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Abstract:
Neuroimaging studies of human working memory (WM) have
suggested that regions in prefrontal cortex (PFC) can be
functionally dissociated based on the type of executive processing
required in a WM task. Using functional Magnetic Resonance Imaging
(fMRI), we tested this hypothesis in an event-related design that
allowed us to vary both maintenance and manipulation demands during
a single trial. Participants were given a set of one, two, or three
letters to maintain over a five second delay. At the end of the
delay period, a manipulation cue indicated whether participants
were to continue to maintain the original set of letters in memory,
or to increment (manipulate) the letters in the original memory set
(by one or two). In this way we were able to determine whether
regions are modulated by variations in maintenance demand
independent of manipulation demand, and whether regions are
modulated by manipulation demand independent of maintenance demand.
Contrary to previous studies, we did not find any regions in PFC
that responded only to variations in maintenance demand, nor did we
find any regions that responded only to variations in manipulation
demand. Rather, it appears that regions in PFC are modulated by
both maintenance and manipulation demands, and these two processes
are not subserved by different regions in PFC.
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