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Abstract:
Abstract: Delayed-response tasks have been used to study
prefrontal cortex (PFC) in both human and nonhuman primates. Here
we used fMRI to examine the role of PFC in delayed-response tasks
with and without memory demands. Eight subjects were studied in 2
conditions. In condition 1, subjects viewed a scrambled pattern
(S1) presented in the center of the screen for 3-sec. A 15-sec
delay period ensued and then a second pattern (S2) was presented to
the right or left of a fixation cross for 3-sec. The subject
indicated the position of S2 with a corresponding left or right
button press. S1 evoked transient activation in the middle frontal
gyrus (MFG) that peaked at 6-sec and returned to baseline by
12-15-sec. A second and much larger response was evoked by S2.
Condition 2 reversed the order of information provided by the
stimuli in that S1 was presented on the right or left and S2 was
presented centrally. Thus, S1 informed the subject of the
appropriate hand for the response, but this response was not
executed until S2. S1 now evoked a much greater MFG response than
in condition 1. Thus, activity in the MFG was strongly influenced
by the information provided by the stimulus - with relatively
little activation when S1 provided no temporal information
(Condition 1) and much greater activation when S1 provided explicit
response information (Condition 2).
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