RESEARCH METHODS IN PSYCHOLINGUISTICS
Psycholinguistics employs a number of ways understand language. These
range from observational studies, speech error analysis to experiments and
neuroimaging techniques. We also use computational models to simulate our theories
about the language system. This section will explore some of the techniques employed
by researchers. However, keep in mind that we are always developing new techniques
to understand how language works.
MENTAL CHRONOMETRY
The study of reaction time on cognitive tasks is a common psychological
paradigm in trying to infer the duration, sequence and content of cognition. As seen in
Figure 1.5, reaction time (or RT) is measured as the time between the onset of a stimuli
and the response by the participant. The mean and the variance of reaction times are
considered useful indices on processing speed. The most common form of reaction
time experiments are button presses. However, eye movements and voice onset (in
repetition and reading tasks) can also be employed. One of the most popular reaction
time paradigms is called priming. Priming is used in almost all areas of psychology.
The basic idea is that if two things share some cognitive or psychological attribute,
they will either facilitate or interfere with each other. However, it they do not share such
similarities, there will be no such effect. For example, it is easier to recognize the word
DOG if you have already seen the word CAT. This can be a kind of sematic priming in
that both words belong to the same semantic category (ANIMAL). Such an effect is
known as facilitation while the interference of slowing down of such an effect if known
as interference. The reasoning behind priming effects can be modelled as a web of
interconnected ideas or concepts in the mind. Concepts that are connected
semantically (dogs and frogs are both animals) or phonologically (dog and bog end
with similar sounds) are more likely to facilitate priming. In Figure 1.6, sematic
connections are indicated with straight lines while phonological connects are indicated
with dotted lines. The idea is that encountering a stimulus (by seeing or hearing it) will
not only activate that concept in the mind but also partially activate connected
concepts to some degree. As such, when any one of those connected concepts is
presented next, they will be retrieved quicker because they have already been partially
activated (or primed) by the previous activation.
LESION STUDIES
As the brain is a vulnerable organ, it can be damaged by external or internal
trauma. If blood flow and oxygen supply is constricted even for a few minutes to
neurons they begin to die. These sites of damage are called lesions. Such trauma can
be from accidents, strokes, brain surgery, or the ingestion of certain toxins. Examining
these lesions and associating them with the behavioural limitations of such patients
can provide valuable information about which regions are responsible for which
behaviour. Cognitive Neuropsychology has contributed to psycholinguistics from the
earliest times. Perhaps the earliest record of this is from case 20 in the Edwin Smith
Papyrus. It is the report of a patient with a head injury which led to the following
observation: “…He is speechless. An ailment not to be cured.” A clear case of speech
loss due to brain injury. Centuries later, Broca and Wernicke continue with such
observation. Cognitive neuropsychology attempts to relate braindamaged behavioural
deficits to models of normal processing. Shallice (1988) overserved that cognitive
neuropsychology has made significant advances in associating neurological disorders
to cognitive model, emphasized the importance of single case studies over group
studies, and contributed to the exploration of impaired brain behaviour as a way
towards understanding unimpaired behaviour. While traditional lesion studies were
conducted by post-mortem examination and backtracking to analyse the behaviour of
the patient while alive, modern neuroimaging techniques allow us to examine lesions
in patients while they are alive and conduct behavioural analysis in real time.
ELECTROENCEPHALOGRAPHY (EEG)
The advent of neuroimaging techniques has led to a flowering of new research
in psycholinguistics. While traditional X-rays are not able to provide much detail on the
brain, other technology such as the measurement of electrical activity in the brain have
provided valuable data. Such techniques include electroencephalography or EEG
which measures the brain’s electrical activity by detecting them from electrodes placed
on the scalp. An amplifier can then amplify the millivoltage differences across the scalp
and provide a continuous reading of brain activity. Psychologists go even further and
measure such electrical activity by tying them to specific events (such as the
presentation of a stimulus). Such event-related potential or ERPs can have positive or
negative polarities. These peaks in ERP readings are labelled according to their
polarity (positive or negative) and the time difference from the stimuli onset (in
milliseconds). Some common ERPs include N400 (detected 400ms after stimulus
onset as a negative voltage) and P300 (detected 600ms after stimulus onset as a
positive voltage). As EEG and ERP are measuring electrical activity, they detect
changes in the brain almost instantly. We can say they have very good temporal
resolution. However, as they are detecting this electrical potential from the scalp, the
signals that are detected tend to be an averaged out one from multiple brain regions
and neurons. Therefore, it is not always possible to pinpoint which brain region was
actually involved in a particular EEG or ERP signal. In other words, these techniques
have poor spatial resolution. Other techniques such as PET and MRI have been
developed as a way to increase the spatial resolution of neuroimaging.
POSITRON EMISSION TOMOGRAPHY (PET)
PET (positron emission tomography) uses radioactive substances as tracers to
produce images of brain activity. As the brain consumes a large amount of energy,
injecting glucose into the body ensures that most of it ends up in brain regions that are
active in a cognitive task. If the glucose contains isotopes that are radioactive, their
emissions can be detected and transformed into images. PET is employed both as a
medical and research tool. As seen in Figure 1.8, a short-lived radioactive isotope is
injected into the participant. The most commonly used is F-18 labeled
fluorodeoxyglucose (FDG). After a waiting period for the active molecule to become
concentrated in the brain tissue (one hour for FDG), the participant is placed inside
the scanner. As the tracer decays, its emissions are collected by the scanner. The
scanner depends on detecting a pair of photons moving in opposite directions.
Photons that do not have a temporal pair are ignored. Computational reconstruction
uses statistical analysis and error correction to produce images such as Figure 1.8
which shows a scan of an unimpaired participant. As you can imagine, the main issue
with PET is the injection of radioactive material into the body. Various jurisdictions set
standards on the maximum amount of radiation that a person can be exposed to in a
year. This means that the same participant can only take part in a small number of
PET scans which limits the amount of data collection possible in psychological studies.
Another factor is the expense of PET scanners and the radioactive tracers.
FUNCTIONAL MAGNETIC RESONANCE IMAGING (fMRI)
An alternative to PET that doesn’t use radioactive substances is Magnetic
Resonance Imaging (MRI). This employed powerful electromagnets to affect hydrogen
atoms. Hydrogen atoms are abundant in humans as water and fat. The atomic nuclei
of hydrogen atoms are able to absorb radio frequency energy when placed in a
magnetic field. The resulting spin polarization can produce a radio frequency signal
that can be detected and analyzed. Varying the parameters of the radio pulse
sequence can produce different contrasts between brain tissues based on the
properties of their constituent hydrogen atoms. Computational processing of the
signals can produce a highly detailed 3D image of the brain. However, this is a static
image of the tissues without any indication of brain activity. Recently, fMRI (functional
magnetic resonance imaging) has come to the forefront as a way to overlap MRI scans
with images of brain activity. This measures the energy released by hemoglobin in the
blood. It is assumed that the areas of the brain that are most active would be the most
likely to take in more blood (for energy). Therefore, the measurement of blood flow
with different brain regions can indirectly show us a measure of their activation during
particular cognitive tasks. This type of scan provides a better temporal and spatial
resolution than PET. However, as there is a 1-5 second lag between brain activation
and detection, the temporal resolution of fMRI is inferior to EEG.
COMPARING BRAIN IMAGING TECHNOLOGY
Neuroimaging is at the forefront of psycholinguistic research into language
processing in the brain. They can tell us about the time course of various cognitive
processes and the extent to which mental processes interact with each other.
However, these techniques are still quite expensive and vary in terms of their temporal
and spatial resolutions. As can be seen in Figure 1.10, different techniques vary in
terms of how accurately they measure timing and active brain regions. EEG can detect
brain activity with high temporal resolution but cannot tell us exactly where they
originated. As signals are all detected on the surface of the head, we cannot be sure
whether they originated in the cortex or areas deeper inside the brain. On the other
hand, PET and fMRI are quite good at providing spatial information. However, as they
rely on the flow of fluids (blood), there is a temporal lag between when a brain region
become active and when the signal is detected by the scanner. Methodological
limitations also exist as most of these techniques require the participant to be still
during the scan. This limits the ability to study overt speech or other movement. In
addition, the use of powerful magnets in fMRI means that participants with any metal
implants cannot take part in such studies (the metal would fly out of their body towards
the scanner).
A more serious limitation of any neuroimaging technique is the difficulty in
interpreting the results. How do we know what is causing a particular activity? We can
see when or where something is happening, but not necessarily how. Observing neural
activity is not the same as observing mental activity. Some studies often average out
the results from multiple participants. How can we be sure that all of them are using
the same brain regions for similar activities? However, even with such limitations,
these methods have opened us to a wide range of insights into the neurological basis
of language. As new methods are developed, we may even see these methods
employed regularly for research and rehabilitation.
1. Reaction Time Experiment
A diagram showing the process of testing someone’s reaction time to seeing a
number on a computer screen and pressing the number on their keyboard:
1. Stimuli: The number 3 appears on the computer screen. The timer starts.
2. Sensory: The eyes see the number 3.
3. Cortical: The stimuli is processed by the brain.
4. Motor: The brain tells the hand to press the number 3 on the keyboard.
2. Positron Emission Tomography Schema
Function of a PET machine. A scanner detects the emissions of the short-lived
radioactive isotope in the brain of the subject, transmits this information to a
Coincidence Processing Unit, which is subsequently used to reconstruct an
image of the subject’s brain activity.
3. fMRI activation in Emotional Stroop Task
fMRI scans of six brains. The first three images display the brain’s response to
expressions, while the last three illustrate the brain’s response to words.
Coloured marks from red to yellow are used to qualitatively assess the strength
of the brain’s response, in addition to the location of brain activity.
4. Comparing Brain Imaging Techniques
A labeled, three-dimensional graph comparing the several brain imaging
techniques on the axes of Temporal Resolution, Portability, and Spatial
Resolution.
Whole brain imaging techniques listed by spatial resolution from low to high:
• Surface EEG: low spatial resolution, medium portability, high temporal
resolution
• MEG: low spatial resolution, high portability, high temporal resolution
• PET: low spatial resolution, low portability, low temporal resolution
• fNIRS: low spatial resolution, high portability, medium temporal resolution
• fMRI: low spatial resolution, low portability, medium temporal resolution
• Functional Ultrasound
Local brain imaging techniques listed by spatial resolution from low to high:
• Optical imaging: high spatial resolution, high portability, high temporal
resolution
• Implanted EEG: high spatial resolution, high portability, high temporal
resolution
