Dissociation of frontal and cerebellar activity in a cognitive task: evidence for a distinction between selection and search.

Human brain imaging studies have found that increases in functional activation in left-frontal cortex during cognitive tasks are often accompanied by similar increases in right-cerebellar regions. The present study used functional magnetic resonance imaging (fMRI) to investigate the distinctive contributions of these regions using a word stem completion task. Stems with many possible completions (MANY condition) were alternately presented with stems that had few possible completions (FEW condition), and subjects were asked to covertly complete each stem with a word and press a response switch for each successful completion. Prominent increases in activation in the MANY, relative to the FEW, condition were observed in the left middle frontal gyrus (Brodmann areas 9/10) and left caudate nucleus. In contrast, portions of the right-cerebellar hemisphere (posterior quadrangular lobule and superior semilunar lobule) and cerebellar vermis exhibited increases in the FEW, relative to the MANY, condition. This double dissociation suggests that the frontal and cerebellar regions make distinctive contributions to cognitive performance, with left-frontal (and striatal) activations reflecting response selection, which increases in difficulty when there are many appropriate responses, and right-cerebellar activation reflecting the search for responses, which increases in difficulty when even a single appropriate response is hard to retrieve.