SERIAL PROCESSING OF THE SOMATESTHETIC INFORMATION REVEALED BY DIFFERENT EFFECTS OF STIMULUS RATE ON THE SOMATOSENSORY-EVOKED POTENTIALS AND MAGNETIC-FIELDS

In order to evaluate information processing in the somatosensory corte x, the effect of two different stimulus rates was investigated by simu ltaneously recording somatosensory-evoked potentials (SEPs) and magnet ic fields (SEFs) in nine healthy adults. During electric stimulation o f the median nerve at the wrist, SEFs were recorded with the helmet-sh aped whole-head coverage magnetometer array with 122 first-order plana r gradiometers while SEPs were simultaneously recorded from seven scal p positions. Interstimulus intervals (ISIs) of 0.9 s and 4 s were comp ared. In all subjects, N20 as well as its magnetic counterpart, N20m, was clearly demonstrated over the contralateral somatosensory area. Su bsequent deflections around 80-200 ms did not make any clear peak and were smaller than those at 20-60 ms (P30m, P40m, N50m and P60m). After 200 ms, SEFs were negligible, whereas SEPs had larger amplitude than those of shorter latencies, constituting a peak around 250 ms (P250). Both SEF and SEP deflections later than 40 ms were decreased in respon ses at the shorter ISI; this diminution was most prominent for P250. T herefore, it is concluded that the tangential currents in the somatose nsory cortex (area 3b) mainly contribute to responses during the first 200 ms after the stimulus, whereas the radially oriented currents (mo st likely in the crown of the postcentral gyrus) take over for subsequ ent information processing. (C) 1998 Elsevier Science B.V.