Psychology is appealing to me partly because it requires so much stealthiness. We can't directly observe mental events, but must instead use indirect methods to test our hypotheses. We must cleverly misdirect the suspicions of our subjects, to ensure they don't guess the hypotheses being tested and thus affect our results. And, as if by magic, psychologically-informed visual illusions can reveal the mind's inner mechanisms right before our eyes.
For those who particularly admire psychology's furtive manner, neuroscience's focus on direct measurement of our mental hardware might seem crude and ordinary, at least in comparison to the artful techniques of experimental psychology. What can neuroscience add, beyond telling us that cognition happens in the brain? One answer to that question comes from a study by Vogel and Machizawa on a topic of perennial debate: the question of how much information people can hold in their short-term ‘working’ memory.
In just three pages, the authors describe how they were able to use electroencephalography (EEG) to identify waves of electrical activity on the surface of the brain that predicted how much visual information someone could hold in their working memory. Subjects first donned an elastic electrode cap, and were then asked to remember just one side (either the left or right) of a display of coloured squares. After a one second delay, subjects were asked to judge whether a second display was the same as or different from the first display.
For analysis, the authors subtracted EEG activity in the cerebral hemisphere on the same side of space (ipsilateral) as the part of the display the participants had tried to remember, from activity in the hemisphere on the opposite side (contralateral). Using this subtraction method, Vogel and Machizawa were able to isolate activity related to effortful memory processes from activity related to passive visual processing. Their results showed a wave of electrical activity over contralateral posterior parietal and lateral occipital electrodes, whose amplitude increased both with the accuracy of the subjects' response, and with the number of items that had been presented.
At first glance, this methodology might seem overly simplistic - what if the amplitude increased merely because of increases in difficulty, and/or changes in executive processing? To those who favour the more nuanced methods of psychology, these shortcomings might reflect a lack of theoretical sophistication in much of cognitive neuroscience.
And yet, a closer look reveals that some alternative explanations could potentially be ruled out: as the number of items to be remembered increases beyond an individual's capacity, one would expect a measure of executive- or difficulty-related activity to continue increasing. However, if this measure truly isolates capacity from difficulty and executive demands, it should level out at an individual's memory capacity limit.
Accordingly, Vogel and Machizawa determined that the observed wave of electrical activity did not increase as display set size increased beyond an individual's capacity, despite the fact that accuracy continued to decrease, suggesting that the EEG wave in question does indeed index the number of items maintained in visual working memory. Going even farther, the authors showed that the magnitude of amplitude increase between set-sizes could be used as an alternate measure of visual working memory capacity, because low-capacity individuals tend to "max out" their capacity sooner, and thus show less increase between supra-capacity set-sizes than higher-capacity individuals.
Some might interpret these results to mean that visual working memory capacity limits can be independent of executive functions, and given the location of the wave, that these limits arise from modality-specific processes. Others might disagree. Perhaps most inspiring, however, is the way these authors integrated the methods of neuroscience with traditional psychology: consider the use of bilateral displays and subtractive logic to equate for visual processing across both hemispheres, or the elimination of alternative explanations with careful quantitative analysis. This study exemplifies how an intelligent fusion of methods from both psychology and neuroscience can help to address questions of central importance – even those that have been hotly debated for nearly a century, such as this one.
Vogel, E.K. & Machizawa, M.G. (2004). Neural activity predicts individual differences in visual working memory capacity. Nature, 15, 428, 748-51.
Christopher H. Chatham is pursuing a PhD in cognitive neuroscience at the University of Colorado.
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