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Divided Attention Can Enhance Memory Encoding: The Attentional Boost Effect in Implicit Memory

Pietro Spataro Sapienza University of Rome

Neil W. Mulligan University of North Carolina at Chapel Hill

Clelia Rossi-Arnaud Sapienza University of Rome

Distraction during encoding has long been known to disrupt later memory performance. Contrary to this long-standing result, we show that detecting an infrequent target in a dual-task paradigm actually improves memory encoding for a concurrently presented word, above and beyond the performance reached in the full-attention condition. This absolute facilitation was obtained in 2 perceptual implicit tasks (lexical decision and word fragment completion) but not in a conceptual implicit task (semantic classification). In the case of recognition memory, the facilitation was relative, bringing accuracy in the divided attention condition up to the level of accuracy in the full attention condition. The findings follow from the hypothesis that the attentional boost effect reflects enhanced visual encoding of the study stimulus consequent to the transient orienting response to the dual-task target.

Keywords: implicit memory, divided attention, attention and memory, repetition priming

The deleterious effects of distraction on memory encoding have been amply documented from the earliest days of psychological research (see Mulligan, 2008, for review). A common experimen- tal technique uses the dual-task paradigm, in which memory en- coding is carried out under full-attention (FA) or divided-attention (DA) conditions (i.e., while simultaneously carrying out a second task designed to compete for attentional resources). The results of numberless studies make it abundantly clear that DA during en- coding degrades later memory on tests such as recognition, free recall, and cued recall (e.g., Craik et al., 1996; Mulligan, 1998, 2008).

Recently, Swallow and Jiang (2010) reported a surprising twist on the usual adverse effects of DA. In these experiments, partic- ipants studied a sequence of pictures, each with a small square superimposed at the center. In the DA condition, participants were instructed to remember all of the images and to monitor the color of the square, pressing the space bar whenever they detected an infrequent white square (targets) among frequent black squares (distractors). In the FA condition, participants were told to ignore the squares and to focus only on encoding the pictures. When memory for the pictures was later tested in a four-choice recogni- tion task, Swallow and Jiang (2010) found that in the DA condi- tion, the images encoded together with the target squares (i.e.,

corresponding to the press response) were recognized significantly better than were those encoded with the distractor squares (the attentional boost effect). In the FA condition, in which the partic- ipants made no response to the squares, no attentional boost effect was found. What is importantly for present purposes is that the attentional boost effect was relative: For pictures accompanied by the more frequent black squares (the distractor trials), the DA condition produced worse picture memory than did the FA condi- tion, a typical DA effect on memory encoding. For pictures ac- companied by the target (white) squares, picture memory was equal in the two attention conditions, which indicated the elimi- nation of the DA effect. Thus, the attentional boost effect reported by Swallow and Jiang (2010) was a relative boost in memory encoding, bringing the typically poor memory produced by DA up to the level of the FA condition.

In later studies, the authors ruled out a number of potential accounts of the effect based on attentional cuing, reinforcement learning, perceptual grouping, oddball processing and distinctive- ness. Rather, Swallow and Jiang (2011, 2012) concluded, the attentional boost effect reflects enhanced visual encoding pro- duced by the opening of an attentional gate consequent to the transient orienting responses triggered by the detection of target squares. According to the event segmentation theory (Zacks, Speer, Swallow, Braver, & Reynolds, 2007), this gating mecha- nism would be implemented in subcortical regions like the locus coeruleus or the nucleus basalis and is involved in alerting the observer to salient environmental changes. Once a modification of the stimulus’ properties occurs (e.g., the color of the target squares becomes red), the gating mechanism is activated, resulting in increased attention to the perceptual properties of the concurrently presented images, as well as in the updating of their internal representations. In other words, the attentional gate postulated by Swallow and Jiang (2010) would act as a filter to sensory input,

This article was published Online First January 28, 2013. Pietro Spataro, Department of Psychology, Sapienza University of

Rome, Rome, Italy; Neil W. Mulligan, Department of Psychology, Uni- versity of North Carolina at Chapel Hill; Clelia Rossi-Arnaud, Department of Psychology, Sapienza University of Rome.

Correspondence concerning this article should be addressed to Neil W. Mulligan, Department of Psychology, University of North Carolina, Cha- pel Hill, NC 27599-3270. E-mail: [email protected]

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Journal of Experimental Psychology: Learning, Memory, and Cognition

© 2013 American Psychological Association

2013, Vol. 39, No. 4, 1223–1231 0278-7393/13/$12.00 DOI: 10.1037/a0030907

1223

facilitating the perceptual processing of the information encoded together with dual-task targets.

Consideration of the attentional boost effect in tandem with research on implicit memory raises the possibility of an absolute, positive effect of DA on later memory. Such a result would be quite surprising given that the presence of a second task at encod- ing virtually always reduces (or, in rare instances, merely fails to reduce) later memory. Enhanced memory encoding under DA would be unprecedented and important to document.

Implicit memory refers to unconscious or unintentional retrieval and is typically assessed on tests of repetition priming, such as lexical decision or word-fragment completion, whereas explicit memory refers to conscious or intentional retrieval reflected in standard memory tests such as free recall and recognition. In general, DA during encoding produces much weaker effects on implicit memory than explicit memory (Mulligan & Brown, 2003; Spataro, Cestari, & Rossi-Arnaud, 2011). Although implicit mem- ory is affected by DA under some conditions (e.g., depending on the type of implicit test, the type of dependent measure, or the precise nature of the dual task; Mulligan & Peterson, 2008), there are numerous examples in which a dual task that impaired later explicit memory left implicit memory unaffected (Mulligan, Duke, & Cooper, 2007; Parkin, Reid, & Russo, 1990). In general, re- search on attention and implicit memory has found that dual tasks with infrequent response selection typically fail to affect implicit memory encoding despite disrupting explicit memory encoding (Mulligan, 2003).

In the present study, we first reproduced the attentional boost effect in a four-choice recognition test (explicit memory), using words instead of pictures. Then we examined the effect in two different perceptual implicit tasks (lexical decision and word- fragment completion). We contrasted a DA condition, in which participants read aloud a series of words and concurrently moni- tored the color (red or green) of a small circle placed below each word, with a FA condition, in which participants read the words but ignored the circles. In agreement with Swallow and Jiang (2010), our approach was to consider the attentional boost effect as the combination of two related factors: a primary task facilitation as a consequence of increased attention and perceptual processing of the words due to target detection, plus a primary task interfer- ence as a consequence of the attentional demands associated with monitoring the circles to determine if a target is present. In the case of implicit memory, the mere monitoring required by the dual task is unlikely to reduce later implicit memory (Mulligan, 2003; Mulligan et al., 2007). This implies that, for words encoded with green (distractor) circles, the FA and DA conditions should pro- duce comparable amounts of priming. In contrast, for words en- coded with red (target) circles, priming should be significantly greater in the DA condition than in the FA condition, because the facilitating effect due to the detection of the infrequent red circles should overcome any small attentional interference produced by the dual task. An alternative possibility stems from the automatic- ity hypothesis (Aloisi, McKone, & Heubeck, 2004; see Lozito & Mulligan, 2010, for discussion), which states that implicit memory is the result of involuntary encoding processes. According to this account, any variation in attentional levels should not influence implicit memory; thus, the attentional boost effect should not be observed either in lexical decision or in the word-fragment com- pletion task.

Experiment 1

Experiment 1 reproduced the attentional boost effect in a four- choice recognition task, using words instead of pictures. In addi- tion, Swallow and Jiang (2010) repeated the presentation of the to-be-remembered images 10 times, whereas in the present exper- iment, the study words were shown only once. Experiment 1 verifies that the attentional boost effect is observed with verbal material and without an extended learning process.

Method

Participants. Thirty-six students from Sapienza University of Rome participated (27 women, mean age � 24.6 years).

Materials and procedure. The critical items were 30 words, seven to nine letters in length, from the LexVar database (Barca, Burani, & Arduino, 2002). They were divided into two lists of 15 words each (A and B), matched on several variables, including length in letters (M � 7.80 vs. M � 7.73), written frequency (M � 74.47 vs. M � 79.33), age of acquisition (M � 4.21 vs. M � 4.13), familiarity (M � 6.01 vs. M � 6.13), imageability (M � 5.22 vs. M � 5.42), and concreteness (M � 5.88 vs. M � 6.08). The mean values of age of acquisition, familiarity, imageability, and con- creteness were obtained from the LexVar database, as measured on 7-point Likert-type scales, whereas the estimates of written fre- quency were taken from the CoLFIS Vocabulary (which includes over 3 million occurrences; Laudanna, Thornton, Brown, Burani, & Marconi, 1995). An additional set of 120 words of medium frequency (between 50 and 100 occurrences) were selected and used as filler items during the study phase.

In the DA condition, participants were told to study (and read aloud) each word while simultaneously monitoring the color of a small circle immediately below the word. Participants were in- structed to press the space bar whenever they saw an infrequent red circle among more frequent green circles. A total of 150 words were presented during the encoding phase, at a rate of 500 ms per word. The study procedure was modeled on Swallow and Jiang (2010). On each trial, one word (Times New Roman, 44 points) and one circle (red or green; 1 cm in diameter) appeared simulta- neously at the center of the screen for 100 ms, with a vertical distance of 1 cm between them, after which only the word re- mained visible for an additional 400 ms. There was no interruption between successive trials. The study list was constructed of 16 blocks of seven items (15 critical blocks preceded by one practice block). Critical words encoded with red circles (and associated with a press response) were always located in the fourth position. Critical words encoded with green circles were placed either in the third position or in the fifth position; this choice followed from the fact that in the study by Swallow and Jiang (2010), memory for the stimuli presented with distractor squares did not vary across encoding positions. For simplicity, the critical words presented on target trials (i.e., with red circles) are referred to as target words and the critical words presented on distractor trials (i.e., with green circles) are referred to as distractor words. However, it should be noted that words on all trials were to be identified. The designation of target and distractor words refers to whether the word co- occurred with a target or a distractor on the dual (circle detection) task.

In sum, only two critical words were presented per block, yielding a total of 30 critical words across all blocks (15 encoded

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1224 SPATARO, MULLIGAN, AND ROSSI-ARNAUD

with red circles [target words] and 15 encoded with green circles [distractor words]). For half of the participants in each condition, eight distractor words were located in the third position, while the other seven were placed in the fifth position; for the remaining participants, the pattern was reversed. All other words in a block were noncritical filler words and were accompanied by green circles. One to five additional filler words, always presented with green circles, were randomly located between blocks to reduce the temporal regularity of the target trials. It should be noted that the block organization of the study list was not apparent to the par- ticipant; all trials were presented as one continuous list. List A and List B words were counterbalanced across participants, so they had the same probability to be encoded with red and green circles. The FA condition was identical except that participants were told to ignore the circles.

After a 5-min distractor task (arithmetic problems), a four- choice recognition task was administered. There were a total of 30 quadruplets, one for each critical word. Each trial included a fixation point for 500 ms, a test slide including four words that remained visible until response, and a 1,500-ms pause. The four words were arranged in different quadrants of the screen, with the studied word being located equally often in each quadrant. Two foils were semantically unrelated to the studied word, while the third foil was a semantic associate (e.g., for the studied word hospital, the semantic associate was surgeon). For each studied word, the corresponding foils were selected from the LexVar database to be matched as closely as possible in terms of length and written frequency (range: 20 –100 occurrences; M � 73.86). Such a manipulation was used to mimic as closely as possible the original study by Swallow and Jiang (2010), where two of the four test images were similar scenes. Participants were required to select the words studied during the encoding phase. For the pur- pose of scoring, the choice of the semantic associates was consid- ered an error throughout.

Results and Discussion

During the encoding phase, participants correctly detected 98.2% of the red circles (mean response time � 419.9 ms). In the present experiment, as well as in the following ones, care was taken to ensure that participants read aloud all the words, so that identification accuracy was always 100%. The proportions of studied words correctly recognized (Figure 1) were submitted to a 2 (word type: target words [encoded with red circles] vs. distractor words [encoded with green circles]) � 2 (attentional condition: FA vs. DA) mixed analysis of variance (ANOVA), in which word type was manipulated within subject and attentional condition was manipulated between subjects. Results revealed (a) a significant effect of word type, F(1, 34) � 9.41, mean square error (MSE) � 0.024, p � .004, �2 � 0.22, indicating that target words were recognized better than distractor words were (M � 0.64 vs. M � 0.53), and (b) a significant interaction, F(1, 34) � 6.55, MSE � 0.024, p � .015, �2 � 0.16. The effect of attentional condition was not significant, F(1, 34) � 1.12, p � .297. An analysis of simple effects showed the typical attentional boost effect: In the DA condition, accuracy was greater for target words than for distractor words, F(1, 34) � 15.82, p � .000, �2 � 0.32, whereas no difference was found in the FA condition, F(1, 34) � 0.13, p � .722). It is important to note that the attentional boost effect was

relative, because it enhanced memory for target words in the DA condition to the same level of the FA condition, F(1, 34) � 1.24, p � .272; in contrast, distractor words were recognized more accurately in the FA condition than in the DA condition, F(1, 34) � 8.19, p � .007, �2 � 0.19, replicating the usual negative effect of a secondary task during encoding on explicit memory. The results did not change when the performance for the target words was conditionalized on prior success at the study-phase detection task (not surprisingly given the very high detection rate): indeed, the critical interaction between word type and attentional condition remained significant, F(1, 34) � 6.12, MSE � 0.023, p � .019, �2 � 0.15. Furthermore, the findings are not explained by differ- ences in the likelihood of (incorrectly) selecting the semantic associates, because a mixed ANOVA with the same factors as above showed no significant effects or interaction on the propor- tion of semantic associates chosen, F(1, 34) � 2.23, p � .14.

In summary, the results of Experiment 1 replicated the relative boost effect observed by Swallow and Jiang (2010), using verbal material (instead of pictures) and a single encoding presentation (instead of multiple presentations). Moreover, our data extend to long-term memory the suggestion that the detection targets need not overlap in space with the to-be-remembered items to produce a significant facilitation (Makovski, Swallow, & Jiang, 2011).

Experiment 2

In Experiment 2, we examined the attentional boost effect in the lexical decision task (LDT), a perceptual implicit test characterized by a strong resilience to the negative consequences of DA (Mul- ligan & Peterson, 2008; Newell, Cavenett, & Andrews, 2008; Spataro, Mulligan, & Rossi-Arnaud, 2011).

Method

Experiment 2 used the methods of Experiment 1 with the following modifications. A new pool of 36 participants took part (21 women, mean age � 24.2 years). A set of 45 critical words, seven to nine letters in length (including the 30 used in Experiment 1), were divided into three sublists of 15 words each. The prop- erties of the additional 15 words were similar to those illustrated for Experiment 1 (M length in letters � 7.67; M written frequency �

Figure 1. Experiment 1: Proportions of correct recognition, as a function of word type and attentional condition. DA � divided-attention; FA � full-attention. Bars represent standard errors.

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1225ATTENTIONAL BOOST EFFECT IN IMPLICIT MEMORY

77.33; M age of acquisition � 4.23; M familiarity � 5.85; M imageability � 5.30; M concreteness � 5.92; M neighborhood size � 0.20). The three sublists were counterbalanced across participants, so that all the critical words had the same probability to be presented in target trials (with red circles), in distractor trials (with green circles) or as unstudied words. Encoding was inciden- tal, because participants were required to read aloud all the words but were not told to remember them for a later memory task.

The test phase consisted of the LDT. Participants were presented with a total of 110 items, including the 45 critical words (30 studied and 15 unstudied) and 45 legal pronounceable nonwords, plus 20 practice items (10 words and 10 nonwords). Each trial included three events: a fixation point for 500 ms, a string of letters until the participant’s response, and a pause of 1,500 ms. The instructions were to decide if each item was a valid Italian word by pressing either S for word or N for nonword: Both speed and accuracy were stressed. The order of presentation was randomized anew for each participant, and no mention was made about the relationship with the study phase.

Results and Discussion

During the encoding phase, participants correctly detected 99.2% of the red circles (mean response time � 401.2 ms). Only reaction times (RTs) for correct responses were analyzed and RTs more than 3 standard deviations from the participant’s mean were removed (Ziegler & Perry, 1998; less than 3% of the data). Mean RTs for words encoded with red circles, words encoded with green circles, and baseline (unstudied) words were 667.08, 701.81, and 743.99 ms in the DA condition and 682.58, 679.40, and 720.25 ms in the FA condition. Priming scores (the difference between RTs for unstudied and studied words) are the dependent measure (see Figure 2). Preliminary t tests indicated that RTs for unstudied words did not differ between the DA and FA conditions, t(34) � – 0.59, p � .55, and that priming scores were significantly greater than zero for target words and for distractor words, t(17)s � 3.28, all ps � 0.01. A mixed 2 (word type: target vs. distractor) � 2 (attentional condition: FA vs. DA) ANOVA, with word type as the within-subject variable and attentional condition as the between- subjects variable, found a significant effect of word type, F(1, 34) �

6.89, MSE � 644.85, p � .013, �2 � 0.17, indicating that priming scores were greater for target words (encoded with red circles) than for distractor words (encoded with green circles; M � 57.2 ms vs. M � 41.5 ms), and there was a significant interaction, F(1, 34) � 10.11, MSE � 644.85, p � .003, �2 � 0.23. The main effect of attentional condition was nonsignificant, F(1, 34) � 1.60, p � .213. Follow-up analyses showed an attentional boost effect in the DA condition, in which repetition priming was higher for target than distractor words, F(1, 34) � 16.86, p � .000, �2 � 0.33, but not in the FA condition, F(1, 34) � 0.15, p � .698. It is important to note that the attentional boost was absolute: Priming scores for target words were significantly greater in the DA condition than in the FA condition, F(1, 34) � 4.73, p � .037, �2 � 0.12. In contrast with Experiment 1, no difference between the two conditions was obtained for distractor words, F(1, 34) � 0.01, p � .939, confirm- ing that a dual task with infrequent response rates did not disrupt implicit memory (Mulligan et al., 2007). Similar results were obtained when the performance for the target words was condi- tionalized on prior success at the study-phase detection task; in particular, the crucial interaction between word type and atten- tional condition remained significant, F(1, 34) � 11.30, MSE � 635.34, p � .002, �2 � 0.25. Error rates ranged from 3.4% to 4.9% and did not vary across word type (target, distractor, and unstud- ied) or attentional condition, F(2, 68) � 0.39, p � .62.

The results of Experiment 2 showed that an absolute attentional boost effect is observed in the LDT. According to Swallow and Jiang (2010), this happens because the attentional orienting re- sponse following target detection facilitates the perceptual encod- ing of the concurrent word, overcoming any negligible interference caused by the dual task. Experiment 3 generalizes this finding to a second perceptual implicit test.

Experiment 3

Word-fragment completion (WFC) was used in Experiment 3 because it shares with the LDT two relevant properties: (a) It is a perceptually driven task (Roediger & McDermott, 1993) and (b) it is minimally affected by DA manipulations (Mulligan & Hartman, 1996; Spataro, Mulligan, & Rossi-Arnaud, 2010).

Method

Experiment 3 used the same materials and procedures as Ex- periment 2 with the following modifications. A new set of 36 participants took part (25 women, mean age � 24.6 years). The test phase consisted of the WFC task. A word fragment was constructed for each critical word by replacing three to five letters with blanks (the proportions of letters retained ranged from 0.43 to 0.57). The critical words were rotated across participants so that they had the same probability to be presented in target trials (with red circles), in distractor trials (with green circles), or as unstudied words. On the WFC test, participants were presented with a total of 70 fragments for 4 s each, corresponding to the 45 critical words (30 studied and 15 unstudied), plus 25 filler words (i.e., additional new words not presented during the encoding phase). Each trial included three events: a fixation point for 500 ms, a fragment for 4,000 ms, and a pause of 1,500 ms. The order of the fragments was randomized anew for each participant, with the constraint that the first 10 fragments were filler items that could not be completed

Figure 2. Experiment 2: Priming scores in the lexical decision task, as a function of word type and attentional condition. DA � divided-attention; FA � full-attention. Bars represent standard errors.

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1226 SPATARO, MULLIGAN, AND ROSSI-ARNAUD

with a critical word. Participants tried to complete the fragments with the first word that came to mind. It should be noted that the time limit of 4 s is lower than that typically used in literature (10 or 12 s; Mulligan, 1998; Roediger, Weldon, Stadler, & Riegler, 1992). This was done to minimize the use of explicit retrieval strategies.

Results and Discussion

During the encoding phase, participants correctly detected 99.4% of the red circles (mean response time � 425.9 ms). The mean proportions of fragments correctly completed for words encoded with red circles, words encoded with green circles, and baseline (unstudied) words were 0.32, 0.21, and 0.13 in the DA condition and 0.26, 0.26, and 0.17 in the FA condition, respec- tively. For the WFC test, priming is computed as the difference between the proportion of correct completions for studied and unstudied words (see Figure 3). Preliminary t tests indicated that the proportions of fragments completed with unstudied words did not differ between the DA and FA conditions, t(34) � –1.02, p � .31, and that priming scores were significantly greater than zero for target words and for distractor words, t(17)s � 2.25, all ps � .05. A mixed 2 (word type: target vs. distractor) � 2 (attentional condition: FA vs. DA) ANOVA showed results identical to those of Experiment 2. Both the main effect of word type and the interaction between word type and attentional condition were significant, F(1, 34) � 4.22, MSE � 0.010, p � .048, �2 � 0.11, and F(1, 34) � 4.82, MSE � 0.010, p � .035, �2 � 0.12, but the main effect of attentional condition was not, F(1, 34) � 0.96, p � .33. An analysis of simple effects showed that priming was greater for target than for distractor words in the DA condition, F(1, 34) � 9.02, p � .005, �2 � 0.21, but not in the FA condition, F(1, 34) � 0.01, p � .921. As in Experiment 2, an absolute boost effect was obtained: For target words, the DA condition produced more priming than the FA condition did, F(1, 34) � 4.67, p � .038, �2 � 0.12; in contrast, for distractor words, the DA and FA conditions produced equivalent priming, F(1, 34) � 0.04, p � .854. The results did not change when the performance for the target words was conditionalized on prior success at the detection task; in particular, the crucial interaction between word type and

attentional condition remained significant, F(1, 34) � 4.52, MSE � 0.010, p � .041, �2 � 0.12.

The results of Experiment 3 replicated the findings of Experi- ment 2 using a different implicit task (WFC). Overall, the present data suggest that DA can actually increase perceptual priming for the words presented with dual-task targets.

Experiment 4

If the attentional boost effect is due to enhanced perceptual encoding (as proposed by Swallow & Jiang, 2010), then this effect should not extend to conceptually driven implicit tests that are relatively insensitive to variation in perceptual processing. In Ex- periment 4, conceptual priming was assessed with a semantic classification task (Brysbaert, Van Wijnendaele, & De Deyne, 2000). Such tasks are largely unaffected by dual-task manipula- tions (Mulligan & Peterson, 2008), so no effect of DA is expected for the words encoded with green circles. The critical issue is whether enhanced memory is found for words encoded with red circles. The current analysis predicts no facilitation.

Method

A new set of 36 participants took part (22 women, mean age � 23.8 years). The critical words, the study phase, and distractor phase were the same as those in Experiments 2 and 3. During the test phase, participants were presented with 100 stimuli, corre- sponding to 45 critical words (30 studied and 15 unstudied), 45 Italian first names, and 10 practice items (five words and five first names). Each trial included a fixation point for 500 ms, a string of letters until the participant’s response, and a pause of 1,500 ms. Participants responded by pressing the A key to classify the stim- ulus as a word and the L key to classify it as a first name. Both speed and accuracy were stressed. The order of presentation was randomized anew for each participant. As in previous experiments, no mention was made about the relationship with the study phase.

Results and Discussion

During the encoding phase, participants correctly detected 98.4% of the red circles (mean response time � 419.1 ms). As in Experiment 2, only the RTs for correct responses were analyzed, and RTs more than 3 standard deviations from the participant’s mean were removed (less than 5% of the data were discarded). Mean RTs for words encoded with red circles, words encoded with green circles, and baseline (unstudied) words were 599.81, 599.54, and 616.79 ms in the DA condition and 613.71, 607.10, and 627.82 ms in the FA condition, respectively. Priming scores were com- puted as the difference between RTs for unstudied and studied words (see Figure 4). Preliminary t tests confirmed that RTs for unstudied words did not differ between the DA and FA conditions, t(34) � – 0.31, p � .75, and that priming scores were significantly greater than zero for target words and for distractor words, ts(17) � 2.49, all ps � .05. In contrast to Experiments 2 and 3, a mixed 2 (word type: target vs. distractor) � 2 (attentional condition: di- vided vs. full attention) ANOVA on priming scores showed no main effects and no interaction between the two, F(1, 34) � 0.48, p � .493. The same results were obtained when the performance for target words was conditionalized on prior success at the de-

Figure 3. Experiment 3: Priming scores in the word fragment completion task, as a function of word type and attentional condition. DA � divided- attention; FA � full-attention. Bars represent standard errors.

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1227ATTENTIONAL BOOST EFFECT IN IMPLICIT MEMORY

tection task, F(1, 34) � 0.59, p � .447. Error rates ranged from 1.8% to 4.6% and did not vary across word type (target, distractor and unstudied) or attentional condition, F(2, 68) � 1.98, p � .15.

Experiment 4 showed no attentional boost effect in a semantic classification task. Given that conceptual priming is relatively sensitive to prior conceptual but not perceptual encoding, this result is consistent with the hypothesis that the facilitation pro- duced by the detection of dual-task targets affects processing of perceptual but not semantic information. However, before accept- ing this conclusion, it is important to consider the statistical power of the critical comparison in this experiment. To this purpose, we computed the unbiased d effect sizes of the attentional boost effect in the DA conditions of Experiments 2 and 3 (Hedges & Olkin, 1985), yielding values of 0.71 and 0.81, respectively. Using the software G�Power3 (Faul, Erdfelder, Lang, & Buchner, 2007), we found that the post hoc power to detect an advantage of target words in the DA condition of Experiment 4 ranged from 0.97 to 0.99. The power to detect an effect less than one third of that observed in Experiments 2 and 3 (d � 0.20, a medium-small effect size, according to Cohen, 1988) was 0.83. In addition, we also performed a Bayesian analysis to test the likelihood that word type interacted with attentional condition, using a method illustrated by Masson (2011), which requires the transformation of the sum-of- squares values generated by the ANOVA. Within this approach, the null hypothesis (interaction absent) and the alternative hypoth- esis (interaction present) are directly contrasted as competing models; then, the Bayes information criterion value is estimated and used to compute a Bayes factor and generate the posterior probabilities for each hypothesis. For Experiment 4, this analysis indicated that the probability of the null model (interaction absent), given the data, p(H0|D), was 0.83. A similar value (0.81) was obtained when analyzing the likelihood of the null hypothesis for the main effect of word type in the DA condition. Both results provide positive evidence for the null hypothesis, following the guidelines proposed by Raftery (1995). Thus, it appears that the current experiment had substantial power to detect an attentional boost effect if one was to be found in the semantic classification task.

A second important point to clarify is that the absence of significant DA effects in Experiment 4 should not be taken as

evidence that the semantic classification task does not reflect conceptual processing. Instead, this finding is consistent with the distinction drawn by Vaidya et al. (1997; see also Gabrieli et al., 1999) between competitive and noncompetitive access to concep- tual knowledge in memory. Competitive tests (e.g., category ex- emplar generation and general knowledge) are those in which the retrieval cues are insufficient to guide the retrieval process to a unique entry in semantic memory and thus initiate a competition between alternative legitimate responses. Performance in this type of task is typically enhanced by conceptual elaboration (Srinivas & Roediger, 1990; Vaidya et al., 1997) and decreased by DA at encoding (Light, Prull, & Kennison, 2000; Mulligan & Stone, 1999). In contrast, noncompetitive tests (e.g., category verification and semantic classification) are those in which the retrieval cues directly specify a unique solution. In such a circumstance, any encoding task will result in full priming (provided that the studied stimuli are correctly identified): As a consequence, noncompetitive tests are relatively unaffected by conceptual elaboration and DA at encoding (Light et al., 2000; Mulligan & Peterson, 2008; Vaidya et al., 1997). However, these forms of priming are not mediated by perceptual processes, because it has been repeatedly demonstrated that they are insensitive to study–test modality changes (Light et al., 2000; Vaidya et al., 1997). Even more cogent evidence comes from neuroimaging studies. Wig, Grafton, Demos, and Kelley (2005), for instance, found that transcranial magnetic stimulation to the left frontal cortex (a cerebral region typically associated with semantic processing) disrupted repetition priming in a semantic classification task, whereas stimulation to the middle and inferior occipital gyri (typically associated with visual processing) had no effect. Likewise, using functional magnetic resonance imaging, Wagner, Koutstaal, Maril, Schacter, and Buckner (2000) reported significant signal reductions in the left inferior prefrontal cortex when participants made semantic decisions about words processed in a semantic manner during the study phase but not when they made decisions about words processed in a nonsemantic way. All of these findings unequivocally confirm that semantic classifica- tion tasks tap conceptual processes, even though they are unaf- fected by DA at encoding.

General Discussion

In the present study, we document the surprising result that dividing attention during encoding can actually enhance memory. Words that accompanied target stimuli from a dual task produced more perceptual priming (in WFC and LDT) than did words encoded under full attention. This stands out as a highly singular result given that the overwhelmingly common result is that DA impairs or at the very least does not enhance memory performance (Mulligan, 2008). The guiding hypothesis was that detection of the target enhances the perceptual encoding of a co-occurring stimulus (the study picture in Swallow & Jiang, 2010; the study word in the present experiments). Consistent with this, the attentional boost effect was found on two perceptual priming tests, so categorized because of their sensitivity to prior perceptual processing, but not on a conceptual priming test, which is relatively insensitive to variation in prior perceptual processing.

More generally, dividing attention during encoding largely dis- rupts elaborative processes (e.g., Craik et al., 1996; Mulligan, 2008). On the one hand, if a memory test is relatively insensitive

Figure 4. Experiment 4: Priming scores in the semantic classification task, as a function of word type and attentional condition. DA � divided- attention; FA � full-attention. Bars represent standard errors.

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1228 SPATARO, MULLIGAN, AND ROSSI-ARNAUD

to variation in elaborative encoding, which is the case for percep- tual implicit tests, then the monitoring requirements of this dual task will have minimal negative impact on later memory perfor- mance. On the other hand, the enhanced perceptual encoding brought about through target detection will produce an enhance- ment that is not offset by the typical negative effects of the dual task, with the consequent surprising result that a DA condition ends up enhancing memory encoding as reflected by perceptual priming. Recognition memory is often assumed to be sensitive to both semantic and elaborative processing on the one hand and perceptual fluency processes on the other (Yonelinas, 2002). This renders recognition memory, according to this analysis, sensitive to both the negative effects of a dual task (the reductions in semantic and elaborative processing) and the positive effects as- sociated with enhanced perceptual analysis of study stimuli co- occurring with targets. This is clearly the case in the picture recognition tests used by Swallow and Jiang (2010, 2011). In the results of the present Experiment 1, the negative effect is reflected in worse memory for words encoded with green (distractor) circles in the DA condition compared with FA condition; however, the positive effect is reflected in the attentional boost pattern, in which words encoded with red (target) circles are at an advantage relative to words encoded with green circles, and equivalent to words presented in the FA condition.

It should be noted that a positive effect of DA on recognition memory was reported by Voss, Baym, and Paller (2008). How- ever, this beneficial effect was a retrieval rather than an encoding phenomenon and was due to a shifting in the type of retrieval strategies used in the FA and DA conditions, with low-confidence guess responses (prevalent in the DA condition) being more ac- curate than both recollection and familiarity responses (prevalent in the FA condition). In contrast, the positive effect produced by the attentional boost effect arises during encoding, due to enhanced perceptual processing of a stimulus (a study word or picture) that appears simultaneously with a dual-task target (Swallow & Jiang, 2010). Thus, the present findings provide evidence that attending to a second task may produce an absolute enhancement to the perceptual processing of study words during the encoding phase. Another recent study (Lin, Pype, Murray, & Boynton, 2010) has reported that memory for images presented at the same time as dual-task targets was enhanced relative to images presented at the same time as distracters. Lin et al. (2010) had participants study a rapid serial visual presentation sequence of letters appearing over full-field urban and natural scenes. They were required to press a key when a target letter was presented and, immediately after the end of the sequence, to indicate whether a probe scene was included in the sequence. Like in the attentional boost effect, Lin et al. (2010) found that memory performance was better for scenes co-occurring with target letters than for scenes co-occurring with distractor letters. Nonetheless, there are important differences with both the present findings and those illustrated by Swallow and Jiang (2010, 2011) that prevent a direct comparison between the two set of findings. Methodologically, the presentation rates of the scenes in the study by Lin et al. (2010) were much faster than those in our experiments (133 ms/item vs. 500 ms/item); participants were tested after single sequences rather than after multiple se- quences; and the test phase was performed immediately after each sequence, without breaks (thus tapping short-term memory rather than long-term memory). Even more important, Lin et al. (2010)

tested source memory, whereas in the presented experiments, our focus was on memory for the scenes themselves. This is a crucial difference, because source and item memory may rely on inde- pendent brain systems (Davachi, 2006). Finally, Lin et al. (2010) found no evidence of dual-task interference, because source mem- ory for images presented together with targets in the DA condition were remembered better than the same images in the FA condition. In contrast, in the present Experiment 1 (explicit recognition), as well as in the studies by Swallow and Jiang (2010, 2011), the performance for words presented at the same time of target circles did not differ between the FA and DA conditions. The latter finding suggests that qualitatively different mechanisms may un- derlie the two phenomena.

It is unlikely that the absolute advantage observed in perceptual implicit memory can be explained by alternative accounts of the boost effect, such as attentional cuing, reinforcement learning, distinctiveness, and oddball processing (see Swallow & Jiang, 2010, 2011, for extensive discussion). The attentional cuing hy- pothesis states that participants might use the red circles as selec- tive cues to attend to the background images, thereby enhancing their processing. Swallow and Jiang (2011, Experiment 1) ruled out this explanation by showing that the attentional boost effect was not obtained when the target squares appeared 100 ms before the to-be-remembered images (instead of being presented concur- rently with them). Similarly, the effect does not reflect the rein- forcement of predictive information in memory, because Swallow and Jiang (2011, Experiment 2) demonstrated that there was no significant advantage for images that preceded the presentation of the target squares by 100 ms (according to the reinforcement hypothesis, any stimulus that consistently anticipates the reward should be reinforced in memory). Given that the attentional boost effect shows the same characteristics in the present experiments as in those reported by Swallow and Jiang (2010, 2011; expect for the absolute nature of the target words’ advantage), the above evi- dence implies that these hypotheses are unlikely explanations for our data. A third possibility is that the attentional boost effect represents nothing more than the classical isolation effect in mem- ory (Hunt & McDaniel, 1993), whereby people tend to have superior memory for an item when it is perceptually or semanti- cally distinct from the other items in the list than when it is not. As outlined by Swallow and Jiang (2010), this hypothesis incorrectly predicts that the attentional boost effect should have occurred in the FA condition, a result that was never observed in our experi- ments. Furthermore, all of the above accounts would expect a significant attentional boost effect in Experiment 4 (with semantic classification), because the encoding phase was the same as those in Experiments 1–3: The fact that we found no advantage in that condition supports an explanation based on the distinction between perceptual and conceptual processes.

Finally, the present results have consequences for theories of implicit memory. In particular, our data are inconsistent with the claim that implicit memory is the result of involuntary, automatic processes (the automaticity hypothesis; Aloisi et al., 2004). What is more interesting is that they do not support the distinction between implicit tests based on identification processes (i.e., in which the retrieval cues have unique solutions) and production processes (i.e., in which the retrieval cues induce a competition between multiple solutions; Gabrieli et al., 1999), because the

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1229ATTENTIONAL BOOST EFFECT IN IMPLICIT MEMORY

attentional boost effect had a differential impact on three implicit tasks than can be all classified as identification tests.

In summary, four experiments illustrated the surprising finding that, in contrast with the well-known interference hypothesis (Craik et al., 1996), when participants are asked to study a list of words while simultaneously detecting infrequent targets in a sec- ond task, performance in the DA condition can be increased over and above the levels obtained in the FA condition. This facilitation occurred in two perceptual implicit tests but not in a semantic classification task. Overall, the results support the hypothesis that the boost effect is caused by the opening of an attentional gate after the detection of infrequent targets, which increases the perceptual encoding of simultaneous stimuli (Swallow & Jiang, 2010, 2011).

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Received June 7, 2012 Revision received September 12, 2012

Accepted September 12, 2012 �

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