Somatosensory evoked potentials elicited by mechanical and electrical stimulation of each single pain or tactile spot of the skin.

Single pain and tactile spots on the dorsum of the right hand in man were stimulated by electric pulses and mechanical taps using a needle to a pain spot and a horse tail bristle to a tactile one. Somatosensory evoked potentials (SEPs) and far field potentials (FFPs) were observed in three volunteers by averaging of 200 or 400 samples of responses recorded from scalp points corresponding to the left and right somatosensory areas (LSA and RSA). For SEP measurements, the difference between responses at LSA and RSA was obtained in order to specify the optimum response at LSA. The patterns of SEPs elicited by mechanical or electrical stimulation to a pain spot were similar to those to a tactile one. The typical SEP to mechanical stimulation consisted of N23, P31, N40, P49, N64, P87, N114 and P147, while that to electrical stimulation did of N24, P33, N42, P52, N66, P95, N122 and P156. Consequently, mean peak latencies of the later components in SEPs elicited by mechanical stimulation were earlier than those to electrical one. The SEP amplitudes for mechanical stimulation were 1-3 microV and were larger than those to electrical one. By mechanical stimulation FFPs could not be obtained, while by electrical stimulation FFPs in peak latency of approximately 14 msec (P14) were seen. No specific components to sensory modality of pain or tactile sensation were observed either in SEPs or in FFPs.