Mechanically and electrically evoked somatosensory potentials in human: scalp and neck distributions of short latency components.

Short latency somatosensory potentials evoked by electrical stimulation of the median nerve as well as by mechanical stimulation on the nail of the index finger were recorded from 10 normal adults using a noncephalic reference (the forearm contralateral to the stimulus). Potentials were recorded from 15 electrode locations extending from the level of the 4th thoracic through the 7th, 4th and 2nd cervical vertebrae to the scalp at Oz, P4, P3, A2, A1, C4, Cz, C3, F4, F3 and Fpz. In general, all the components of potentials evoked by mechanical stimulation had electrically evoked counterparts with comparable surface distributions and variations between subjects. Some of the electrically evoked components, which were low in amplitude and variable in occurrence between subjects, did not have mechanically evoked counterparts. Possible generators of the components detected are discussed based on their surface distribution and polarity reversals. A comparable study on patients with well localized lesions must be performed in order to support or disprove the generators proposed.