color o n trial n + 1,

178 COLIN M. MACLEOD

interference was e n h a n c e d . Conversely, when the i n k color o n t r i a l n was t h a t o f the irrelevant w o r d on trial n + 1, interference was r e d u c e d (Effler, 1977b). H e t h e n c o n f i r m e d a n d e x t e n d e d these results in a variety o f ways (Effler, 1978b, 1980; Ettler & R a b e n s t e i n , 1979).

T h e first o f these findings m a k e s sense i f it is a s s u m e d t h a t the w o r d o n trial n m u s t be suppressed, a n d t h e n has to b e given a d d i t i o n a l activation to b e c o m e the response on trial n + 1. The second result c a n be e x p l a i n e d as an i n s t a n c e where having j u s t m a d e a p a r t i c u l a r response o n the last trial m a k e s it easier to d i s c a r d t h a t as a possible response o n this trial. O f course, such explanations are easier after having seen the data.

N e i l l (1977) e x t e n d e d t h e D a l r y m p l e - A l f o r d a n d B u d a y r (1966) observation to the i n d i v i d u a l - t r i a l version o f the S t r o o p task, finding the s a m e result t h a t they a n d Efller (1977a) found: greater interference i f the w o r d o n trial n m a t c h e d t h e i n k color on trial n + 1. Neill labeled the p h e n o m e n o n the distractor-sup- pression effect. We now k n o w t h a t the p h e n o m e n o n is fairly general (e.g., Westberry, 1984), a n d extends to letter n a m i n g (Allport, Tipper, & C h m i e l , 1985; T i p p e r & C r a n s t o n , 1985) a n d picture n a m i n g (Allport et al., 1985; Tipper, 1985). In the case o f pictures, even n o n i d e n t i c a l m e m b e r s o f the s a m e cate- g o r y can p r o d u c e the d i s t r a c t o r - s u p p r e s s i o n effect.

N e i l l (1978) o b s e r v e d t h a t w h e n t h e w o r d o n t r i a l n + 1 m a t c h e d the i n k color j u s t n a m e d o n trial n, there was facilita- tion, this t i m e replicating Effler (1977b). He t o o k this as evi- dence t h a t the set o f highly activated c o m p e t i n g responses was m a d e smaller b y the overlap, hence reducing response c o m p e t i - t i o n , as o p p o s e d to t h e s u g g e s t i o n I offered in d i s c u s s i n g Etfler's (1977b) study.

Lowe (1979, 1985) first o b t a i n e d this successive-trials effect when consecutive items were not all i n c o n g r u e n t items o f the s t a n d a r d t y p e u s e d b y Neill, thereby increasing its generaliza- tion. H e t h e n m a n i p u l a t e d t h e p r o p o r t i o n o f trials o f various t y p e s in subsequent experiments, showing t h a t the distractor- suppression effect c o u l d be e l i m i n a t e d o r even t u r n e d into facili- tation. This clearly implicates subject strategies in the p h e n o m e - non, a n d Lowe was u n c o n v i n c e d o f the n e e d to p r o p o s e an inhibitory m e c h a n i s m o f the sort favored b y NeiU o r Tipper. He

;~uggested that subjects m a y have b e e n t r y i n g to m a t c h succes- sive stimuli, thereby n o t treating trials as i n d e p e n d e n t , b u t later

I rescinded this a r g u m e n t (Lowe, 1985). Efforts to m a k e succes- sive stimuli less matchable, by using different cases a n d by vary- ing list c o m p o s i t i o n a n d SOA, d i d not noticeably alter the dis- t r a c t o r - s u p p r e s s i o n effect. Still r e s i s t i n g a n i n h i b i t i o n argu- ment, Lowe m a i n t a i n e d t h a t the p r o b l e m was in c o o r d i n a t i n g the c o l o r a n d w o r d codes.

M o r e recently, N e i l l a n d W e s t h e r r y (1987) m a n i p u l a t e d s p e e d - a c c u r a c y i n s t r u c t i o n s a n d intertrial interval. U n d e r a c c u - racy instructions, subjects showed the effect, but it d i s a p p e a r e d as emphasis o n speed increased. As well, the effect persisted with u p to a l-s intertrial interval, b u t dissipated between 1 a n d 2 s. Neill a n d W e s t b e r r y i n t e r p r e t e d these findings as consistent with a selective-inhibition account: A f t e r b r o a d activation, se- lective inhibition is used to restrict processing to j u s t the rele- vant information, a n i d e a very m u c h in a c c o r d with the work o n p r i m i n g o f a m b i g u o u s words discussed u n d e r S e m a n t i c Vari- ation (e.g., O d e n & Spira, 1983).

The work on the d i s t r a c t o r - s u p p r e s s i o n effect suggests a gen-

eralization b e y o n d the S t r o o p effect. I n a d d i t i o n to activation, there may well b e an i n h i b i t o r y process t h a t helps to c o n s t r a i n the i n f o r m a t i o n selected for m o r e extensive processing. F o r the present, this is seen as a n active strategy o f inhibition. W h e t h e r the facilitation f r o m successive trials has to d o with a functional s h r i n k i n g o f the set o f c o m p e t i n g responses o r a suppression o f responses recently uttered is less clear. My own bias, in o p p o s i - t i o n to Neill (1978), is to invoke a suppression i d e a so t h a t t h e facilitation a n d interference effects as a result o f item sequence have a c o m m o n grounding.

As a footnote, it is interesting to c o n t r a s t these results with those involving a slightly different variation. T h o m a s 0 9 7 7 ) r e p e a t e d the w o r d c o m p o n e n t over successive pairs o f c o l o r - w o r d stimuli, a n d found t h a t w o r d repetition decreased b o t h interference a n d facilitation. Eftler (1980) c o n f i r m e d the inter- ference decrease, b o t h when the w o r d n a m e d an ink color in the response set a n d when the w o r d n a m e d a n i n k color not in the response set. However, there were no repetition effects for n o n - c o l o r words. Subsequently, Ettler (1981) t r i e d to d e t e r m i n e whether a m o u n t o f repetition over trials was i m p o r t a n t , b u t his o d d n o n m o n o t o n i c p a t t e r n provides little insight.

Ellison a n d L a m b e r t (1968) t o o k a rather different tack. T h e y had subjects r e p e a t the c o l o r words continuously before per- forming the interference trials, with the goal o f reducing the m e a n i n g o f the words a n d hence t h e i r potential for interference. A l t h o u g h p e r f o r m a n c e o n the interference c a r d s p e d up, they could not d e t e r m i n e whether this was due to "semantic satia- tion." A n u m b e r o f a d d i t i o n a l controls would have b e e n desir- able in this work. Still t h e i r basic finding is in line with Eftler's (1980).

These two different o u t c o m e s having to d o with the relation o f the word o n trial n - 1 to the critical stimulus on trial n actually fit together quite nicely, so I offer the following conclu- sion: W h e n the irrelevant word on trial n - 1 is the n a m e o f the target i n k color on trial n, interference with color n a m i n g will be e n h a n c e d temporarily; when the i n k color on trial n - 1 m a t c h e s the word on trial n, there will be s o m e facilitation o f color n a m i n g on trial n. I f the word on trial n - I is repeated on trial n, then the w o r d is already s u p p r e s s e d a n d will c a u s e less interference in n a m i n g a different i n k color on trial n. A n i n t e r e s t i n g s t u d y would b e to m i x these two t y p e s o f repetition effects in the s a m e e x p e r i m e n t , directly c o m p a r i n g their size.

P r e t r i a l C u e s

Very closely related to studies o f the effects o f one trial on the next are studies o f the effects o f a cue j u s t before a Stroop t r i a !. Nealis (1974) t a c k l e d this p r o b l e m by presenting color words in colored ink before a t o - b e - r e a d word. He was able to p r o d u c e a reverse Stroop effect in t h a t there was interference evident in reading the target word. H i n t o n (1976) found t h a t p r i o r presen- t a t i o n o f the w o r d whose i n k c o l o r was to b e n a m e d on the u p c o m i n g target trial t e n d e d to decrease color-naming t i m e ( b u t see Trial Sequence for conflicting results). However, when that w o r d had to b e rehearsed for later recall, response t i m e to its i n k c o l o r increased. H i n t o n t o o k the facilitation effect as e v i d e n c e o f r e d u c e d c o m p e t i t i o n for e n t r y i n t o a r e s p o n s e buffer; the increased interference effect was h e l d to b e the result o f increased p r o b l e m s in response selection.

THE STROOP EFFECT 179

Neill (1978) u s e d the p r e c u e as a sort o f " w a r n i n g ; the m o s t prevalent use o f this procedure. The target was a s t a n d a r d in- c o n g r u e n t c o l o r - w o r d c o m b i n a t i o n . N e i l l o b s e r v e d r o u g h l y equivalent response t i m e s after a neutral cue o r the relevant c o l o r word. However, interference was increased by cueing ei- t h e r the w r o n g ink-color n a m e o r the irrelevant c o l o r word. A c c o r d i n g to Neill, the n u m b e r o f c o m p e t i n g responses a n d t h e i r strength d e t e r m i n e d interference. T h e results were incon- sistent with the i d e a t h a t increasing a r e s p o n s e s availability c o u l d m a k e it easier to reject.

Using the hi-lo analog, H a r v e y (1984) v a r i e d the interval by which a w a r n i n g p r e c e d e d the critical stimulus. T h e w a r n i n g stimulus signaled the subject whether to r e s p o n d to the word's m e a n i n g o r its position. Oddly, only w o r d r e a d i n g showed inter- ference, which d e c l i n e d as the t i m e between w a r n i n g a n d criti- cal stimulus increased. H a r v e y t o o k this as evidence t h a t when subjects could b e t t e r focus t h e i r attention o n the a p p r o p r i a t e d i m e n s i o n because t h e y were w a r n e d , they were better able to c o p e with i n c o m p a t i b l e d i m e n s i o n s . This i d e a seems worthy o f further study.

L o g a n a n d Z b r o d o f f (1982), in t h e i r spatial analog o f the S t r o o p task, also u s e d the precue technique. W h e n the cue p r o - v i d e d advance i n f o r m a t i o n a b o u t whether the u p c o m i n g trial was c o m p a t i b l e o r incompatible, response t i m e on c u e d trials was faster t h a n o n u n c u e d trials. W h e n only one cue could appear, its benefit was greater t h a n when two cues could ap- pear. P r a c t i c e w i t h c u e s p e r m i t t e d s u b j e c t s to b e n e f i t f r o m t h e m even with s h o r t e r warnings, b u t d i d not increase the bene- fit. Overall, these results were t a k e n to reflect successful sub- ject-initiated c o n s t r u c t i o n a n d use o f strategies. Because the S t r o o p t a s k is often seen as a h a l l m a r k index o f automaticity, such d e m o n s t r a t i o n s o f the role o f strategic control are i m p o r - tant.

Logan, Zbrodoff, a n d W i l l i a m s o n (1984) p u r s u e d this using the c o l o r - w o r d task. By m a k i n g the ink color p r e d i c t a b l e f r o m the w o r d a n d t h e n p r o v i d i n g advance knowledge o f the word, they found t h a t subjects c o u l d p e r f o r m the color-naming t a s k m o r e r a p i d l y with two i n k colors b u t n o t with four. T h e i r result with only two responses is r e m i n i s c e n t o f Shor's (1975); the fact that the advantage o f the cue d i s s i p a t e d with four responses suggests t h a t the strategy c o u l d not h a n d l e the greater uncer- t a i n t y with m o r e stimuli.

P r o b a b l y t h e m o s t p r o v o c a t i v e r e c e n t results a r e t h o s e o f M a r c e l (1983, E x p e r i m e n t 3) on unconscious p e r c e p t u a l p r o - cessing. He first d e t e r m i n e d the interval required between a s t i m u l u s a n d a p a t t e r n m a s k such that a subject could not dis- c r i m i n a t e a w o r d f r o m a blank. T h e n he ran blocks o f trials in which a cue w o r d was either m a s k e d with this interval, to pre- vent conscious perception, o r m a s k e d with a very long interval, so t h a t it w o u l d b e easily seen. H e included congruent, incon- gruent, neutral (e.g., the w o r d water), a n d no-word control trials, a n d also v a r i e d over blocks whether the word was presented simultaneously with the c o l o r patch o r 400 m s earlier. Marcel found that p e r f o r m a n c e in the no-word control a n d the neutral control was essentially identical. A l t h o u g h facilitation a n d inter- ference effects were a little larger i f the subject was a w a r e - - p a r - ticularly when the w o r d p r e c e d e d the c o l o r b y 400 m s - - b o t h were present even when the subject c o u l d not r e p o r t whether a

w o r d cue h a d b e e n present. M a r c e l argued against the view t h a t interference arose in response production.

As M a r c e l p o i n t e d out, his was not the first study o f " s u b l i m i - nal perception" in the SWoop task. Severance a n d D y e r (1973) t r i e d b u t d i d not o b t a i n a s u b l i m i n a l S t r o o p effect. A l t h o u g h s i m i l a r to Marcel's in s o m e ways, t h e i r study used extremely b r i e f w o r d exposures followed b y a bright b l a n k field to o b t a i n chance p e r f o r m a n c e in pretesting. M a r c e l argued that t h e i r en- ergy m a s k i n g has its effect m u c h m o r e peripherally t h a n d o e s his p a t t e r n masking, so the difference in results is n o t t h a t sur- prising.

C h e e s m a n a n d Merikle (1984) e x a m i n e d M a r c e r s s u b l i m i n a l S t r o o p effect using a m o r e stringent forced-choice m e t h o d for o b t a i n i n g the o p t i m a l threshold o f awareness o f the cue word. Then, using four colors, they v a r i e d the cue-word d e t e c t a b i l i t y from 25% (chance) to 100% (unmasked prime). At 25%, there was no effect o f the cue word; as cue-word detectability in- creased, so d i d facilitation a n d interference. T h e y c l a i m e d t h a t t h e i r findings refuted Marcel's c l a i m o f unconscious p e r c e p t i o n in the S t r o o p task.

C h e e s m a n a n d M e r i k l e (1984) went on to d e m o n s t r a t e t h a t t h e y could o b t a i n effects o f the cue at several SOAs when they used a subjective threshold in which subjects c l a i m e d not to see the cue word. Subsequently, C h e e s m a n a n d M e r i k l e (1986) d e m - o n s t r a t e d that subjective thresholds d i d convey s o m e i n f o r m a - tion a b o u t processing. First, they showed t h a t the cue w o r d h a d an effect whether it was above o r b e l o w subjective threshold (so long as it was above objective threshold). T h e y then v a r i e d the p r o p o r t i o n o f trials in which the cue a n d target were congruent, a n d discovered that subjects c o u l d use this variation to assist t h e i r processing, but only i f the cue w o r d was above subjective threshold. In other words, a strategy c o u l d b e i m p l e m e n t e d only when subjects were aware o f the primes, e x t e n d i n g the conclu- sions o f Logan a n d Z b r o d o f f (1982).

Although there is not a massive a m o u n t o f evidence here, I will dignify the clear picture with a conclusion: Advance cues conveying information about the upcoming Stroop trial can be used to establish processing strategies that improve performance i f these cues are above the level o f subjective awareness and i f a very small set o f cues is used consistently. T h e m a j o r i m p l i c a t i o n o f the work in this section is that foreknowledge can p e r m i t subjects to c o n s t r u c t a beneficial, task-wide strategy. A t t e n - t i o n a l - a l l o c a t i o n policy is a critical element in the Stroop task.

S t i m u l u s Onset A s y n c h r o n y (SOA)

Many explanations o f Stroop interference have relied on the relative speed o f processing o f the two c o m p e t i n g stimulus di- mensions. Intuitively, i f a subject processes two d i m e n s i o n s si- multaneously, one c o u l d interfere with the other, especially i f the d i m e n s i o n that is p r o c e s s e d faster is the one that is sup- p o s e d to be ignored (see D u n b a r & M a c L e o d , 1984, for a dis- cussion o f the "horse-race" metaphor). Logically, then, giving the slower c o l o r d i m e n s i o n a h e a d s t a r t should reduce the i m - p a c t o f the o r d i n a r i l y faster w o r d processing, thereby decreas- ing interference (and facilitation). W i t h the individual-trial p r o - cedure, such studies can be p e r f o r m e d by m a n i p u l a t i n g the SOA between the word a n d color.

T h e first study to v a r y SOA was by D y e r (197 lc). H e used a

180 COLIN M. MACLEOD

large SOA range (0-500 ms) because he thought it possible that short SOAs would lead t o increased interference b u t longer SOAs would lead to decreased interference. With four c o m m o n colors and an X X X X - t y p e control, he explored both incon- gruent and congruent combinations by having a word appear in black at 1 o f 10 intervals before that same word appeared in color. The subject's task was to n a m e the ink color.

Dyer (197 l c) found facilitation for congruent trials (26 ms) and interference for incongruent trials (75 ms), relative to the control, at all word-preexposure intervals. However, the time by which the word preceded the ink color also mattered. Interfer- ence was about 90 m s with a 0-ms SOA, increased to I 18 m s at a 40-ms SOA, fell to 68 m s at a 200-ms SOA, and then further decreased to 32 m s at a 500-ms SOA. Although a n o n m o n o t o - nicity is suggested, in fact Dyer's data only allowed the conclu- sion that interference decreased with SOA. Nevertheless, Dyer claimed that interference peaked in the neighborhood o f a 50- m s preexposure o f the word. He also noted that facilitation increased somewhat from the 0-ms to the 200-ms SOA.

R a t h e r t h a n preexposing a color w o r d in black a n d t h e n showing that color word in an ink color, Dyer and Severance (1973) preexposed a color word in Mack and then showed a X X X X - t y p e stimulus in an ink color. Thus, the color and word were now not integrated. Relative to the earlier study, this cut interference almost in half and eliminated facilitation. Further- more, varying SOA from 0 to 100 ms had no effect on interfer- ence. W h e n Severance and Dyer (1973) tried to replicate these findings with a "subliminal" preexposure o f the irrelevant word, they found no interference.

In 1974, Dyer tried using much longer SOAs between the word and the ink color. Interference decreased to a minimal level with a 2-s SOA, but then increased again as SOA grew beyond 2 s. Facilitation looked to follow the same pattern but with d i m i n i s h e d magnitude. Relative to r a n d o m i z e d SOAs, blocking SOA produced faster color naming at short SOAs, but the effect o n interference was minimal. Taken together, all o f Dyer's SOA results suggest an elongated S-shaped time course in the Stroop effect: Interference initially increased with greater SOA, then decreased, a n d finally increased again at very long SOAs. Such a pattern is not easily reconciled with any simple relative speed-of-processing interpretation.

In a thesis modeled after Dyer's work, T h o m a s (1977) varied SOA from the ink color leading the word by 300 ms to the word leading the ink color by 300 ms. Interference with color naming was greater for shorter SOAs. This result was also extended to facilitation a n d confirmed by N e u m a n n (1980), Goolkasian (1981), and Long and Lyman (1987). Precisely the same pattern was reported by Flowers (1975) in a sorting version o f the color- w o r d task; Flowers a n d Wilcox (1982) extended this t o the flanker procedure using two letters o r digits. In Thomas's study, facilitation occurred whenever the word preceded or even when it closely followed the corresponding ink color (up to 50 ms after). Overall, the evidence was not consistent with a simple differential rate-of-processing model.

Lassen (1975) extended SOA studies to the p i c t u r e - w o r d task. Interference with picture naming peaked when the incon- gruent word led the picture by about 100 ms. O n the other hand, although pictures did cause a small a m o u n t o f interfer- ence with word reading, manipulation o f SOA had no effect o n

reading words. These results are reminiscent o f the data found in the standard Stroop task, as are Magee's (1982) data o n SOA manipulations in the picture-word task.

The definitive pair o f studies on the relation between SOA and interference has been reported quite recently Glaser and Glaser (1982, Experiment 1) used the color-word task and in- cluded incongruent, control, and congruent trials. W h e n the task was to read the word, there was no interference o r facilita- tion regardless o f whether ink color or word was presented u p to 400 ms earlier: SOA had no effect on word reading. However, SOA had a powerful effect o n ink-color naming. I f the word was presented before or even as late as 100 ms after the ink color, both interference and facilitation occurred. Interference was maximal in a 100-ms window a r o u n d simultaneous, consistent with Dyer's results.

In other experiments, Glaser and Glaser (1982) showed that increasing the frequency o f congruent trials could lead to a reverse Stroop effect (ink color interferes with word reading), with both interference and facilitation in word reading when the word preceded the ink color by 200 m s or more. They ar- gued that the high probability o f the "irrelevant" word also being the name o f the ink color led subjects to prepare the word for output. However, their m a i n conclusion was that the SOA pattern did not readily fit a simple speed-of-processing model. They maintained that there was a fast, strong inhibition plus slower facilitation and inhibition deriving from response bias.

Glaser and Diingelhoff (1984) varied SOA in the p i c t u r e - word task. The pattern was very similar to that in the color- word task: Word-reading time was unaffected by SOAs o f u p to 400 ms in either direction, whereas picture naming showed a strong influence. Again, when the word appeared within 100 ms o f the picture, both interference and facilitation were evi- dent in picture naming. W h e n the word appeared more than 100 m s before the picture, there was considerable facilitation but little interference, as T h o m a s (1977) reported for the color- word task. Differing patterns o f facilitation and interference in response to SOA manipulation have also been reported in the flanker task (Grice, Boroughs, & C a n h a m , 1984; Grice & Gwynne, 1985).

Basically, the SOA pattern seems quite consistent in both the color-word and picture-word tasks. This permits the following generalization to be made: When the color (or picture) is to be named, maximal impact o f a congruent or incongruent word will be observed when the two dimensions begin within 100 m s o f each other. Facilitation may extend to longer SOAs than interference when the word comes first. Manipulating SOA has virtually no impact on word reading unless a very high proportion o f con- gruent trials biases use o f the color to initiate response production. T h e major significance o f the SOA studies is that they call into question the idea that interference is due simply to words being processed faster than ink colors. I f that were true, there ought to be a point at which an ink color presented sufficiently before a word would cause reverse Stroop interference (and facilitation). Concerted empirical efforts have failed to find any evidence o f such a point.

T h e Reverse Stroop Effect

I f a word normally interferes with n a m i n g a color o r a pic- ture, but a situation is created wherein the color or picture inter-

THE STROOP EFFECT 181

feres with reading the word, this is an instance o f a reverse Stroop effect. Such findings are o f theoretical interest because they relate to the idea that interference depends o n the relative speed o f processing o f each o f the two dimensions.

T h e first r e p o r t o f a reverse S t r o o p effect was by S t r o o p (1935b, Experiment 3) himself in the seminal article. This effect arose only after considerable training and was quite transient. Since then, others have been more successful in creating stable reverse Stroop situations, but their success has usually relied on substantially changing the task, m a k i n g it difficult to compare their results to the standard Stroop situation. Still the existence o f the reverse Stroop effect is now widely accepted.

As an illustration, Palef and Olson (1975; see also Seymour, 1973) had subjects respond either to the meanings o f the words above and below or to their spatial locations on the screen. In E x p e r i m e n t 1, spatial position was m a d e easier (and hence faster) to process than word meaning. In this case, incongruent spatial position interfered with a keypress decision about word meaning, but incongruent word meaning did not interfere with a keypress decision about spatial position. In Experiment 2, where spatial position was made harder to process, the pattern o f interference reversed. In general, the faster dimension inter- fered with the slower one but not vice versa.

Quite a n u m b e r o f other studies have demonstrated reverse Stroop effects as well (e.g., Abramczyk, Jordan, & Hegel, 1983; Chen & Ho, 1986; Chmiel, 1984; D u n b a r & MacLeod, 1984; Francolini & Egeth, 1980; Glaser & Dolt, 1977; M a c L e o d & Dunbar, 1988; Martin, 1981; Morikawa, 1981; Nealis, 1974; Pritchatt, 1968; Shor, 1975; Warren & Marsh, 1979). Many o f these studies used nonverbal responses, although it is not clear how crucial response m o d e was because the stimuli generally were rather different from the basic Stroop task as well.

T h e two studies generally cited as illustrative o f reverse Stroop interference are those o f G u m e n i k and Glass (1970) and Dyer and Severance (1972). Both involved reducing the readabil- ity o f the words by partial obliteration. Both found a reverse Stroop effect in that ink colors influenced word reading for hard-to-read words. Unfortunately, like many o f the others in the list, these two did not examine interference in both "direc- tions" in the same experiment.

It seems possible, then, to have interference occur from di- mension A o n dimension B and vice versa. However, m a n y o f these e x p e r i m e n t s are quite far r e m o v e d f r o m the s t a n d a r d task. Cases where exactly the same stimuli and responses were used with only a n i n s t r u c t i o n a l difference are rare, leaving r o o m for more work in this area. Yet interference can occur in both directions when the only change is from naming colors to reading words (e.g., D u n b a r & M a c L e o d , 1984), conflicting with the belief that the faster process interferes with the slower one, and not vice versa. For this reason, the following conclu- sion is added to the list, although it must remain a little vague at this point: A reverse Stroop effect (i.e., interference with word read- ing caused by an incompatible, irrelevant ink color) appears to be possible, but this effect is not simply a consequence o f the relative speeds o f processing each dimension.

Practice

A n obvious and theoretically critical question is: Does the Stroop effect change as a consequence o f (type o f ) practice at

the task? I f so, how does it change, and what does this tell us about the cause o f the phenomenon? Stroop and his predeces- sors realized the importance o f this factor; now I will examine what has been found out since then in studies manipulating practice.

O n the view that words interfere with color naming because o f our extensive reading experience, Stroop interference is a direct consequence o f differential practice. Thus, variation in practice should have direct impact on the task. Stroop (1935b, Experiment 3) recognized this, although he thought that consid- erable practice might be needed to modify existing response tendencies. It is unfortunate, then, that the relevant studies of- ten involve very little practice (e.g., Alperson, 1967, used the range o f 3 to 50 trials). It is hardly surprising that such studies conclude that practice is not a particularly critical variable. Per- haps the Jensen and Rohwer (1966) claim that almost all prac- tice effects occur within the first 5 trials is responsible for this. I believe they were wrong.

Intuitively, extended practice with the Stroop task should lead to reduced interference as subjects develop a strategy for coping more successfully with the task. This result has indeed been observed (e.g., Etfler, 1978a; Ogura, 1980), but not always (e.g., Harbeson et al., 1982; Shor et al., 1972; White, 1978). Further- more, this is not the whole s t o ~ As one illustration o f how much more complex the situation is, the stimuli and responses c h o s e n are also i m p o r t a n t in training studies. Flowers a n d Stoup (I 977) found that interference dissipated with practice for nonintegrated stimuli when the task was sorting, but not when it was oral naming. Nielsen (1975) found initially greater inter- ference for a vocal than for a manual response, but then found a greater decline in interference over practice for the manual than for the vocal response. The results o f Roe, Wilsoncraft, and Griffiths (1980) confirm that manual responding is affected more quickly by practice, which is interesting given that manual responding is faster to begin with and therefore closer to a "per- formance floor."

It also seems that there is a good deal o f specificity to practice effects. M6nard-Buteau and Cavanagh (1984) found that, for items like "blue banana," there was interference in saying "blue" to the word or the picture in color. However, extended practice on the word version did not transfer to the picture version. They argued that the semantic element o f interference did not coincide in the two tasks, and that interference might be more at the encoding stage for pictures and the response stage for words. This seems worth pursuing, particularly because it suggests that the c o m m o n assumption o f process identity in these two tasks may be at least partly incorrect.

In contrast to M6nard-Buteau a n d Cavanagh (1984), Reis- berg, Baron, and Kemler (1980) did obtain transfer along se- mantic lines. They used the counting task where the items to be counted are themselves digits. With practice, interference de- clined but in a very specific way. Subjects who had practiced with the digits 2 and 4 to be ignored showed no benefit on a counting test where the digits I and 3 or the words to and for were to be ignored. However, there was good transfer when the words two and four were to be ignored. Thus, meaning appears to be involved.

Regan (1977) compared a well-practiced to an unpracticed task. She had English-speaking subjects learn the names o f Ar-

182 COLIN M. MACLEOD

m e n i a n letters. She t h e n c o m p o s e d large English a n d A r m e - n i a n letters out o f small English a n d A r m e n i a n letters, a n d re- quired subjects to n a m e the small letters while ignoring the large letters, using the Navon (1977) procedure. Interference was e q u i v a l e n t for t h e v e r y f a m i l i a r set a n d for t h e newly l e a r n e d set. This result speaks to what is m e a n t by "automatic" processing: A l t h o u g h such processing a p p e a r s to be involun- tary, it a p p a r e n t l y d o e s not require extensive practice.

Reasoning that a novel o r i e n t a t i o n would slow d o w n w o r d processing, Liu (1973) had subjects rotate t h e i r Stroop cards upside d o w n a n d t h e n n a m e the i n k colors. Interference was r e d u c e d c o m p a r e d with n o r m a l l y o r i e n t e d words. However, us- ing several unique w o r d orientations, D u n b a r a n d M a c L e o d (1984) failed to c o n f i r m Liu's result. In fact, we found equiva- lent interference regardless o f the word's orientation, so t h a t previous practice differences could not b e the critical factor.

M o s t recently, M a c L e o d a n d D u n b a r (1988) t o o k o n the prac- tice issue directly. We h a d subjects l e a r n to call each o f four unique shapes b y a different c o l o r n a m e (green, p i n k , orange, o r blue). We c o u l d t h e n p r e s e n t e a c h s h a p e in a n e u t r a l c o l o r (white), a c o n g r u e n t color, o r a n i n c o n g r u e n t color, a n d require s u b j e c t s to n a m e t h e s h a p e o r t h e i n k color. W i t h a s m a l l a m o u n t o f s h a p e - n a m i n g p r a c t i c e , i n k c o l o r i n t e r f e r e d w i t h shape n a m i n g b u t shape n a m e d i d not interfere with ink-color naming. W i t h m o d e r a t e practice, interference was equivalent in b o t h directions. W i t h extensive practice, only shape n a m e s in- terfered with ink-color n a m i n g a n d not vice versa. Practice h a d a systematic i m p a c t , interestingly in a task with no "word" di- m e n s i o n at all a n d where shape n a m i n g never b e c a m e faster t h a n ink-color naming.

Practice warrants further investigation. A l o n g the way, vari- ables such as intertrial interval (see Doten, 1955) could also be e x a m i n e d . In o u r work ( M a c L e o d & Dunbar, 1988), we m a n i p u - lated practice o n one d i m e n s i o n , leaving the o t h e r in its "natu- ral state"; it w o u l d be informative to covary practice o n the two d i m e n s i o n s , a project t h a t D i a n e W i l l i a m s a n d I have u n d e r way in my laboratory. For the present, my general conclusion a b o u t practice is p r o b a b l y too general: Degree of practice in processing each of the dimensions of a multidimensional stimu- lus is influential in determining the extent of interference from one dimension on another. The greater the practice in processing a dimension, the more capable that dimension is of influencing the processing of another dimension. Practice may t u r n out to be one o f the m o s t effective m a n i p u l a t i o n s for disentangling the- ories o f the Stroop effect.

This ends the section o n m a n i p u l a t i o n s over multiple trials. O f course, there are isolated articles e x a m i n i n g m o r e esoteric effects. O n e e x a m p l e is studies in which feedback is m a n i p u - lated, as when vocal feedback is m i n i m i z e d (Breslow, G r a n d , & F r e e d m a n , 1980; G r a n d , Breslow, & F r e e d m a n , 1980) o r de- layed ( D o e h r m a n , L a n d a u , & O ~ o n n e l l , 1978) o r when re- sponse feedback is v a r i e d in s o m e o t h e r way (e.g., H o c h m a n , 1973). A n o t h e r is the case o f m a n i p u l a t i o n o f effort in the task (e.g., M a c K i n n o n , G e i s e l m a n , & Woodward, 1985). However, the m a i n findings are those o u t l i n e d previously. I now shift the focus from stimulus m a n i p u l a t i o n s to those concentrating on the response side o f the task.

Response-Related Manipulations U n d e r the view that interference derives from a l o g j a m at a

l i m i t e d - c a p a c i t y response buffer because the irrelevant w o r d

has entered before the relevant i n k color (e.g., M o r t o n , 1969; M o r t o n & Chambers, 1973), the locus o f interference is p l a c e d squarely in the response stage. Thus, m a n i p u l a t i o n s affecting how o r when the response is made, o r how m a n y responses might possibly b e made, b e c o m e very i m p o r t a n t in testing such a view.

Order o f Response

Few studies have required subjects to m a k e two responses to the s a m e stimulus as, for example, when consecutive responses to the i n k color a n d the w o r d (or vice versa) are required. T h e p r o t o t y p e work in this d o m a i n is that o f Schweickert (1978, 1983), who argued t h a t only one decision c a n be m a d e at a t i m e in the Stroop task. As illustrations, he cited several studies (e.g., G h o l s o n & Hohle, 1968a; H o c k & Egeth, 1970; R i e c k & Coates, 1977), all involving only one response. To d e m o n s t r a t e t h a t at least s o m e o f the processing o f the w o r d a n d the i n k c o l o r is concurrent, he cited G r e e n w a l d (1972). Here subjects saw an a r r o w p o i n t i n g left o r right a n d h e a r d the w o r d left o r right simultaneously. T h e y h a d to r e s p o n d orally to one d i m e n s i o n a n d m a n u a l l y to the other. G r e e n w a l d v a r i e d whether one o r b o t h a r r o w s o r w o r d s c o u l d o c c u r in a b l o c k o f trials, a n d whether there was conflict between the dimensions. His d a t a showed clear evidence o f interaction a m o n g these variables.

S c h w e i c k e r t ' s (1983) e x p e r i m e n t u s e d a c o l o r e d r e c t a n g l e above a color word. There were 2 subjects. O n e was to press a key i n d i c a t i n g t h e c o l o r o f t h e r e c t a n g l e b e f o r e r e a d i n g t h e w o r d aloud; her d a t a suggested that she d e c i d e d a b o u t the i n k color first. T h e o t h e r subject was to press a key for the w o r d a n d t h e n n a m e the i n k color aloud; h e r d a t a suggested t h a t she d e c i d e d a b o u t the w o r d first. This o u t c o m e is difficult to h a n - dle u n d e r a differential speed-of-processing account, which ex- plains the usual a s y m m e t r y as being due to the decision a b o u t the w o r d preceding the decision a b o u t the i n k color. This ap- pears to be t r u e when the subject n a m e s the i n k color. However, the decision o r d e r is reversed when the w o r d m u s t b e read, so why is there a n a s y m m e t r y in the s t a n d a r d Stroop task?

Work involving two responses (see also Virzi & Egeth, 1985, E x p e r i m e n t 4) may help to explain how the various processes are scheduled. O f course, as Schweickert was well aware, the d a n g e r is that scheduling is influenced b y whether one o r m o r e t h a n one response is required. This m a y also be reflected in memory. Corballis a n d Luthe (1971) presented subjects with three i n c o n g r u e n t Stroop items in r a p i d succession, a n d v a r i e d whether subjects h a d to recall on a n i t e m by item basis (i.e., the i n k color a n d w o r d for each item) o r d i m e n s i o n a l l y (i.e., all three ink colors a n d t h e n all t h r e e words). W h e n words were r e p o r t e d before colors, d i m e n s i o n a l recall was b e t t e r t h a n item-based recall. W h e n colors were r e p o r t e d before words, the p a t t e r n reversed. A g a i n scheduling o f processes a p p a r e n t l y was driven b y t a s k d e m a n d s . We n e e d to k n o w m o r e a b o u t how sequencing is governed.

Response Modality: Oral Versus Manual

I f interference occurs at o r n e a r response output, it seems reasonable to e x a m i n e the form o f response. The critical con- t r a s t has b e e n between vocal-response o u t p u t - - n a m i n g - - a n d m a n u a l - r e s p o n s e o u t p u t - - a keypress. The question is whether

THE STROOP EFFECT 183

interference differs over m o d e o f response in the S t r o o p task. In particular, d o e s the c o m p a t i b i l i t y o f the verbal response a n d the irrelevant w o r d a c c o u n t for s o m e o r all o f the interference, as studies such as those o f G h o l s o n a n d Hohle (1968b) a n d Belier (1975) might suggest?

P r i t c h a t t (1968) suggested that the Stroop effect was a conse- quence o f verbal o u t p u t because the effect was r e d u c e d when a m a t c h i n g response was used. However, W h i t e (1969) was the first to m a k e the explicit c o m p a r i s o n o f verbal a n d nonverbal responses. F o r the s t a n d a r d Stroop task, he r e p o r t e d less inter- ference with the nonverbal keypress response t h a n with the oral response. Unfortunately, he u s e d as his d e p e n d e n t variable the ratio o f i n c o n g r u e n t t i m e to control time, so it is difficult to tell exactly what the p a t t e r n l o o k e d like in the i n d i v i d u a l c o n d i - tions.

O n e o f the b e s t k n o w n response-related studies was b y Keele (1972), although he used only m a n u a l responses. He r e a s o n e d that i f interference arises d u r i n g m e m o r y retrieval, t h e n any w o r d should interfere with n a m i n g i n k c o l o r be it a n o n c o l o r w o r d o r a color word. In contrast, i f n o n c o l o r words d o not interfere b u t color words do, t h e n the retrieval stage c a n n o t be the locus o f interference. H i s results s u p p o r t e d the latter predic- tion, leading h i m (and Posner & Snyder, 1975) to a late-selection t h e o r y o f attention. In fact, though, the Keele (1972) study does not directly address the issue o f response modality; his study is m o r e a replication o f Klein's (1964) logic using keypresses in- s t e a d o f oral responses. 8

M o r e crucial are studies where the two response m o d e s are directly c o m p a r e d in the s a m e e x p e r i m e n t . Nielsen (1974) var- i e d t h e i r r e l e v a n t s t i m u l u s f r o m c o n t r o l Xs t o c o l o r words, roughly following K l e i n (1964). Interference much like Klein's was o b s e r v e d for b o t h vocal a n d keypress responses, although interference was greater for vocal responses. Even with practice, vocal-response interference exceeded m a n u a l - r e s p o n s e interfer- ence. Using s i m i l a r logic, Majeres (1974) h a d subjects either say o r write their response (only the first letter had to be written). T h e r e was a clear g r a d i e n t o f interference for b o t h response modes, b u t less interference when writing t h a n when saying the response.

R e d d i n g a n d G e r j e t s (1977) d e m o n s t r a t e d that i n c o n g r u e n t words p r o d u c e d 177 m s o f interference when the response to i n k c o l o r was oral a n d only 98 m s when the response was m a n - ual; c o n g r u e n t words p r o d u c e d 23 m s o f facilitation for oral responses a n d 67 m s for m a n u a l responses. Curiously, there a p p e a r e d to be less interference b u t m o r e facilitation when the response was m a n u a l c o m p a r e d with oral. W h e t h e r this general- izes r e m a i n s to be seen.

Perhaps the pivotal question is whether interference consis- tently a p p e a r s when r e s p o n d i n g is manual. In general, the an- swer a p p e a r s to b e " y e s " Several articles have c o n f i r m e d inter- ference with a m a n u a l response (e.g., Logan et al., 1984; Roe et al., 1980; Schmit & Davis, 1974; Virzi & Egeth, 1985; Warren & Marsh, 1979). Some, such as t h a t o f Roe et al. (I 980), even found no overall differences between the two response modes. The only case where there ~vas n o interference with a m a n u a l re- sponse a p p e a r s to b e the e x p e r i m e n t b y M c C l a i n (1983b) de- s c r i b e d shortly.

A feature o f any t a s k that c o n t r i b u t e s to its difficulty is the degree to which the responses m a p readily onto the various d i m e n s i o n s o f the stimulus. Fitts a n d Posner (1967) referred to

this as the s t i m u l u s - r e s p o n s e c o m p a t i b i l i t y issue. T r e i s m a n a n d F e a r n l e y (1969) stressed this in t h e i r account o f the Stroop effect. Put simply, does the w o r d interfere with n a m i n g the c o l o r because the verbal nature o f the w o r d matches that o f the re- sponse?

M c C l a i n m a n i p u l a t e d c o m p a t i b i l i t y in b o t h the s t a n d a r d S t r o o p t a s k a n d its a u d i t o r y a n a l o g . I n t h e c o l o r - w o r d t a s k (McClain, 1983b), interference in c o l o r n a m i n g d e c l i n e d as the r e s p o n s e m o v e d f r o m o r a l to p r e s s i n g b u t t o n s l a b e l e d w i t h c o l o r n a m e s to pressing b u t t o n s labeled with color patches. In fact, interference was not significant in the last condition, b u t such c o m p l e t e absence o f interference is an unusual result. 9 In t h e a u d i t o r y e x p e r i m e n t ( M c C l a i n , 1983c), s u b j e c t s h a d to identify either the p i t c h (high o r low) o r the w o r d (high o r low). W h e n pitch was to be identified, interference o c c u r r e d for an oral o r a keypress response, but not for a h u m m e d response. W h e n the w o r d was to be identified, the p a t t e r n reversed.

A l t h o u g h M o r t o n a n d C h a m b e r s (1973) q u e s t i o n e d the role o f s t i m u l u s - r e s p o n s e compatibility, it seems to m a t t e r (Flowers et al., 1979; S i m o n & S u d a l a i m u t h u , 1979; Z a k a y & Glicksohn, 1985). O f course, c o m p a r i n g response t i m e s across oral a n d m a n u a l responses can be a t h o r n y interpretive p r o b l e m (see, e.g., S t a n o v i c h & P a c h e l l a , 1977). R e c a l l t h a t N e i l l (1977) showed that i f the irrelevant w o r d on trial n - 1 n a m e d the relevant i n k color o n trial n, this slowed oral r e s p o n d i n g (rela- tive to unrelated items). Yet when response m o d a l i t y was m a n - ual, the s a m e sequential relation actually s p e e d e d responding. W h y ? Neill argued that suppression o c c u r r e d only when the response had to b e spoken, whereas activation in the m a n u a l task helped e l i m i n a t e the w o r d faster from contention as a possi- ble response. S i m o n a n d S u d a l a i m u t h u (1979) r e p o r t e d a re- lated effect o b t a i n e d within single trials.

Such results are puzzling. Nevertheless, there is sufficient convergence to p e r m i t the following conclusion: Although still significant, interference (but perhaps not facilitation) is reduced when response modality is switched from oral to manual. Stimu- lus-response compatibility matters; if the normal processing of the irrelevant dimension leads to a response in the mode desig- nated for the relevant dimension, interference is likely to be heightened. However, neither response m o d e alone n o r the in- teraction o f stimulus a n d response m o d e s can account for the Stroop effect. The effect is due to m o r e t h a n a queuing p r o b l e m at the finish line.

Response Set Size and Composition

It frequently is argued that p e o p l e r e a d the irrelevant w o r d in S t r o o p variations because it usually is an eligible response (e.g., K l e i n , 1964; Proctor, 1978; see Stimulus Set Size). I f interfer-

8 It should be noted that Keele (1972) did report substantial interfer- ence when comparing incongruent stimuli to control stimuli despite the manual response mode.

9 McClain used the difference between incongruent and congruent times as her measure of interference, which unfortunately combines interference and facilitation. Wheeler (1977) found no difference be- tween color naming and word reading in the incongruent condition using a manual response, but this is a very unconventional way to contrast response modes.

184 COLIN M. MACLEOD

ence occurs in responding, then manipulations o f the set o f responses might be expected to have some effect.

Both Klein (1964) and Scheibe et al. (1967) reported twice as much interference when the color word was the name o f an- other possible ink color in the set. Even controlling for word frequency and association, Proctor (1978) still found 24 to 30 ms more interference when the words and ink colors coincided. Other studies support this conclusion in the color-word task (e.g., Stifling, 1979) and in the picture-word task (Lupker & Katz, 1981). Lupker and Katz found more interference when irrelevant words n a m e d the categories o f possible p i c t u r e names than when they did not. D u n b a r (1986) provided an extensive discussion o f the relevant p i c t u r e - w o r d literature, and La Heij (1988) reported careful studies showing that re- sponse-set members produce heightened interference. Studies using the flanker task (e.g., La Heij et al., 1985) also found that a target word suffers more when a nearby word is a potential response than when it is not.

There are two logical possibilities for what should happen as set size increases. First, interference might increase because more potential responses are vying for output, m a k i n g manag- ing the contents o f working m e m o r y more effortful. Second, interference might decrease because the probability that any particular response is actually in working m e m o r y would de- crease with larger response set size. Nielsen (1974) found that interference increased with increasing set size, R a y (1974) found that it was unaffected, and L a Heij et al. (1985) found that it decreased. There are other studies that could be aligned with each o f these findings (see also Stimulus Set Size). The manipu- lation o f response set size seems so straightforward that these conflicting findings are frustrating. Thus, the recent emphasis on this issue in the work o f L a Heij and colleagues (e.g., La Heij, 1988; La Heij et al., 1985) is timely.

The findings o n response set are clear with respect to t h e overlap o f the two dimensions, but not with respect to the effect o f set-size variation. This is captured in the following conclu- sion: When the irrelevant dimension o f a set o f stimufi includes n a m e s that are eligible responses f o r the relevant dimension, more interference results than when the sets are nonoverlapping Although variations in response set size might be expected to affect interference, existing results are unclear.

Individual Differences

A great many articles have examined the relation between Stroop interference and some individual differences parameter. The purpose o f m o s t o f these clearly is not to explain the Stroop effect, so for this reason I omit many o f these studies. Instead, I select only a few dimensions o f individual variation to review here. As Kareev (1982) illustrated, certain analyses at the level o f the individual have considerable potential in helping us to understand the Stroop effect.

Sex Differences The most obvious difference between individuals is sex, so

much effort has been directed to exploring sex differences (of. Maccoby & Jacklin, 1974). The Stroop task has not escaped: There are now over a dozen studies o f how men and w o m e n

differ in degree o f interference. Even before Stroop, Ligon (1932; see also Brown, 1915) noted that girls n a m e d colors faster than did boys, although there was no difference in word-read- ing speed. Stroop (1935b) confirmed this, but critically showed that men and w o m e n did not display differential interference. This result has been replicated often (e.g., Golden, 1974b; Sar- manay, 1977; Sladekova & Daniel, 1981), yet investigators still pursue the matter (e.g., Peretti, 1969, 1971).

Naish (1980) suggested that m e n m a y encode irrelevant words both orthographically and phonemically when sorting the colors in which they are written, whereas women encode only phonemically. However, he also suggested that this strat- egy difference might be unique to certain Stroop-like situations and not evident in normal reading.

Several studies have examined the relation between Stroop interference and some test o f interest, with sex differences as a subsidiary analysis. In a factor-analytic study o f extraversion and field dependence, loadings o n a Stroop factor were higher for m e n than for women (Bone & Eysenck, 1972). In a study o f the relation between a test o f matching familiar figures and the Stroop test (Boyden & Gilpin, 1978), errors on the two tests were positively correlated for m e n but not for women, which the authors took as an indication o f distractibility and impulsivity being related in men.

Jorgenson and her colleagues (Davis, Jorgenson, Kritselis, & Opella, 1981; Davis, Jorgenson, & Opella, 1983; Jorgenson, Da- vis, Opella, & Angerstein, 1980, 1981; Jorgenson, Davis, Wil- bon, & Opella, 1983) have been especially interested in hemi- spheric differences and sex differences in the Stroop task. In brief, sex differences in both grade-school and college students did not appear to account for much o f the variance in Stroop performance in their studies.

In grade-school children, girls have been shown to be gener- ally faster but not better at color recognition and not different in terms o f interference, the most interesting measure theoreti- cally (Dash & Dash, 1982). Bettner, Jarvik, and Blum (1971) studied an aged population and found poorer Stroop perfor- mance in m e n than in women (average age o f 84 years).

O n the whole, research has failed to find m u c h difference in Stroop interference between men and w o m e n at any age. Al- though women may be somewhat faster, especially in naming colors, this relates to general response speed (see Jensen, 1965), not to the derived measure o f interference. Here is a s u m m a r y statement: There are no sex differences in Stroop interference at any age. Perhaps this is too strong, but I remain to be con- vinced.

A g e Differences

The development o f color and word-form processing with age was o f interest before the Stroop task was created (e.g., Brian & Goodenough, 1929; Ligon, 1932). Basically, more recent stud- ies make similar points. Thus, Cramer (1967) supported Ligon's claim that form processing dominates over color processing, although preschoolers may show the teverse preference (Aro- chova, 1971), having not yet learned to recognize letter forms very well.

Interest in development with regard to the Stroop task itself did not arise until the mid-1960s. There are some three dozen

THE STROOP EFFECT 185

pertinent studies, the first being a life-span study by Comalli, Wapner, and Werner (1962; see also Rand et al., 1963). Testing subjects from ages 7 to 80 years, they observed greatest interfer- ence in the young children; interference declined into adult- hood and then increased again with advanced age. They saw young children and older adults as having relatively more diffi- culty in screening out interfering stimuli. Ehri and Wilce (1979) produced an experimental analog o f this result by training first and second graders on a set ofwords to be used in the picture- word task. Initially, as the subjects were learning to read the words, interference increased; however, once the words were learned and further training simply improved response speed to the words, interference decreased.

Schiller (1966) helped to clarify the nonmonotonicity in inter- ference with age in children. Interference was minimal for chil- dren in Grade 1, younger than the youngest subjects in the Comalli et al. (1962) study. Interference then became maximal in Grades 2 and 3 (Comalli et al's youngest subjects), and de- d i n e d thereafter. The strong suggestion is that this early rise and then fall in interference reflects the onset o f reading skills. Dash and Dash (1982) confirmed Schiller's result (see also Friedman, 1971). Using the picture-word task, Ehri (1976) showed a similar pattern, except that poor readers in the second grade did not show interference, behaving rather like normal first graders in the other studies. Attempts to create Stroop-like situations for younger, prereading children have not been very informative (e.g., Butollo, Bauer, & Riedl, 1971; C a m m o c k & Cairns, 1979). Furthermore, attempts to categorize subgroups o f development in the task have not been successful (e.g., Rand et al., 1963; Silverstein & Franken, 1965).

The decrease in interference through adulthood and before the age o f 60 that Comalli et al. (1962) observed has been con- firmed (e.g., Wise, Sutton, & Gibbons, 1975), as has the increase in interference for adults older than 60 years (see Cohn, Dust- man, & Bradford, 1984; Panek, Rush, & Slade, 1984). There may be exceptions (e.g., Baumler, 1969), but even studies o f aging twins reveal the increased interference with age greater than 60 years (Bettner et al., 1971; Jarvik, Blum, & Varma, 1972), together with the suggestion that identical twins show more similar interference than do fraternal twins. Attempts at remediation o f the exaggerated interference shown by older adults have not been notably successful (e.g., White, 1978).

Many o f the developmental Stroop studies focus on reading skill (e.g., Bakan & Shotland, 1969; Corbitt, 1978; Fournier, Mazzarella, Ricciardi, & Fingeret, 1975; Frey, 1971; Ludwig & Lazarus, 1983; West & Stanovich, 1978; Wilder, 1969). The Corbitt (1978) dissertation provides a good example. He showed that reading ability and interference were closely related: Inter- mediate and good readers read words faster than they named colors, and showed a Stroop effect without a reverse Stroop effect. Poor readers named colors faster than they could read words and showed the opposite pattern.

How about reading comprehension? Using the picture-word task, Golinkoff and Rosinski (1976) showed that good versus poor comprehenders in Grades 3 and 5 did not differ in the degree o f interference shown in naming pictures containing single words. Similar findings were reported by Ehri (1977) and Underwood et al. (1984). Rosinski (1977) showed that the se- mantic gradient for words that differentially relate to colors was

also consistent from Grade 2 through college. Merrill et al. (1981) went on to show that, with sentence contexts, good com- prehenders in Grade 5 displayed interference only for targets appropriate to the context, whereas p o o r comprehenders showed interference regardless o f context appropriateness. A study by Kareev (I 980) further investigated development o f sen- tence encoding in children using the color-word task.

Interestingly, children with reading disabilities often show robust Stroop interference (Alwitt, 1966), as do autistic children (Bryson, 1983), hyperactive children (DeHaas & Young, 1984), aphasics (Cohen, Meier, & SchuLze, 1983), and retardates (Bas- sett & Schellman, 1976; Tschopp & Jorswieck, 1976). Recent work by Ellis, Woodley-Zanthos, Dulaney, and Palmer (1989) suggests that automatic word reading may be even harder for retardates to control than it is for normals, causing greater inter- ference in retardates. Das (1970) reported a similar result, al- though the pattern reversed when very poor readers were in- cluded in the retarded group (Das, 1969).

The overall picture o f development, then, can be summa- rized fairly concisely by the Comalli et al. (1962) result. Here is the conclusion: Interference begins early in the school years, ris- ing to its highest level around Grades 2 to 3 as reading skill de- velops. With continued development o f reading, interference de- clines through the adult years until approximately age 60, at which point it begins to increase again. Virtually everyone who can read shows a robust Stroop effect from an early age.

Hemispheric Differences

A number o f investigators have examined laterality in the Stroop task, using Dyer's (1973a) study as a springboard. Basi- calls he showed that presenting the word and the color sepa- rately to the left and right o f fixation did not destroy the Stroop effect: Both interference and facilitation were still evident. Fur- thermore, the effect does not seem to be altered by presenting the words vertically (McCown & Arnoult, 1981), although it may be somewhat reduced in the periphery as opposed to fo- veally (Goolkasian, 1978). Such findings opened the door for hemispheric-differences research.

Schmit and Davis (197z~) tested subjects with Stroop stimuli displayed in only the left or right visual field using a buttonpress response. They observed greater interference in the left hemi- sphere, consistent with that hemisphere's dominant role in ver- bal processing. Guiard (1981) corroborated this using a man- ual-response task. The finding was also confirmed in the stan- dard oral-response procedure by Tsao, Feustel, and Soseos (1979), although only in their error data (see also Newman, 1981). However, not all investigators have agreed: Warren and Marsh (1978) found equivalent interference for the two hemi- spheres using color-word stimuli, whereas Long and Lyman (1987) found greater interference when the word was processed in the right hemisphere (with the color presented foveally).

Tsao, Wu, and Feustel (1981) showed that the Schmit and Davis pattern was reversed for Chinese speakers, consistent with the idea that both logographs and colors are handled by the right hemisphere. Analogously, Morikawa (1981) showed that idiographic Kanji characters produced greater right-hemi- sphere interference in Japanese subjects. Hatta (1981; see also Hatta, Katoh, & Aitani, 1983) confirmed the result for Kanji,

186 COLIN M. MACLEOD

but found no hemispheric difference for orthographic Kana. Interestingly, Morikawa emphasized the overall reduced size of interference effects with Japanese subjects, itself a result worth pursuing.

Cohen and Martin (1975) presented auditory stimuli to the left or right ear using the Hamers (1973) high/low procedure. Again, the Stroop effect was larger in the left hemisphere, and was accentuated by simultaneous irrelevant distraction in the other ear (see also Vaid & Lambert, 1979, with the same task, and Pieters, 1981, using an auditory left/right position versus word-discrimination task). Alivisatos and Wilding (1982) used the global/local-letters task (with a manual response) to show that the local level of a foveally presented first stimulus inter- fered with processing a second stimulus in the right hemisphere but not in the left. The global aspect o f the first stimulus inter- fered with the second stimulus in both hemispheres, with a tendency to more interference in the right hemisphere.

Toma and Tsao (1985) obtained greater interference in the left hemisphere for the picture-word task. Using the same task, Lupker and Sanders (1982) hypothesized that the verbal-re- sponse competition in the left hemisphere gave way to a percep- tual type o f interference in the right hemisphere, a hypothesis worthy o f further investigation. In particular, though, more work is needed where the two dimensions are presented sepa- rately to the two hemispheres rather than together to the same hemisphere. SOA studies might also be informative by control- ling which hemisphere is stimulated first and by which dimen- sion of the stimulus.

Evoked (event-related) potentials are part of a new technology for studying the Stroop effect. Scott, Hoffman, and Bickford (1967) were pioneers in this area, demonstrating that lambda waves in the parietal-occipital region showed increased ampli- tude in the interference condition, although they admitted that this Could relate to muscle potentials as easily as to mental activ- ity. The technology has advanced a great deal since then (e.g., Johnston & Venables, 1982). More in the current mold o f such studies, Warren and Marsh (1979) took their neural-activity measurements to suggest that standard Stroop interference was due to response-selection processes, but that the reverse Stroop effect was due to complex encoding interactions.

Duncan-Johnson and KopeU 0980,1981) used oral response time together with P300 latency (cf. Pritchard, 1981) in the discrete-trials version of the Stroop task with congruent, incon- gruent, and neutral words and two ink colors. Blocks of trials required word reading or ink-color naming. The usual pattern was observed in the response times, but P300 was unaffected by experimental condition. Because P300 is taken to be an index of encoding, the conclusion was that interference must stem from response competition.

Aine and Harter (1984a, 1984b) adapted the study of evoked potentials to examining hemispheric differences in the Stroop task. Basically, they claimed that the activity associated with interference occurs in the left hemisphere, converging on the response time and error-rate measures already described. Tak- ing all o f this evidence together, the following conclusion is offered: The left hemisphere generally shows more interference than the right. A study by Posner, Walker, Friedrich, and Rafal (1984) suggests that the left parietal lobe may be particularly involved in disengaging attention, although the left frontal lobe

may also play a role (Perret, 1974). Undoubtedly, we will see more work on the topic of localization in the near future.

Language Differences: The Case o f the Bilingual

If Stroop interference stems from an irresistible urge to read the word, what would happen if you were naming ink colors in one language but the words were in another language? About two dozen studies have been directed at this question. This work began with Dalrymple-Alford (1968), but his items were either incongruent items in the same language or congruent items in different languages, a confound that does not permit us to obtain a clear picture o f the Stroop effect within versus be- tween languages. For this, we must examine studies by Preston and Lambert (1969) and by Dyer (197 la).

Preston and Lambert (1969) found substantial interference whether the ink colors were to be named in the same language as the distracting words appeared or in the other language. En- glish-Hungarian bilinguals showed roughly 68% of the interfer- ence across languages that they showed within a single lan- guage; for English-French bilinguals, the overlap was 95%. Pres- ton and Lambert concluded that interference between languages could be as great as that within languages, but that it depended on relative familiarity with the two languages in ques- tion.

Dyer (1971 a) showed that English monolinguals naming col- ors in English displayed maximal interference when the words were in English and declining interference as the similarity of the irrelevant words to English decreased. Using Spanish-Eng- lish bilinguals in his second experiment, Dyer found maximal interference when the naming and distracting languages coin- cided, but there was also substantial interference when the lan- guages did not coincide. Roughly, the different-language condi- tions showed about 63% of the interference seen in the same- language conditions.

Shortly after Dyer's study, Hamers (1973; Hamers & Lam- bert, 1972) used her auditory Stroop analog with French-Eng- lish bilinguals and reached the same conclusion: The different- language condition produced about 76% of the interference seen in the same-language condition. This pattern recurs for Spanish-English bilinguals in the picture-word task (Ehri & Ryan, 1980), French-English bilinguals in the flanker-word task (Guttentag et al., 1984), and diverse languages in the color-word task (e.g., Chinese-English and Japanese-English; Fang, Tzeng, & Alva, 1981; Turkish-English: Kiyak, 1982). Fang et al. (1981) also confirmed that the ratio of between-language to within- language interference declines as language similarity decreases.

Complicating matters, the weightings of interference within and between languages are also affected by relative proficiency in the two languages. M~igiste (1984,1985) observed a changing pattern with the development of the individual's second lan- guage. Initially, her subjects were German dominant and showed more interference when responding in German regard- less of whether the words were Swedish or German. Gradually, as their experience with Swedish increased, they reached a point of equivalence; then, as Swedish came to dominate, they showed most interference when naming in Swedish. Chen and Ho (1986), using Chinese-English bilinguals, suggested that within-language interference was always greater for their Chi-

THE STROOP EFFECT 187

n e s e - d o m i n a n t subjects, b u t t h a t the p a t t e r n when n a m i n g in English shifts w i t h i n c r e a s e d E n g l i s h - l a n g u a g e e x p e r i e n c e . F r o m initially greater between-language interference, t h e i r sub- j e c t s m o v e d t o finally g r e a t e r w i t h i n - l a n g u a g e i n t e r f e r e n c e . T h e r e is also evidence to suggest that this difference in interfer- ence d e p e n d s on the d o m i n a n c e o f the n a m i n g language. I f the n a m i n g l a n g u a g e is t h e n o n d o m i n a n t one, i n t e r f e r e n c e b e - tween a n d within languages tends to be close to identical (e.g., Dornic, 1982; D o r n i c & Wirberg, 1983).

F u r t h e r m o r e , c e r t a i n languages, such as C h i n e s e , m a y be m o r e vulnerable to interference t h a n others. B i e d e r m a n a n d Tsao (1979; see also M o r i k a w a , 1981, using J a p a n e s e subjects) showed m u c h greater interference for C h i n e s e students n a m i n g the i n k colors o f logographic characters t h a n for English stu- dents n a m i n g t h e i n k colors o f words. T h e y c o n c l u d e d that the processes involved in r e a d i n g C h i n e s e are m o r e s i m i l a r to those involved in n a m i n g colors t h a n are those involved in reading English. Tsao et al. (1981; see also M o r i k a w a , 1981, using Japa- nese subjects) p r o v i d e d s o m e s u p p o r t for this c l a i m by showing that m o r e interference o c c u r r e d in the right h e m i s p h e r e for C h i n e s e speakers, u n l i k e the usual p a t t e r n o f m a x i m a l interfer- ence in the left h e m i s p h e r e for speakers o f r o m a n c e languages. However, Smith a n d K i r s n e r (1982) o b t a i n e d m o r e s y m m e t r i - cal d a t a using Chinese-English a n d French-English bilinguals, so this issue has yet to b e resolved.

W h a t can we conclude a b o u t S t r o o p interference in bilin- guals for the present? Interference between the two languages o f a bilingual, although not as great as that within either one o f the languages, is very robust: Between-language interference typi- cally is about 75% o f within-language interference. Furthermore, a dominant language has more potential for interfering than does a nondominant one. T h e r e m a y also b e differences in the pro- cessing o f o r t h o g r a p h i c a n d i d i o g r a p h i c languages. Overall, the cross-language s e m a n t i c c o n t r i b u t i o n to Stroop interference is substantial.

Theoretical Accounts o f the Stroop Effect

It seems only a p p r o p r i a t e to preface discussion o f current t h e o r i e s with a c o m m e n t on how S t r o o p h i m s e l f i n t e r p r e t e d his data. Here is what he c o n c l u d e d over a half-century ago:

The associations that have been formed between the word stimuli and the reading response are evidently more effective than those that have been formed between the color stimuli and the naming response. Since these associations are products of training, and since the difference in their strength corresponds roughly to the difference in training in reading words and naming colors, it seems reasonable to conclude that the difference in speed in read- ing names o f colors and in naming colors may be satisfactorily accounted for by the difference in training in the two activities. (Stroop, 1935b, pp. 659-660)

Stroop's (1935b, 1938) general view c o r r e s p o n d e d closely to one o f the two prevalent accounts o f the S t r o o p e f f e c t h t h e relative s p e e d - o f - p r o c e s s i n g v i e w - - a n d was c o m p a t i b l e with the o t h e r o n e - - t h e a u t o m a t i c i t y view. Because o f t h e i r p r e e m i - nence in the literature, a n d b e c a u s e t h e y are conceptually close cousins, these two explanations are e x a m i n e d first.

Relative S p e e d o f Processing

In its s i m p l e s t form, the relative speed-of-processing view begins with the fact t h a t words are r e a d faster t h a n colors are

n a m e d (e.g., Cattell, 1886; Fraisse, 1969). This s p e e d difference is seen as particularly critical when two p o t e n t i a l responses (e.g., one f r o m a w o r d a n d one f r o m an i n k color) c o m p e t e to b e the response actually p r o d u c e d . The t i m e c o s t o f this c o m p e t i t i o n is "interference." This general i n t e r p r e t a t i o n is referred to as response c o m p e t i t i o n o c c u r r i n g at the e n d o f a horse race, be- cause the two codes are seen as racing to control final output.

A q u a r t e r o f a c e n t u r y ago, K l e i n (1964) saw interference as resulting from the n e e d to "restimulate" with the i n k c o l o r to overcome the strong t e n d e n c y to p r o d u c e the word, although the m e c h a n i s m o f r e s t i m u l a t i o n was not specified. Klein's view was not altogether inconsistent with Stroop's, a n d it character- ized earlier a t t e m p t s to explain the SWoop effect in t e r m s o f "habit strength" a n d related associationistic ideas o f learning. Today m o s t psychologists t h i n k o f the Stroop task as a h a l l m a r k measure o f attention, not learning. Thus, T r e i s m a n (1969) saw the p r o b l e m as difficulty in d e c i d i n g whether to a t t e n d to the i n k color o r to the word analyzers when t h e y led to different potential responses, an i d e a D y e r (1973c) a d o p t e d in his review.

I d e a s l i k e T r e i s m a n ' s u n d e r l i e w h a t c a m e to b e t h e b e s t k n o w n realizations o f response competition: those o f M o r t o n (1969; M o r t o n & C h a m b e r s , 1973) a n d P o s n e r a n d S n y d e r (1975). It was this general r e s p o n s e - c o m p e t i t i o n i d e a t h a t D y e r (1973c) e n d o r s e d in his review. In the words o f M o r t o n a n d C h a m b e r s (1973),

The various stimulus analyzers [for word and ink color] are sup- posed to operate in parallel and will each give rise to an appro- priate naming response. If multiple responses are available they will compete for entry into the single channel exit from the Logo- gen S y s t e m . . . the crucial variable is the relative speed of naming the various attributes o f the stimuli. (p. 388)

This is a very clear s t a t e m e n t o f the relative speed-of-processing view, including its r e s p o n s e - c o m p e t i t i o n element.

Very s i m i l a r ideas are c o n t a i n e d in the account offered by Posner a n d Snyder (t975):

First, the usual Stroop effect arises because of response competi- tion between vocal responses to the printed word and the ink c o l o r . . . . Second, the direction o f interference depends upon the time relations involved. Words are read faster than colors can be named, thus a color naming response receives stronger interfer- ence from the word than the reverse . . . . Third, words often facili- tate the vocal output to colors with which they share a common name . . . . These three results suggest that color naming and read- ing go on in parallel and without interference until close to the output. (p. 57)

Again, the elements o f relative speed a n d interference at the stage o f response o u t p u t are highlighted. These are the two essential elements o f the relative speed-of-processing account o f the Stroop effect. A p p e n d i x B s u m m a r i z e s the 18 m a j o r re- sults that m u s t b e h a n d l e d by any successful theory. Let us consider now how this evidence accords with this account.

Certainly, the relative speed-of-processing i d e a allows for nu- m e r o u s analogs o f the t a s k (Result 1); all t h a t is required is differential processing t i m e for two c o m p e t i n g stimuli. Regard- ing the effects o f n o n s e m a n t i c relations (Result 2), this view holds that greater s i m i l a r i t y between the n a m e s o f two c o m p e t - ing responses should m a k e it h a r d e r to select the correct one at the r e s p o n s e - p r o d u c t i o n stage. Regarding s e m a n t i c effects (Re-

188 COLIN M. MACLEOD

suit 3), disambiguation at the response stage should also be more difficult for more highly related responses.

Posner and Snyder (1975) pointed out that words more highly related to the concept o f color would be primed by processing o f the ink-color names, increasing the likelihood o f those words reaching the response stage first. This logic explains the fact that overlap o f response set for the relevant and irrelevant di- mensions influences interference (Result 14); the two dimen- sions are priming each other. Priming also explains why non- color words preceded by related words cause more interference in color naming (Result 4). Basically, words are processed faster when primed and hence are more likely to win the race to the response stage and to interfere with color naming. O f course, the p r i m i n g m e c h a n i s m is n o t f u n d a m e n t a l to the relative- speed hypothesis per se. It must be grafted on, and represents an additional element to this explanation.

Facilitatory congruence effects (Result 5) should occur be- cause the response to both dimensions is the same, so produc- tion can be based o n whichever arrives first at the response stage. Indeed, they probably even prime each other. Increased interference from mixing c o n g r u e n t with incongruent trials (Result 7) might be thought o f as being due to greater uncer- tainty at the response stage regarding which dimension (on in- congruent trials) is to provide the output. O f course, this re- quires an additional assumption beyond the two main ones mentioned earlier, but it seems a reasonable one.

Although the fact that integrated stimuli result in more inter- ference than nonintegrated stimuli (Result 6) is not a direct prediction from the relative speed-of-processing view, it can be handled. Assume that it is easier to ascertain which potential response derived from which stimulus dimension when the di- mensions are more easily separated. Dealing with the fact that m a n u a l r e s p o n d i n g displays less interference t h a n oral re- sponding (Result 13) is even more straightforward. Simply as- s u m e that there are different r e s p o n s e - o u t p u t routines (or buffers) for different modalities, which also fits with effects o f stimulus-response compatibility.

Pretrial cues should assist processing (Result 9) because they can be used to "tune" the response-output system. These cues help in deciding which dimension is salient and perhaps also what value to expect o n that dimension. However, too many so-called cues would probably lead the subject to ignore them, as has been found. W h a t about sequential trial effects (Result 8)? Two problems arise in the system. First, there may be some trace o f the two potential responses from the previous trial still resident in the response buffer when the subsequent trial be- gins. Second, there is probably priming o f the responses on trial n by the responses on trial n - 1. With some jockeying, this horse race can be made sensitive to neighboring trials.

Although silent o n sex differences (Result 15), the relative speed view would predict greater left hemisphere involvement (Result 17) because o f the importance o f reading. Both the de- velopmental pattern (Result 16) and the bilingual pattern (Re- sult 18) make sense under this view because both development a n d language proficiency ought to affect w o r d processing speed.

So far, so good. Fifteen o f the 18 results are comfortably ac- commodated. However, the remaining t h r e e - - w i t h o u t doubt the most direct predictions o f the relative speed-of-processing

v i e w - - a p p e a r to be wrong. The most obvious prediction is that the relative speed o f processing each o f the two dimensions o f the c o m p o u n d stimulus should determine interference. T h e naive intuition is that the faster dimension should interfere with the slower one and not vice versa; interference should always be completely asymmetrical. O f course, this extreme position need not be true, in fact, because we are dealing with distributions o f processing times over trials, and any overlap in those distribu- tions could produce interference. Figure I portrays this point; it is apparent that orderly patterns o f interference are predicted.

I n fact, though, interference is not an orderly function o f relative speed o f processing: The careful SOA studies o f Glaser and Glaser (1982) and Glaser and Dttngelhoff (1984) convinc- ingly make this point (see SOA). Previewing the slower dimen- sion does not lead to the clear reversal o f the direction o f inter- ference expected, thereby violating two major results (Results 10 and 11). Also relevant are the studies conducted by Kevin D u n b a r and myself (Dunbar & MacLeod, 1984; MacLeod & Dunbar, 1988): Direct manipulation o f the speed o f processing o f a dimension through reorientation o r practice (Result 12) does not produce results consistent with this theoretical per- spective.

The relative speed-of-processing hypothesis contains three key assumptions. First, there is assumed to be parallel process- ing o f the two dimensions o f the stimulus at differential speeds. Second, there is a limited-capacity response channel into which only one o f the two potential responses can be admitted at a time; priority is determined by speed. Third, there is potential for priming o f possible responses from several sources, includ- ing preceding trials and other response-set elements. This is actually quite a powerful set o f assumptions. Together they are capable o f a c c o m m o d a t i n g 15 o f the 18 critical findings in Ap- p e n d i x B, although s o m e corollary a s s u m p t i o n s were intro- duced along the way. However, they fail in the face o f the m o s t direct test o f the view: explicit manipulations o f relative speed o f processing by SOA or by practice. O n this basis, I contend that the relative speed-of-processing hypothesis m u s t be re- jected.

Automaticity

T h e s e c o n d prevalent explanation is the automaticity ac- count, which was rooted in Cattell's (1886) work over a century ago. Here the basic idea is that processing o f one dimension requires much more attention than does processing o f the other dimension. Thus, naming the ink color draws more heavily on attentional resources than does reading the irrelevant word. Moreover, reading the word is seen as obligatory, whereas n a m - ing the ink color is not. Presumably, this imbalance derives from our extensive history o f reading words as opposed to n a m - ing ink colors. Under this view, the a s y m m e t r y that is the fun- damental characteristic o f the Stroop task must occur. Words are read very automatically; colors require considerably more attention to be named.

This description is based o n the theorizing o f LaBerge and Samuels (1974), P o s n e r a n d Snyder (1975), Shiffrin a n d Schneider (1977), a n d L o g a n (1978) a m o n g others (see, e.g., Hunt & Lansman, 1986). Contrary to the all-or-none view o f

THE STROOP EFFECT 189

Figure 1. Relative speed of processing and interference. As the latency distributions for the processing of the two dimensions shift, so should the patterns o f interference.

a u t o m a t i c i t y p o p u l a r in s e c o n d a r y sources, all o f these investi- gators saw a u t o m a t i c i t y as a g r a d i e n t t h a t d e v e l o p e d with learn- ing. Thus, w o r d r e a d i n g was very automatic, a n d c o l o r (or pic- ture) n a m i n g was m u c h less automatic. M o r e a u t o m a t i c process- ing c o u l d t h e n interfere with less a u t o m a t i c processing, but not vice versa. T h e S t r o o p t a s k is a n interesting case expressly be- cause the two d i m e n s i o n s differ so m u c h in how a u t o m a t i c a l l y t h e y are processed.

Let us c o n s i d e r how such a view c o m e s to grips with the 18 key results in A p p e n d i x B. C e r t a i n l y whenever the irrelevant d i m e n s i o n o f a t w o - d i m e n s i o n a l stimulus is a word, the p o t e n - tial for interference exists. Thus, m a n y analogs o f the S t r o o p c o l o r - w o r d t a s k (Result 1) are expected. A l t h o u g h an all-or- n o n e version o f a u t o m a t i c i t y would have t r o u b l e h a n d l i n g inter- ference in situations where neither d i m e n s i o n was a word, this

would not be the case for the c o n t i n u u m version (e.g., M a c L e o d & Dunbar, 1988).

T h e real strength o f a u t o m a t i c i t y theories lies in t h e i r ability to c a p t u r e p r i m i n g situations. Recall that p r i m i n g required a n a d d i t i o n a l a s s u m p t i o n for the speed-of-processing account. In contrast, it is at the very h e a r t o f the a u t o m a t i c i t y explanation, as is evident in r e a d i n g Posner a n d Snyder (1975), for example. P r i m i n g occurs through the a u t o m a t i c spread o f activation, es- sentially the s a m e m e c h a n i s m u s e d to explain interference.

Thus, orthographic/acoustic effects (Result 2) should result f r o m a u t o m a t i c activation o f p a r t s o f the relevant a n d irrelevant response words. In like manner, s e m a n t i c effects should be evi- dent, particularly given t h a t Posner a n d Snyder a i m e d to ex- p l a i n lexical facilitation with the s a m e m e c h a n i s m as SWoop interference (see also Logan, 1980, for a s i m i l a r idea). As the

190 COLIN M. MACLEOD

irrelevant words b e c o m e more color related, they automatically activate more c o m m o n features with the relevant ink colors, resulting in interference (Result 3). Primes should be able to accomplish this in a prime-target procedure (Result 4), and previous trials should be able to do it in a continuous-trials procedure (Result 8). Set overlap should also influence the ex- tent o f automatic activation o f competing features o n any given trial through priming (Result 14). Finally, facilitation from con- gruent dimensions ought to occur (Result 5). In sum, automatic priming is the most "natural" consequence o f the automaticity account.

Also a straightforward o u t g r o w t h o f the automaticity ac- count is the fact that practice affects interference (Results 12). Practice can also be seen as encompassing the changes that occur with the development o f reading skill (Result 16) and with increasing language proficiency in bilinguals (Result 18). Auto- matic processes (e.g., reading words) have to be learned; the more automatic a process becomes with practice, the more it is capable o f causing interference with a less automatic process. This also helps to explain apparent instances o f reversed Stroop interference (Result I l): W h e n a normally more automatic pro- cess associated with one stimulus dimension is altered by an experimental manipulation, the normally less automatic pro- cess associated with the other dimension may b e c o m e relatively more automatic. In such an instance, the direction o f interfer- ence reverses.

The modality effect--that interference is reduced when the response mode is switched from oral to manual (Result 1 3 ) - makes sense because, although it may be quite automatic to process a word for oral output, it may be less automatic when the required response is in some other modality. O f course, this is an additional assumption, but not an unreasonable one.

The effects o f S P A (Result 10) are m u c h less nettlesome for the automaticity view, which does n o t hinge o n processing speed. Indeed, the fact that S P A manipulation has little impact o n word reading fits with the automaticity view. Unfortunately, this advantage is offset by the fact that tests o f automaticity are less direct than tests o f speed. Probably for this reason, Stroop researchers have not measured the automaticity o f the two di- mensions directly (cf. the discussion in MacLeod & Dunbar, 1988, p. 134).

Like the speed o f processing view, the automaticity account would predict a greater role for the left hemisphere (Result 17), because reading is what is becoming automated. There is also nothing in this account that predicts sex differences (Result 15).

The remaining three results in Appendix B pose more serious threats to the automaticity account. First, as long as both di- mensions o f a display are perceivable simultaneously, it might Seem that whether they are separate or integrated should not matter very much. In either event, once seen, a word would be processed automatically. Yet integration versus separation does matter (Result 6); generally interference effects are substantially larger in the integrated version o f the task (e.g., K a h n e m a n & Henik, 1981).

Because automatic processes do not require attention, strate- gies should have little effect on their execution. Yet particular mixtures o f congruent and incongruent trials alter the degree o f interference (Result 7), as can cues about what to expect on an u p c o m i n g trial (Result 9). These straightforward attentional-al-

location effects r u n against the grain o f the automaticity idea. Perhaps the best illustration o f this is the study by K a h n e m a n and Chajczyk (1983) in which they observed diluted Stroop effects when irrelevant additional words occurred in the display. How could reading a word, which is claimed to require no re- sources, draw resources away from color naming? Why should an extra, unrelated w o r d draw away further resources? These questions are n o t resolved b y the a u t o m a t i c i t y hypothesis.

It seems, then, that automaticity fails to provide a compre- hensive account o f the Stroop effect. However, this failure may apply only to the strong, all-or-none view ofautomaticity. Kah- n e m a n and Chajczyk (1983) described a process as strongly automatic if it is unaffected by attentional-allocation strategy. A process is partly automatic if it can occur largely without atten- tion, but nevertheless is affected by attention. They suggested that the partly automatic designation is more appropriate for the Stroop situation, leaving r o o m for attentional-allocation strategies to exert some influence.

MacLeod and D u n b a r (1988) reemphasized the view o f a u t o - maticity as a continuum, not a dichotomy. This position harks back to earlier explanations o f the Stroop effect by reinstating learning h i s t o r y as a critical element in interference. Better learned processes will be more automatic and will lead to such consequences as interference. Although response-speed im- provements may also be an outgrowth o f practice or experience, speed per se will not be the important factor. A well-learned but slow process could interfere with a less well-learned but faster process because interference arises throughout the course o f processing, not just at some late response stage.

Unfortunately, Stroop experiments rarely have provided any independent measure o fautomaticity. The specificity o f the rela- tive speed-of-processing view allowed it to be put to stringent test (and to fail). To provide a reasonable test o f automaticity views, we must be able to identify a priori which processes are more automatic so that conclusions about interference asym- metries are not entirely ad hoe. Studies carefully controlling learning history and using methodologies such as the second- ary-task procedure hold promise in this regard. At present, auto- maticity a c c o u n t s r e m a i n potentially viable, but in n e e d o f greater specification and more stringent test.

Perceptual Encoding

As Dyer (1973c, p. 114) noted, "Most o f these explanations have considered the p h e n o m e n o n in terms o f response compe- tition" Such views are often referred to as "late selection" ac- counts, in that the conflict occurs late in processing at a re- sponse stage as opposed to "early selection" at encoding, for example. Preference for late-selection accounts has been consis- tent throughout the history o f Stroop research; relative speed and automaticity are just the two most prevalent examples. However, it is worthwhile to consider the early selection idea.

The best k n o w n version o f early selection was put forward by Hock and Egeth (1970): the perceptual-encoding account. The basic idea is that perceptual encoding o f ink-color information is slowed by incompatible information from a color word as opposed to a neutral control. H o c k and Egeth (1970) presented evidence from a short-term m e m o r y scanning task, which they

THE STROOP EFFECT 191

saw as incompatible with a late, response-conflict account. Us- ing oral y e s - n o responses (to avoid a semantic relation with the printed words), they suggested that color-related words are rec- ognized earlier and thereby more likely to distract from encod- ing ink color.

However, D a l r y m p l e - A l f o r d a n d A z k o u l (1972) a n d D y e r (1973c) questioned their interpretation, arguing that Hock and Egeth (1970) failed to distinguish between identification and covert naming, and that their conclusions rested o n accepting the null hypothesis. The only other finding consistent with the H o c k and Egeth (1970) p o s i t i o n - - t h a t o f Tecce and D i m a r t i n o 0 9 6 5 ) that words spoken at the time o f ink-color encoding can either facilitate o r inhibit that e n c o d i n g - - w a s also criticized by Dyer (1973c, pp. 114-116)as relying o n a questionable assump- tion about the rates o f processing word versus color informa- tion, The perceptual encoding account has not been very promi- nent since then. At any rate, it would have trouble with several o f the key results in Appendix B (e.g., Results 7 and 14 a m o n g others). Processing subsequent to encoding definitely seems to matter to the SWoop effect.

Parallel Models. Accruing Evidence Toward a Decision

Virtually all earlier theories o f the Stroop task were sequen- tial. Information was encoded from each dimension and then analyzed, and then a response was produced, perhaps with an additional disambiguation stage. Processing at one stage had to be complete (or very close to complete) before the next stage could begin. Even if paraUel processing did appear in a model, it was limited to occurring within a stage; stage transitions were sequential. Thus, M o r t o n and Chambers (1973) saw processing within the identification stage as parallel, but transition to the response stage as sequential. Interference occurred at the entry to the response stage.

In recent years, models have begun to relax this strict sequen- tial emphasis, d i s c a r d i n g the idea o f a limited-capacity re- sponse stage (e.g., Eriksen & Schultz, 1979; Flowers & Wilcox, 1982; L o g a n , 1980; McClelland & R u m e l h a r t , 1981 ; Taylor, 1977). Capacity limitations seen as system-resource limits (e.g., K a h n e m a n , 1973) lead to quite different conceptions than did limitations seen as stage-specific bottlenecks (e.g., Broadbent, 1958).

Logan (1980, pp. 528-529) cast his model o f the Stroop effect as a decision process gathering evidence. Evidence accumulates over time until a response threshold is reached; evidence from each dimension is processed at a rate governed by its weight. Two weights determine each dimension's contribution to the decision (in terms o f evidence): a stable, automatic weight and a flexible, strategic attentional weight. Total evidence at thresh- old is the sum o f all evidence from all dimensions. I f the evi- dence from other dimensions is consistent with the desired di- mension, this reduces the threshold and hence the processing time for the desired dimension. However, if irrelevant dimen- sions provide evidence conflicting with the desired dimension, response speed will be slowed. The extent o f intrusion o f an- other dimension will be a function o f its weights; those with larger weights will have a greater impact on the composite deci- sion process.

Logan's model provides a reasonable prototype for such paral-

lel models. In general, evidence is accrued for a response with- out requiring a "response stage y Instead, interference arises because o f the a m o u n t o f priming received by competing re- sponses (i.e., the a m o u n t o f evidence accrued for them). This interference occurs during the gathering o f evidence, a continu- ous process. Some models assume independent evidence gather- ing along the dimensions, but m a n y allow interaction (e.g., McClelland & Rumelhart, 1981; Taylor, 1977), greatly increas- ing the ability o f these models to capture data.

Nevertheless, two problems arise in Logan's model (and pre- sumably in similar parallel models). First, the model appears to predict symmetrical facilitation and interference; Logan cer- tainly portrayed the model that way in 1980.~° Yet facilitation is virtually always substantially smaller than interference. Logan's model can a c c o m m o d a t e this by assuming that equal changes in activation for the congruent and incongruent conditions, rep- resented geometrically as equal angles o f departure from the neutral condition, intersect the response threshold at different angles because the point o f origin is not perpendicular to the response threshold (see, e.g., Ratcliff, 1978, Figure 4, p. 65). Note that such a model still must predict that the same manipu- lations will affect the magnitudes o f both facilitation and inter- ference, a prediction definitely worthy o f empirical test.

The second problem with Logan's model is familiar: Asym- metrical interference for color naming versus word reading is explained by differential time o f availability o f the evidence f r o m the two dimensions. As L o g a n (1980, p. 543) put it, "sources available s o o n e r c a n influence (speeded) decisions about sources that b e c o m e available later, but not vice versa:' This is tantamount to a relative speed-of-processing assump- tion, which I have already argued against. How serious a prob- lem is this for this class o f models?

As it turns out, this is not a necessary consequence o f such models, particularly those allowing interaction across dimen- sions. To illustrate with the bulletin-board metaphor, the provi- sional evidence from every dimension is continuously updated on a bulletin board visible to all processors and studied by the decision maker. Although the desired dimension may be closer to complete processing on a given trial, partial evidence still will be present from other dimensions and will contribute to the composite decision. Thus, even the "early returns" from a dimension being processed more slowly can influence output based on a dimension processed relatively quickly. There is still a speed element in this account (e.g., a very slow process might not return any evidence early enough to have any effect), but the strict speed o f processing assumption need not hold. Indeed, the variability in finishing times inherent in this stochastic rep- resentation also permits deviations from strict speed o f process- ing predictions.

Logan's model handles the major data in Appendix B quite well. He would expect many analogs o f the Stroop effect when multiple dimensions are being processed in parallel (Result 1). Because evidence would be collected at different levels o f analy- sis, both orthographic (Result 2) and semantic (Result 3) influ-

to Jonathan Cohen, Kevin Dunbar, and Gordon Logan all indepen- dently pointed this out to me. The possible solution offered here was provided in a letter from Gordon Logan.

192 COLIN M. MACLEOD

ences should make their presence known. This implicates a greater role for the linguistic processing of the left hemisphere (Result 17), but without any anticipated sex differences (Re- sult 15).

Prior activation o f some information should provide a head start to the evidence-gathering process, increasing the weight of that dimension (Result 4). Facilitation follows naturally: Evi- dence collected from different dimensions points to the same response and makes the decision easier (Result 5). Perceptual integration should make it difficult to assign differential weights to dimensions, whereas separation should make it eas- ier (Result 6). Mixing congruent and incongruent trials (Result 7) should also disrupt the assigning of weights from trial to trial because sometimes it will be helpful to incorporate informa- tion from other dimensions and sometimes it will be a hin- drance. The logic is similar for successive trial effects (Result 8) and for advance cueing (Result 9), both of which will cause temporary adjustments o f dimensional weights.

A reverse Stroop effect (Result I l) occurs when some manipu- lation of the usual situation changes the direction o f interfer- ence. In Logan's model, the relative weights of the two dimen- sions would also shift, as might the speed of gathering evidence for each. Practice (Result 12) should also affect the priority of each dimension and hence its weight. Once again, the develop- ment o f reading (Result 16) and of language ability (Result 18) can be seen as instances o f naturally occurring practice. Be- cause evidence is being gathered for one o f a set of possible responses, the overlap of this set across the two or more dimen- sions would be important (Result 14).

Only the SOA and the response modality effects do not flow easily from the Logan model, although they can be handled. The word-reading task might not be affected by SOA (Result 10) because it is difficult to alter the weight of the evidence drawn from the word; it is already highly weighted in the standard Stroop task. Apparent perceptual integration may also play a role in the largest interference being seen at the shortcst SOAs. As for response mode (Result 13), less evidence may be required for a manual response than for an oral one, although this specu- lation should be tested.

With some fine tuning, Logan's model can encompass the existing data. However, parallel models expressed only at the conceptual level tend to have more "free parameters" than do sequential models, which may be part o f why they appear to bc more successful. What is required is a parallel model expressed formally, so that its predictions can be tested and evaluated.

A Parallel Distributed Processing Model

The most exciting recent development in models of cognition is the parallel distributed processing framework (e.g., McClcl- land, 1979; Rumelhart, Hinton, & McClelland, 1986). Cohen,

Dunbar, and McCleUand (1990) used this framework to build a model o f the Stroop effect. It incorporates many of the virtues o f automaticity and relative speed of processing but few of the liabilities. Furthermore, it is quite a natural extension o f the kinds o f ideas embodied in the parallel models just described.

At the core of the Cohen et al, (1990) model is the idea that processing occurs in the system through activation moving along pathways o f different strength. Consequently, relative

speed-of-processing predictions need not always hold; it is strength, not speed, that is basic. Nor is strong automaticity appropriate; a gradient of automaticity makes sense (cf. Logan, 1985; MacLcod & Dunbar, 1988) wherein degree of automatic- ity is a function of the strength of each pathway.

Processing is performed in a system comprised of intercon- nected modules. Within each module are continuously operat- ing elementary processing units responsible for accepting in- puts from other units and providing output. Knowledge is repre- sented as a pattern of activation over units, which can change with time in a continuous, nonlinear manner. Processing oc- curs by the spread of activation along connections that exist both within modules and between modules. For simplification, Cohen ctal. (1990) assumed that information flows in one direc- tion-bottom up--from input to output.

When the model is instructed to perform a task, it selects a pathway that includes some or all of the units in one or more modulcs. The set of connections in this pathway specifies its strength, and the choice of pathway therefore determines both the speed and accuracy of processing. Individual units can be members of more than one pathway, allowing interactions be- tween processes when their pathways intersect. Thus, if two pathways arc active simultaneously and produce conflicting ac- tivation at their intersection, interference results; if they pro- duce coinciding activation, facilitation results. Such intersec- tions can occur anywhere in processing, and there can be multi- ple intersections. This realizes the essential details of the Logan model. An important difference from Logan's model is the non- linearity o f the processing units, permitting asymmetry be- tween facilitation and interference.

One of the nice features of this model is its incorporation of a clear role for attention. Attention tunes, or modulates, the oper- ation of processing units in a pathway. However, attention ac- complishes this tuning simply as another source of information would; it has no privileged status. This can be seen in Figure 2, which presents a minimalist view o f the model.

In this simple case, there are two pathways--one for ink-color information and one for word information--that share a re-

"red" "green"

Output Units

Hidden Unite

Input Units

red greencolor Word red green INK COLOR \Naming Readingj. WORD

Y TASK DEMANDS

Figure 2. A parall¢! distributed processing model of the Stroop effect (after Cohen, Dunbar, & McClel]and, 1990).

THE STROOP EFFECT 193

sponse mechanism. Each pathway has a set o f input units, each o f which connects to every intermediate unit. In turn, each intermediate unit connects to all output units. Processing be- gins with the input units and feeds upward to the response units, one o f which will eventually accrue sufficient activation to exceed threshold and produce a response. The only other element is the task-specific attentional units attached to the task-appropriate intermediate units and capable o f tuning at- tention by gating information flow.

Cohen et al. (1990) represented each potential input and out- put as a single unit in the model, although they pointed out that this o n e - t o - o n e m a p p i n g is n o t essential. Like Logan's, the model works by accruing evidence forward through the system, and the level o f activation o f a unit is a weighted sum o f the inputs reaching it. Learning occurs by changing connection strengths based o n minimizing mismatch between the desired response and the actual response produced. Successive cycles, o r trials, allow these strength adjustments to occur. In this way, the network can be trained to produce particular responses u n d e r p a r t i c u l a r sets o f c i r c u m s t a n c e s . F u r t h e r m o r e , their learning algorithm shows the power function widely seen in automaticity research.

A response occurs when a particular output unit's threshold is exceeded. A unit's response strength is the ratio o f its activa- tion to total activation. Attention, also a pattern o f activation over some set o f units, picks the pathway that determines the response, based on task instructions. Ordinarily, intermediate units rest near zero activation, but relevant attentional connec- tions (stemming from task demands) push these units toward a higher resting state, where they are more responsive to inputs. It is through this kind o f priming that attention tunes pathways.

Cohen et al. (1990) simulated a n u m b e r o f key results, includ- ing not only the basic Stroop asymmetry, but also the fact that interference usually exceeds facilitation (e.g., D u n b a r & Mac- Leod, 1984), the S P A effects o f Glaser and Glaser (1982), the practice effects o f M a c L e o d and D u n b a r (1988), the response- set effects o f Klein (1964), and other more general patterns such as the power law with practice (e.g., Logan, 1988) and a reason- able response-time distribution (Ratcliff, 1978). Although there are problems (e.g., when the word precedes the color and the task is to name the color, the word locks in and produces mas- sive interference regardless o f SOA), the model fares very well. W h a t is particularly compelling is that they set out to deal pri- marily with the MacLeod and D u n b a r training studies, yet the model does a fine j o b o f reproducing a diverse group o f results, even in instances where speed o f processing and strong automa- ticity have made incorrect predictions.

T h e Cohen et al. (1990) model provides a promising avenue for testing ideas about attention, and is eminently testable in its own right. As with the Logan model, all 18 o f the results in Appendix B are readily a c c o m m o d a t e d . All that is required are the n o t i o n s o f relative strength a n d attentional t u n i n g o f strength o f c o n n e c t i o n s between units in various pathways, coupled with nonlinearity o f processing units. That it is a func- tioning model that makes explicit the underlying assumptions is surely a mark in its favor. O f course, it would be m o s t convinc- ing to simulate all 18 critical results and to derive novel (and correct) predictions, but the model's initial success is still im- pressive. No doubt it will be put to more extensive test.

How different is the parallel distributed processing model f r o m earlier explanations? My impression is that it a m a l g a - mates the speed-of-processing and continuum-of-automaticity ideas. Like Logan's account, it rests o n parallel processing, but defeats the speed problem by emphasizing strength o f connec- tions. It also makes clear that strength determines the extent o f automaticity o f a process. Finally, it recognizes the importance o f response processes in the Stroop task without requiring a limited-capacity response channel. I n this way, the model suc- cessfully retains the powerful ideas o f earlier explanations with- out being saddled with their flaws.l~

T h e F i n a l W o r d

The first five sections set out the data base to be explained. Then the final section surveyed existing theories, evaluating t h e m against this evidence. The two predominant ones--rela- tive speed o f processing and automaticity--were seen as inade- quate. Ultimately, a new theoretical framework relying on paral- lel processing was considered. This is a ratherdifferent perspec- tive than the one put forth by Dyer (1973c) that is still widely accepted. It will be interesting to see how such theories fare when put to empirical test in u p c o m i n g years. I strongly sus- pect that the Stroop effect will continue to be a challenging p h e n o m e n o n for cognitive psychologists to explain for many years to come. For now, I look forward to the progress that will be examined in the subsequent review o f the Stroop literature some time early in the next millennium.

'J Another connectionist model appeared subsequent to the final version of this review. Phaf, van der Heijden, and Hudson (1990) use an architectural property--direct connections between compatible stim- uli and responses--instead of the differential weights favored by Co- hen et al. (1990) to explain the Stroop effect.

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Appendix A

Organizational Structure o f the Review

Topic Page Topic Page

Variations on the Stroop procedure The Stroop Color-Word Interference Test The individual stimulus version o f the

color-word task Sorting and matching versions o f the color-

word task The picture-word interference task Auditory analogs o f the Stroop task Other analogs o f the Stroop task

Manipulations o f information on critical trials Hue variation Acoustic variation Semantic variation

Semantic variation and the irrelevant word Semantic variation induced by priming

Congruency effects Integration o f the two dimensions

Experiment-wide manipulations o f information

Probability o f various trial types Stimulus set size

165 165

166

166 167 168 168 170 170 170 172 172 173 174 175

176 176 177

Trial sequence 177 Pretrial cues 178 Stimulus onset ~ynchrony 179 The reverse Stroop effect 180 Practice 181

Response-related manipulations 182 Order o f response 182 Response modality: oral versus manual 182 Response set size and composition 183

Individual differences 184 Sex differences 184 ~ge differences 184 Hemispheric differences 185 Language differences: the case o f the

bilingual 186 Theoretical accounts o f the Stroop effect 187

Relative speed o f processing 187 Automaticity 188 Perceptual encoding 190 Parallel models: accruing evidence toward a

decision 191 A parallel distributed processing model 192

T H E S T R O O P EFFECT

Appendix B

Eighteen Major Empirical Results That Must be Explained by Any Successful Account of the Stroop Effect

203

1. T h e Stroop effect is observed with lists o f stimuli, with single stim- uli, a n d with m a n y variations o n the response required. Similar d a t a patterns are evident in numerous Stroop analogs, such as the picture-word task.

2. Both orthographic a n d particularly acoustic/articulatory relations between the irrelevant word (or p a r t o f the word) a n d the to-be- n a m e d ink color contribute to the interference.

3. C o m p a r e d with n a m i n g the ink color alone, irrelevant verbal stim- uli that are unrelated to the concept o f color interfere only mini- mally with color naming. However, as the word's semantic associa- tion to the concept o f color increases, so does its power to interfere.

4. A color-unrelated word c a n b e m a d e to cause greater interference (or facilitation, or both) with color n a m i n g i f its m e a n i n g is acti- vated by a related word or phrase shortly before the color-naming trial.

5. Congruence between the irrelevant word a n d the to-be-named ink color often produces facilitation. However, this facilitation is m u c h less t h a n the corresponding interference in the incongruent condi- tion, a n d the choice o f control condition may be crucial.

6. I f the to-be-named color a n d the to-be-ignored word are presented in separate spatial locations, interference will b e reduced (but not eliminated) relative to the s t a n d a r d , integrated version o f the task. Locational u n c e r t a i n t y makes a n i m p o r t a n t contribution in n o n - integrated situations.

7. The presence o f congruent trials a m o n g the i n c o n g r u e n t a n d con- trol trials will t e n d to invoke the tactic o f splitting attention over the two dimensions, thereby increasing interference on incongruent trials.

8. W h e n the irrelevant word o n trial n - l is the n a m e o f the target ink color o n trial n, interference with color n a m i n g will be enhanced temporarily; when the ink color o n trial n - l matches the word o n trial n, there will be some facilitation o f color n a m i n g o n trial n. I f the word on trial n - 1 is repeated o n trial n, t h e n the word is already suppressed a n d will cause less interference in n a m i n g a different ink color on trial n.

9. Advance cues conveying information about the upcoming Stroop trial c a n be used to establish processing strategies that improve performance i f these cues are above the level o f subjective aware- ness a n d i f a very small set o f cues is used consistently.

10. W h e n the color (or picture) is to b e n a m e d , m a x i m a l impact o f a c o n g r u e n t or i n c o n g r u e n t word will be observed when the two dimensions begin within 100 ms o f each other. Facilitation may extend to longer SOAs t h a n interference when the word comes first. Manipulating SOA has virtually n o impact on word reading unless a very high proportion o f c o n g r u e n t trials biases use o f the color to initiate response production.

11. A reverse Stroop effect (i.e., interference with word reading caused by a n incompatible, irrelevant ink color) a p p e a r s to b e possible, but this effect is not simply a consequence o f the relative speeds o f processing each dimension.

12. Degree o f p r a c t i c e in processing each o f t h e dimensions o f a multi- dimensional stimulus is very influential in d e t e r m i n i n g the extent o f interference from one d i m e n s i o n o n another. The greater the practice in processing a dimension, the more capable t h a t d i m e n - sion seems o f influencing the processing o f a n o t h e r dimension.

13. Although still significant, interference (but perhaps not facilita- tion) is reduced when response modality is switched from oral to manual. Stimulus-response compatibility matters; i f the n o r m a l processing o f the irrelevant d i m e n s i o n leads to a response in the m o d e designated for the relevant dimension, interference is likely to be heightened.

14. W h e n the irrelevant dimension o f a set o f stimuli includes names that are eligible responses for the relevant dimension, more interfer- ence results t h a n when the sets are nonoverlapping. Although varia- tions in response set size might be expected to affect interference, existing results are unclear.

15. There are n o sex differences in Stroop interference at any age. 16. Interference begins early in the school years, rising to its highest

level a r o u n d Grades 2 to 3 as reading skill develops. With contin- ued d e v e l o p m e n t o f reading, interference declines t h r o u g h the adult years until approximately age 60, at which point it begins to increase again.

17. The left hemisphere generally shows more interference t h a n the right.

18. Interference between the two languages o f a bilingual, although not as great as that within either one o f the languages, is very ro- b u s t : Between language i n t e r f e r e n c e t y p i c a l l y is a b o u t 75% o f within-language interference. Furthermore, a d o m i n a n t language has more potential for interfering t h a n does a n o n d o m i n a n t one.

R e c e i v e d A u g u s t 18, 1989 R e v i s i o n r e c e i v e d M a y 29, 1990

A c c e p t e d J u n e 8, 1 9 9 0 •