Functional dissociations within the inferior parietal cortex in verbal working memory

Abstract

Neuroimaging studies of working memory have revealed two sites in the left supramarginal gyrus that may support the short-term storage of phonological information. Activation in the left dorsal aspect of the inferior parietal cortex (DIPC) has been observed in contrasts of working memory load, whereas activation in the ventral aspect of the inferior parietal cortex (VIPC) has been found primarily in contrast of information type (verbal vs. nonverbal). Our goal was to determine whether these two areas are functionally distinct or if instead they are part of a homogeneous region with large variations in the focus of peak activity. Toward this end, we used fMRI to assess the neural response in two working memory tasks (N-back and item recognition) in which we also manipulated memory load and the type of information to be recalled (verbal vs. nonverbal). We found both DIPC and VIPC activation in the same group of subjects and further demonstrated that they have differential sensitivity to our experimental factors. Only the DIPC showed robust load effects, whereas only the VIPC showed reliable effects of information type. These results help to account for the differences observed in between-subject comparisons, and they indicate that the two regions are functionally dissociable. In contrast to the DIPC, activity of the VIPC was also recruited in the fixation and low-load conditions, a surprising result that has not been fully explored in prior studies. Despite their distinctive patterns of performance, neither of these regions displayed a pattern of activity that entirely corresponds to common assumptions of a dedicated phonological short-term store (STS). Instead, we hypothesize that the DIPC may support domain-general executive processes, while the VIPC may support phonological encoding-recoding processes central to a variety of language tasks.