Functional MRI investigation of verbal selection mechanisms in lateral prefrontal cortex.

Response selection activates appropriate response representations to task-relevant environmental stimuli. Research implicates dorsolateral prefrontal cortex (dlPFC) for this process. On the other hand, studies of semantic selection, which activates verbal responses based on the semantic requirements of a task, implicate ventrolateral PFC (vlPFC). Despite this apparent dissociation, the neurocognitive distinction between response and semantic selection is controversial. The current functional MRI study attempts to resolve this controversy by investigating verbal response and semantic selection in the same participants. Participants responded vocally with a word to a visually presented noun, either from a memorized list of paired associates (response selection task), or by generating a semantically related verb (semantic selection task). We found a dissociation in left lateral PFC. Activation increased significantly in dlPFC with response selection difficulty, but not semantic selection difficulty. Conversely, semantic, but not response, selection difficulty increased activity significantly in vlPFC. Activity in left parietal cortex, on the other hand, was affected by difficulty increases in both selection tasks. These results suggest that response and semantic selection may be distinct cognitive processes mediated by different regions of lateral PFC; but both of these selection processes rely on cognitive mechanisms mediated by parietal cortex.