Maintaining structured information: an investigation into functions of parietal and lateral prefrontal cortices

Working memory--including simple maintenance of information as well as manipulation of maintained information--has been long associated with lateral prefrontal cortex (PFC). More recently, evidence has pointed to an important role for posterior parietal cortex (PPC) in supporting working-memory processes as well. While explanations have emerged as to the nature of parietal involvement in working-memory maintenance, the apparent involvement of this region in working-memory manipulation has not been fully accounted for. We have hypothesized that parietal cortex, through its representation of spatial information, in conjunction with dorsolateral PFC, supports organization of information (manipulation) and the maintenance of information in an organized state. Through computational modeling, we have demonstrated how this might be achieved. Presently, we consider a pair of fMRI experiments that were designed to test our hypothesis. Both experiments involved simple working-memory delay tasks with contrasts between maintenance of information in organized and unorganized states, as well as contrasts between high and low working-memory load conditions. Two different kinds of organization, associative (grouping) and relational, were employed in the two studies. Across both studies, superior parietal cortex (BA 7) demonstrated a significant increase in activity associated with maintenance of information in an organized state, over and above any increases associated with increased working-memory load. During the delay period, dorsolateral PFC (BA 9) exhibited similar increases for both organization and load; however, this region was particularly engaged by organization demand during the initial cue period. Functional connectivity analysis indicates interaction between dorsolateral prefrontal cortex (DLPFC) and superior parietal cortex, especially when organization is required.