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Abstract:
Left inferior prefrontal cortex (LIPC) is consistently active
during performance of semantic processing tasks. LIPC has been
hypothesized (a) to mediate the selection of task-relevant
knowledge from among competing task-irrelevant knowledge or (b) to,
more broadly, guide the controlled retrieval of long-term semantic
knowledge. To assess the controlled semantic retrieval hypothesis,
fMRI indexed the sensitivity of LIPC to controlled retrieval
demands even when selection demands were held constant. During
scanning (BOLD EPI, 1.5T, 21 axial slices, TR=2 sec), subjects
decided which of either two or four response words was most
semantically related to a cue word. Controlled retrieval demands
were varied by manipulating the degree of pre-experimental
association between the cue and the target response, being either
strong or weak. Behaviorally, response times were greater and
accuracy was lower for weakly compared to strongly associated
targets, and when retrieval demands were high (four) compared to
low (two). Functionally, activation in the anterior and posterior
extent of LIPC was greater for weakly compared to strongly
associated targets, and when there were four compared to two
response alternatives. This pattern held even when RTs were
matched. Thus, when selection demands are held constant, LIPC
activity is modulated by the extent to which semantic retrieval
requires more controlled processing.
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