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Abstract:
We used event-related fMRI to measure brain activity in
younger adults while they performed the Digit Symbol Substitution
task from the Wechsler Adult Intelligence Scale, a test known to be
sensitive to individual differences in processing speed. On each
trial subjects saw an "answer key," a set of digit-symbol pairings,
across the top of the screen. In the answer key, the digits 1-9
appeared with a set of nonsense figures below them. A single
digit-symbol pairing appeared below the answer key. The subject's
task was to determine whether the pairing that appeared below the
answer key matched (right-thumb button press) or did not match
(left-thumb button press) one of the entries in the answer key.
Regression analyses of individual subjects' reaction time and
cortical activity indicated that reaction time (RT) accounted for
considerable variance in fMRI activity in a number of cortical
regions including dorsolateral prefrontal cortex and parietal
cortex. These analyses indicated a positive correlation between RT
and fMRI activity in dorsolateral PFC (r=.93, p<.002, r2 = .88)
but a negative correlation in parietal cortex (r=-.79, p<.03,
r2=.63). These results suggest that individual differences in
performance speed are related to neural efficiency in anterior, but
not posterior cortical systems.
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