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Abstract:
Left temporal and prefrontal cortex are two cortical regions
that have been consistently implicated in both lesion studies and
neuroimaging studies of semantic memory. We hypothesized that
prefrontal and temporal cortex play distinct roles in semantic
memory: temporal cortex subserves the retrieval of semantic
knowledge whereas prefrontal cortex enables selection among
competing information. To test this hypothesis, we developed a
paradigm in which items in a word generation task were primed with
either relevant or irrelevant (competing) information. In a series
of behavioral experiments, we demonstrated that both relevant and
irrelevant primes result in a facilitation effect that results from
the repetition of semantic processing. Thus relevant and irrelevant
primes decrease semantic retrieval demands, but only relevant
primes decrease demands for selection among competing alternatives;
irrelevant primes should increase competition and thereby increase
selection demands. In an fMRI experiment we identified regions of
left temporal cortex that showed decreased activity for both
relevant and irrelevant primes relative to unprimed items. However,
in left prefrontal cortex, in the inferior frontal gyrus, we found
that relative to the unprimed items there was a decrease in
activity for the relevant prime condition but an increase in
activity for the irrelevant prime condition. These results
confirmed our hypothesis that activity in temporal cortex depends
on retrieval demands whereas activity in prefrontal cortex depends
on selection demands.
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