SEP TOPOGRAPHIES ELICITED BY INNOCUOUS AND NOXIOUS SURAL NERVE-STIMULATION .1. IDENTIFICATION OF STABLE PERIODS AND INDIVIDUAL-DIFFERENCES

Scalp potential topographies evoked by innocuous and noxious sural ner ve stimulation were obtained from 15 human subjects. The SEP scalp top ography could be separated into 6 different stable periods (SP), that is, consecutive time points where there were no major changes in the t opographic pattern. SP1 (occurring 58-90 msec post stimulus) was chara cterized by a contralateral frontal positivity and a central negativit y oriented ipsilateral to the evoking stimulus; SP2 (92-120 msec) by a bilateral frontal positivity and a symmetrical central negativity; SP 3 (135-158 msec) by a widespread negativity with a minimum at the cont ralateral temporo-frontal region; and SP4 (178-222 msec), SP5 (223-277 msec) and SP6 (282-339 msec) by a widespread positivity with a maximu m located along the centro-parietal midline. SP4, SP5, and SP6 could b e distinguished by changes in the orientation of the isovoltage contou r lines and/or by changes in the location of the maximum. The stable p eriods had similar onset and offset latencies and the same major featu res across subjects. However, the topographic patterns were not identi cal across subjects. These individual differences are likely due to th e expected variability in the orientation of the equivalent regional d ipole sources generating these potentials.