INTERSTIMULUS-INTERVAL HAS NO EFFECT ON A MIDLATENCY SCALP POTENTIAL GENERATED BY INNOCUOUS-RELATED ACTIVITY IN THE PRIMARY SOMATOSENSORY CORTEX

The sural nerve-evoked somatosensory evoked potential (SEP) scalp topo graphy was separated into stable periods, where a stable period refers to consecutive time points with the same topographic pattern. The sti mulus intensity-amplitude function, conduction velocity measurements, and a dipole source localization analysis of one of these stable perio ds, SP1 (60-90 ms post-stimulus), strongly suggests that it is generat ed by the response of neurons in the primary somatosensory cortex (SI) to inputs arising from the innocuous A beta peripheral afferents. Int erstimulus intervals (ISI) ranging between 2.5 s and 10.0 s had no eff ect on SP1 amplitude. This contrasts with an earlier report from this laboratory demonstrating that subjective magnitude ratings and the amp litude of another stable period that appears at about 160-180 ms post- stimulus and that is also generated in SI, increase with decreasing IS I. Thus, ISI appears to affect perception and the late but not the mid -latency responses in SI.