Somato-sensory response to mechanical stimulation as recorded in the human EEG.

Abstract Tapping various sites of the body will evoke well defined responses in the EEG from the somato-sensory scalp area in normal human subjects. The responses evoked by tapping are very similar to those evoked by peripheral nerve stimulation in the same subject, but they mostly have longer latencies for the characteristic peaks. They also lack the initial negative wave, which is characteristic of the latter responses. Summated nerve action potentials evoked by peripheral nerve stimulation are single synchronous volleys of comparatively large amplitude, while those evoked by tapping have a smaller amplitude and tend to be polyphasic. The origin of this difference and its possible effect on the similarities and dissimilarities of the EEG responses are discussed. The responses evoked by tapping are rather widespread, but tend to have maximal amplitudes in accordance with the known somatotopic organization. Responses to taps on the proximal parts of the extremities and on the trunk and face have unexpectedly large amplitudes and seemingly wide distributions. Latencies of the initial parts of the response are shorter when evoked by proximal than when evoked by distal taps. Later waves do not behave so regularly; they may in individual cases have longer latencies for proximal than for distal taps. The studies of distribution and latency suggest the existence of a dual complementary inflow responsible for the evoked potential, at least when evoked from proximal parts of the extremities and trunk.