A comparison of rCBF patterns during letter and semantic fluency

To evaluate the functional neuroanatomies underlying letter and category fl uency, 18 normal controls were studied with oxygen-15 water regional cerebr al blood how positron emission tomography. Three counterbalanced conditions each consisted of 6 trials (45 s each): letter fluency (generating words w hen cued with a particular letter), semantic fluency (generating words when cued with a particular category), and a control condition (generating days of the week and months of the year). Relative to the control, participants activated similar brain regions during both fluency tasks, including the a nterior cingulate, left prefrontal regions, thalamus, and cerebellum; reduc tions were found in parietal and temporal regions. In a direct comparison o f the 2 fluency tasks, inferior frontal cortex and temporoparietal cortex ( hypothesized to participate in a phonologic loop for accessing word pronunc iation) were activated more during letter than semantic fluency, whereas le ft temporal cortex (associated with access to semantic storage) was activat ed more during semantic than letter fluency. This study identifies subtle d ifferences in the neural networks underlying letter and semantic fluency th at may underlie the dissociation of these abilities in patients.