COACTIVATION OF PREFRONTAL CORTEX AND INFERIOR PARIETAL CORTEX IN WORKING-MEMORY TASKS REVEALED BY 2DG FUNCTIONAL MAPPING IN THE RHESUS-MONKEY

Functional studies of the dorsolateral prefrontal cortex and the infer ior parietal lobe of the rhesus monkey have implicated the former in s patial mnemonic function and the latter in visuospatial processing. We used the C-14-2-deoxyglucose (2DG) method to assess the contribution of these cortical regions to the cognitive performance of monkeys on w orking memory tasks. In these experiments, one group of monkeys (WORK) was trained to perform tasks (delayed spatial alternation, spatial de layed response, or delayed object alternation) that specifically engag ed working memory processing. Local cerebral glucose utilization (LCGU ) rates in the WORK group was compared with LCGU rates for a second gr oup of monkeys (CONT) tested on one of two tasks (visual pattern discr imination or sensory-motor) that relied upon associative memory. The r esults showed that in comparison to the CONT group, working memory per formance significantly enhanced LCGU by 19% in the principal sulcus re gion of prefrontal cortex and by 11-20% in regions of the inferior par ietal cortex corresponding to areas 7A, 7B, 71P, and 7M. By contrast, LCGU in the auditory cortex was similar for both groups. In all areas examined, metabolic activation peaked in lower layer III where the maj ority of associational and callosal neurons lie. Correlation analyses of LCGU and behavioral task parameters indicated that LCGU in the pari etal subdivisions was significantly related either to the accuracy of performance or to the number of trials completed on the 2DG test. In c ontrast, LCGU in the principal sulcus was positively correlated with t ask difficulty. These findings suggest that the enhancement of LCGU in the principal sulcus was primarily influenced by the mnemonic compone nts of the tasks whereas LCGU in the inferior parietal cortex was infl uenced by their sensory-motor demands. These are the first results sho wing concurrent metabolic activation of the prefrontal and parietal co rtex in monkeys performing working memory tasks and they support the s uggestion that these cortical regions represent two important nodes in a neural network mediating spatial working memory in the monkey (Gold man-Rakic, 1988). Further, the present report reinforces the power of the 2DG method for functional mapping as these areal and laminar resul ts could not be readily appreciated at this resolution in any other me thodological context.