SEP topographies elicited by innocuous and noxious sural nerve stimulation. I. Identification of stable periods and individual differences.

Scalp potential topographies evoked by innocuous and noxious sural nerve stimulation were obtained from 15 human subjects. The SEP scalp topography could be separated into 6 different stable periods (SP), that is, consecutive time points where there were no major changes in the topographic pattern. SP1 (occurring 58-90 msec post stimulus) was characterized by a contralateral frontal positivity and a central negativity oriented ipsilateral to the evoking stimulus; SP2 (92-120 msec) by a bilateral frontal positivity and a symmetrical central negativity; SP3 (135-158 msec) by a widespread negativity with a minimum at the contralateral temporo-frontal region; and SP4 (178-222 msec), SP5 (223-277 msec) and SP6 (282-339 msec) by a widespread positivity with a maximum located along the centro-parietal midline. SP4, SP5, and SP6 could be distinguished by changes in the orientation of the isovoltage contour lines and/or by changes in the location of the maximum. The stable periods had similar onset and offset latencies and the same major features across subjects. However, the topographic patterns were not identical across subjects. These individual differences are likely due to the expected variability in the orientation of the equivalent regional dipole sources generating these potentials.