Task-specific repetition priming in left inferior prefrontal cortex

Previous neuroimaging studies have shown that activation in left inferior p refrontal cortices (LIPC) is reduced during repeated (primed) relative to i nitial (unprimed) stimulus processing. These reductions in anterior (simila r to PA 45/47) and posterior (similar to BA 44/6) LIPC activation have been interpreted as reflecting implicit memory for initial semantic or phonolog ical processing. However, prior studies do not unambiguously indicate that LIPC priming effects are specific to the recapitulation of higher-level (se mantic and/or phonological), rather than lower-level (perceptual), processe s. Moreover, no prior study has shown that the patterns of priming in anter ior and posterior LIPC regions are dissociable. To address these issues, th e present fMRI study examined the nature of priming in LIPC by examining th e task-specificity of these effects. Participants initially processed words in either a semantic or a nonsemantic manner. Subsequently, participants w ere scanned while they made semantic decisions about words that had been pr eviously processed in a semantic manner (within-task repetition), words tha t had been previously processed in a nonsemantic manner (across-task repeti tion), and words that had not been previously processed (novel words). Beha viorally, task-specific priming was observed: reaction times to make the se mantic decision declined following prior semantic processing but not follow ing prior nonsemantic processing of a word, Priming in anterior LIPC parall eled these results with signal reductions being observed following within-t ask, but not following across-task, repetition. Importantly, neural priming in posterior LIPC demonstrated a different pattern: priming was observed f ollowing both within-task and across-task repetition, with the magnitude of priming tending to be greater in the within-task condition. Direct compari son between anterior and posterior LIPC regions revealed a significant inte raction. These findings indicate that anterior and posterior LIPC demonstra te distinct patterns of priming, with priming in the anterior region being task-specific, suggesting that this facilitation derives from repeated sema ntic processing of a stimulus.