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Abstract
Verbal fluency tasks have been widely used to evaluate language and executive control
processes in the human brain. FMRI studies of verbal fluency, however, have used either
silent word generation (which provides no behavioral measure) or cued generation of
single words in order to contend with speech-related motion artifacts. In this study,
we use a recently developed paradigm design to investigate the neural correlates of
verbal fluency during overt, free recall, word generation so that performance and
brain activity could be evaluated under conditions that more closely mirror standard
behavioral test demands. We investigated verbal fluency to both letter and category
cues in order to evaluate differential involvement of specific frontal and temporal
lobe sites as a function of retrieval cue type, as suggested by previous neuropsychological
and neuroimaging investigations. In addition, we incorporated both a task switching
manipulation and an automatic speech condition in order to modulate the demand placed
on executive functions. We found greater activation in the left hemisphere during
category and letter fluency tasks, and greater right hemisphere activation during
automatic speech. We also found that letter and category fluency tasks were associated
with differential involvement of specific regions of the frontal and temporal lobes.
These findings provide converging evidence that letter and category fluency performance
is dependent on partially distinct neural circuitry. They also provide strong evidence
that verbal fluency can be successfully evaluated in the MR environment using overt,
self-paced, responses.