Characterizing the role of prefrontal cortex in working memory and in high-level cognition

Lesion studies have shown that the, prefrontal cortex (PFC) plays a critical role in reasoning and working memory (WM). In our studies, functional magnetic resonance imaging (fMRI) was used in mapping out the role of different PFC regions in WM and in high-level cognitive paradigms. The brain thus served as an interface for linking high-level reasoning constructs in terms of simpler WM constructs.; In our initial study, participants solved visuo-spatial pattern completion problems requiring analytic and figural or visuo-spatial reasoning. Right PFC and bilateral parietal regions involved in spatial and object WM were activated by figural reasoning. Bilateral PFC and left posterior regions involved in verbal and domain-independent associative and executive WM processes were activated additionally by analytic reasoning. These results suggest that reasoning is mediated by a composite of WM systems.; In our second study, participants solved mathematical word problems which required different levels of mathematical reasoning and text processing. Problems which required extensive mathematical reasoning yielded major activations in bilateral PFC regions similar to other problem-solving tasks, indicating that the processes mediated by these regions subserve many forms of reasoning.; In the third and last study, we examined (1) whether PFC areas can be recruited with maintenance but no overt manipulation WM requirement and (2) whether older adults show a corresponding decline in PFC recruitment with decline in WM respectively. Participants performed low and high load verbal WM tasks. Inferior frontal gyrus was activated in both young and old adults in these WM tasks. High WM load additionally recruited middle and superior PFC areas bilaterally in younger adults. Thus, WM maintenance alone without overt manipulation requirements, resulted in the recruitment of wide-spread frontal-lobe regions, whereas aging failed to recruit these regions.; Our fourth study identified regions preferentially involved in maintaining integrated versus un-integrated information in WM. For equal amounts of verbal and spatial information, PFC was activated to a greater degree for maintaining integrated relative to un-integrated representations, while posterior regions showed the opposite pattern. This result provides direct evidence for frontal-lobe specialization in maintaining integrated WM representations essential for high level cognition demanding flexible mental representations.