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Abstract:
Abstract: The ability to generate verbal responses according
to some rule (i.e., semantic or phonemic categories) has been
linked to prefrontal cortex (PFC) on the basis of both functional
neuroimaging and neuropsychological evidence. Specifically, it has
been suggested that PFC may play a specific role in switching
between concepts during verbal fluency tasks (Troyer et al., 1997,
1998). This hypothesis is related to findings that PFC lesions
impair selection among competing responses on a wide variety of
linguistic tasks (e.g., Robinson et al., 1998, Thompson-Schill et
al., 1998). Of particular relevance to the current study, a patient
with left PFC damage was impaired in generating members of the
category "animals" but was unimpaired in generating members of the
more constrained category "farm animals" (Randolph et al., 1993).
In the present study, we used functional magnetic resonance imaging
(fMRI) to examine the relative involvement of PFC in verbal fluency
tasks with response sets that were more or less constrained. During
semantic fluency, subjects generated members of categories such as
"animals" and "furniture" (less constrained) or categories such as
"farm animals" and "bedroom furniture" (more constrained). During
phonemic fluency, subjects generated words that began with "S" or
"T" (less constrained) or words that began with "STA" or "TRA"
(more constrained). In both cases, PFC activity was greater in less
constrained fluency conditions than during more constrained fluency
conditions. This pattern of results is consistent with a number of
neuropsychological observations and supports the hypothesis that
PFC subserves selection among competing sources of
information.
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