GENERATORS OF SOMATOSENSORY-EVOKED POTENTIALS INVESTIGATED BY DIPOLE TRACING IN THE MONKEY

Generators of somatosensory evoked potentials, elicited by electrical stimulation of the median nerve in anaesthetized monkeys (Macaca fusca ta), were investigated by submitting a three-dimensional reconstructed brain model to dipole tracing, which can equate surface potential dis tributions to an approximate corresponding equivalent dipole. The foll owing components of the somatosensory evoked potentials were simultane ously recorded from 21-27 epidural electrodes: P7 (the letter indicate s positive or negative polarity; the number indicates the approximate latency of the peak in ms) was recorded widely from various locations on both the left and right hemispheres, P10 was recorded near the ante rior side of the central sulcus contralateral to the stimulation side, N10 was recorded near the posterior side of the contralateral central sulcus, P12 was recorded on both sides of the contralateral central s ulcus, and P18 was recorded posterior to the contralateral central sul cus. Current source generators (dipoles) of each component of somatose nsory evoked potentials were localized by dipole tracing: a dipole for P7 was located in the thalamus contralateral to the stimulation side; a dipole for P10 and N10 in the posterior wall of the contralateral c entral sulcus (area 3b); a dipole for P12 in the contralateral post ce ntral gyrus (areas 1 and 2); and a dipole for P18 in the anterior wall of the contralateral intraparietal sulcus (area 5). The locations and latencies of dipoles that generated cortical components of somatosens ory evoked potentials, estimated by dipole tracing, were confirmed by direct cortical surface recording from a 16-25 electrode array placed directly on the cortical surface; and multiple unit recording from the anterior and posterior parietal cortices. After excision of area 5, P 18 and N18 were abolished, whereas P10, N10, and P12 were not affected . The results suggest that dipoles for somatosensory evoked potentials progressed from the thalamus to area 5 via the primary somatosensory area. This progress is consistent with the hierarchical sequence of so matosensory information processing.