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.