Topographical displays of somatosensory evoked potentials

The distribution of somatosensory evoked potentials (SEPs) after stimulation of the median nerve at the wrist was examined in 10 normal subjects using isopotential maps. The latencies of continuous negative and positive peaks were measured in each lead. The differences of the potentials at these latencies were measured in all the leads and the isopotential maps were constructed. The distribution of P0-NI was all similar. The latencies of P0 were almost the same in all the leads at about 13 msec. The distribution of NI-PI-NII was divided into three types--N16-P20-N28 localized in the frontal region, N17-P22-N30 localized in the central region and N19-P25-N33 distributed in the parieto-occipito-temporal regions. The distributions of NII-PII and PII-NIII were all similar, with high amplitudes in the central region. The latencies of PII and NIII were almost the same in all the leads at about 45 msec and 68 msec.