SEP TOPOGRAPHIES ELICITED BY INNOCUOUS AND NOXIOUS SURAL NERVE-STIMULATION .2. EFFECTS OF STIMULUS-INTENSITY ON TOPOGRAPHIC PATTERN AND AMPLITUDE

The effects of innocuous and noxious sural nerve stimulation on the SE P scalp topography were examined in 15 human subjects. This analysis f ocused on the 6 stable periods (i.e., consecutive time points where th e topography did not change) that were identified in the companion pap er (Dowman 1994). Stable period 1 (SP1: 58-90 msec post stimulus), SP4 (178-222 msec) and SP5 (223-277 msec) showed amplitude-stimulus inten sity relationships that are similar to those of neurons involved in th e sensory-discriminative aspects of innocuous somatosensation. The SP1 topographic pattern showed little or no change across the innocuous a nd noxious stimulus levels, which together with the amplitude data sug gests that SP1 is largely generated by neurons involved in innocuous s omatosensation. The SP4 topographic pattern did not change appreciably across the innocuous and noxious stimulus levels, but its amplitude d ecreased with increasing noxious stimulation. These data suggest that SP4 is generated by neurons involved in innocuous somatosensation and that noxious inputs inhibit these cells. There were differences in the SP5 topographic patterns evoked at the innocuous and the noxious stim ulus levels, which suggest SP5 also receives a contribution from neuro ns involved in noxious somatosensation. SP3 (135-157 msec) and SP6 (28 2-339 msec) are probably generated by neurons invoked in noxious somat osensation. The topographic patterns of both were different at innocuo us and noxious levels. SP3's amplitude-stimulus intensity function sug gests that it is generated by neurons that respond to noxious inputs i n a non-graded fashion. The amplitude and offset latency of SP6 increa sed with increasing noxious stimulation, which suggests that SP6 is ge nerated by neurons that respond to noxious inputs in a graded fashion.