NEURAL GENERATORS OF EARLY CORTICAL SOMATOSENSORY-EVOKED POTENTIALS IN THE AWAKE MONKEY

Controversy continues to exist regarding the generators of the initial cortical components of the somatosensory evoked potential (SEP). This issue was explored by detailed epidural and intracortical mapping of somatosensory evoked activity in Old World monkeys. In depth recording s, 3 complementary procedures were utilized: (1) the intracortical and subcortical distribution of SEPs was determined from approximately 40 00 locations; (2) concomitant profiles of multiple unit activity (MUA) were recorded as an estimate of local action potential profiles; (3) 1-dimensional calculations of current source density (CSD) were used t o outline the timing and pattern of regional transmembrane current flo w. Our analysis confirms the participation of multiple cortical areas, located on either side of the central sulcus, in the generation of th e initial cortical SEP components. Earliest activity, P10, was localiz ed to area 3, followed within milliseconds by activation of areas 1, 2 (P12), and 4 (P13). In SI (Brodmann's areas 3, 1 and 2), the initial SEP components reflect the depolarization of lamina 4 stellate cells a nd the subsequent activation of adjacent pyramidal cells in laminae 3 and 5. The genesis of later cortical components (P20, N45) represents the composite of activity distributed across multiple cortical laminae and the interaction of overlapping excitatory and inhibitory events. These findings have direct implications for the clinical interpretatio n of SEP waveforms.