Prefrontal contributions to executive control: fMRI evidence for functional distinctions within lateral prefrontal cortex

The prefrontal. cortex (PFC) plays a fundamental role in internally guided behavior. Although it is generally accepted that PFC subserves working memo ry and executive control operations, it remains unclear whether the subregi ons within lateral PFC support distinct executive control processes. An eve nt-related fMRI study was implemented to test the hypothesis that ventrolat eral and dorsolateral PFC are functionally distinct, as well as to assess w hether functional specialization exists within ventrolateral PFC. Participa nts performed two executive control tasks that differed in the types of con trol processes required. During rote rehearsal, participants covertly rehea rsed three words in the order presented, thus requiring phonological access and maintenance. During elaborative rehearsal, participants made semantic comparisons between three words held in working memory, reordering them fro m least to most desirable. Thus, in addition to maintenance, elaborative re hearsal required goal-relevant coding of items in working memory ("monitori ng") and selection from among the items to implement their reordering. Resu lts revealed that left posterior ventrolateral PFC was active during perfor mance of both tasks, whereas right dorsolateral PFC was differentially enga ged during elaborative rehearsal. The temporal characteristics of the hemod ynamic responses further suggested that dorsolateral activation lagged vent rolateral activation. Finally, differential activation patterns were observ ed within left ventrolateral PFC, distinguishing between posterior and ante rior regions. These data suggest that anatomically separable subregions wit hin lateral PFC may be functionally distinct and are consistent with models that posit a hierarchical relationship between dorsolateral and ventrolate ral regions such that the former monitors and selects goal-relevant represe ntations being maintained by the latter. (C) 2001 Academic Press.