FUNCTIONAL MRI STUDIES OF SPATIAL AND NONSPATIAL WORKING-MEMORY

Single-unit recordings in monkeys have revealed neurons in the lateral prefrontal cortex that increase their firing during a delay between t he presentation of information and its later use in behavior. Based on monkey lesion and neurophysiology studies, it has been proposed that a dorsal region of lateral prefrontal cortex is necessary for temporar y storage of spatial information whereas a more ventral region is nece ssary for the maintenance of nonspatial information. Functional neuroi maging studies, however, have not clearly demonstrated such a division in humans. We present here an analysis of all reported human function al neuroimaging studies plotted onto a standardized brain. This analys is did not find evidence for a dorsal/ventral subdivision of prefronta l cortex depending on the type of material held in working memory, but a hemispheric organization was suggested (i.e., left-nonspatial; righ t-spatial). We also performed functional MRT studies in 16 normal subj ects during two tasks designed to probe either nonspatial or spatial w orking memory, respectively. A group and subgroup analysis revealed si milarly located activation in right middle frontal gyrus (Brodmann's a rea 46) in both spatial and nonspatial [working memory-control] subtra ctions. Based on another model of prefrontal organization [M. Petrides , Frontal lobes and behavior, Cur. Opin. Neurobiol., 4 (1994) 207-211] , a reconsideration of the previous imaging Literature data suggested that a dorsal/ventral subdivision of prefrontal cortex may depend upon the type of processing performed upon the information held in working memory. (C) 1998 Elsevier Science B.V. All rights reserved.