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Abstract:
A central issue in cognitive neuroscience is the functional
role of prefrontal cortex (PFC) in human cognition. We conducted
speeded-choice experiments using functional MRI at 1.5T to
investigate whether PFC activation was elicited by changes in the
subject's response strategy. We induced response strategy changes
in two experiments. In the first experiment, stimuli were circles
or squares appearing on the left or right of the screen. Subjects
pressed one button or another depending on the shape of the
stimuli; position was irrelevant to the judgment. On most trials
(90%), the stimuli appeared on the same side as the hand of
response; on the remaining trials (10%), they appeared on the
opposite side. Significant PFC activation, in the middle and
inferior frontal gyri, was associated with the infrequent trials
that required a shift in response strategy. This result extends
previous "oddball" studies by showing that PFC activation results
from changes in response strategy, even if the same response is
made as on previous trials. In the second experiment, we
investigated response strategy changes by examining whether PFC
activation was sensitive to stimulus sequence. The pattern of
prefrontal activation depended upon the particular sequence of
preceding stimuli. These results provide additional evidence
associating PFC function with changes in subjects' behavioral
strategies.
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