A light changes from green to yellow, and you jam your foot on the brake. It's almost an automatic impulse, a no-brainer, you might say, guided by the knowledge, planted somewhere, that moving-vehicle violations are to be avoided. But where, exactly, is that knowledge planted? How does perception transform in an instant to action? Where's the owner's manual with the wiring diagram of the mind that shows all the connections?
The functional MRI experiments conducted by Cohen investigate a concept known in cognitive psychology as working memory. Each subject's brain is scanned while they perform a working memory task and a control task. In the control task, the subject sees a random sequence of letters one at a time on a visual display. They are instructed to press a button whenever the letter "X" shows on the display. In the memory task, subjects see a similar sequence of letters, but they are instructed to press the button only when a letter repeats after exactly one intervening letter. For example, A-F-A should prompt a response, but not A-A or A-Q-G-A.
Both tasks, explains Cohen, require subjects to visually monitor sequences of letters presented one at a time, to evaluate their identity and respond by pressing a button. The memory task, however, requires in addition that the subject keep in mind both the identity and order of the two previous letters and continuously update this mental record as the sequence progresses.
The MRI machine records data from six slice locations in the prefrontal cortex of each subject (above left). A set of activation images for one subject (panels 1-6 above) shows the brain areas significantly activated during the memory task and not during the control task. Results to date from these studies, says Cohen, "support the idea that the prefrontal cortex becomes engaged when recently presented information must be represented and actively maintained to perform a task."go back to the main screen