HIGH-RESOLUTION EVOKED-POTENTIAL IMAGING OF THE CORTICAL DYNAMICS OF HUMAN WORKING-MEMORY

High resolution evoked potentials (EPs), sampled from 115 channels and spatially sharpened with the finite element deblurring method, were r ecorded from 8 subjects during working memory (WM) and control tasks. The tasks required matching each stimulus with a preceding stimulus on either verbal or spatial attributes. All stimuli elicited a central P 200 potential that was larger in the spatial tasks than in the verbal tasks, and larger in the WM tasks than in the control tasks. Frequent, non-matching stimuli elicited a frontal, positive peak at 305 msec th at was larger in the spatial WM task relative to the other tasks. Irre spective of whether subjects attended to verbal or spatial stimulus at tributes, non-matching stimuli in the WM tasks also elicited an enhanc ed P450 potential over the left frontal cortex, followed by a sustaine d potential over the superior parietal cortex. A posterior P390 potent ial elicited by infrequent, matching stimuli was smaller in amplitude for both spatial and verbal WM tasks compared to control tasks, as was a central prestimulus CNV. These results indicate that WM is a functi on of a distributed system with both task-specific and task-independen t components. Lesion studies and coarse temporal resolution functional imaging methods, such as PET and fMRI, tend to paint a fairly static picture of the cortical regions which participate in the performance o f WM tasks. In contrast, the fine grain time resolution provided by im aging brain function with EP methods provides a dynamic picture of sub second changes in the spatial distribution of WM effects over the cour se of individual trials, as well as evidence for differences in the ac tivity elicited by matching and non-matching stimuli within sequences of trials. This information about the temporal dynamics of WM provides a critical complement to the fine-grain spatial resolution provided b y other imaging modalities.