Redefining the functional organization of working memory processes within human lateral prefrontal cortex

It is widely held that the frontal cortex plays a critical part in certain aspects of spatial and non-spatial working memory. One unresolved issue is whether there are functionally distinct subdivisions of the lateral frontal cortex that subserve different aspects of working memory. The present stud y used positron emission tomography (PET) to demonstrate that working memor y processes within the human mid-dorsolateral and mid-ventrolateral frontal regions are organized according to the type of processing required rather than according to the nature (i.e. spatial or non-spatial), of the informat ion being processed, as has been widely assumed. Two spatial working memory tasks were used which varied in the extent to which they required differen t executive processes. During a 'spatial span' task that required the subje ct to hold a sequence of five previously remembered locations in working me mory a significant change in blood-flow was observed in the right mid-ventr olateral frontal cortex, but not in the anatomically and cytoarchitectonica lly distinct mid-dorsolateral frontal-robe region. By contrast, during a '2 -back' task that required the subject to continually update and manipulate an ongoing sequence of locations within working memory, significant blood f low increases were observed in both mid-ventrolateral and mid-dorsolateral frontal regions. When the two working memory tasks were compared directly, the one that emphasized manipulation of information within working memory y ielded significantly greater activity in the right mid-dorsolateral frontal cortex only. This dissociation provides unambiguous evidence that the mid- dorsolateral and mid-ventrolateral frontal cortical areas make distinct fun ctional contributions to spatial working memory and corresponds with a frac tionation of working memory processes in psychological terms.