THE ROLE OF PARIETAL CORTEX IN VERBAL WORKING-MEMORY

Neuroimaging studies of normal subjects and studies of patients with f ocal lesions implicate regions of parietal cortex in verbal working me mory (VWM), yet the precise role of parietal cortex in VWM remains unc lear. Some evidence (Paulesu et al., 1993; Awh et al., 1996) suggests that the parietal cortex mediates the storage of verbal information, b ut these studies and most previous ones included encoding and retrieva l processes as well as storage and rehearsal of verbal information. A recent positron emission tomography (PET) study by Fiez et al. (1996) isolated storage and rehearsal from other VWM processes and did not fi nd reliable activation in parietal cortex. This result suggests that p arietal cortex may not be involved in VWM storage, contrary to previou s proposals. However, we report two behavioral studies indicating that some of the verbal material used by Fiez et al. (1996) may not have r equired phonological representations in VWM. In addition, we report a PET study that isolated VWM encoding, retrieval, and storage and rehea rsal processes in different PET scans and used material likely to requ ire phonological codes in VWM. After subtraction of appropriate contro ls, the encoding condition revealed no reliable activations; the retri eval condition revealed reliable activations in dorsolateral prefronta l, anterior cingulate, posterior parietal, and extrastriate cortices, and the storage condition revealed reliable activations in dorsolatera l prefrontal, inferior frontal, premotor, and posterior parietal corti ces, as well as cerebellum. These results suggest that parietal region s are part of a network of brain areas that mediate the short-term sto rage and retrieval of phonologically coded verbal material.