Localization, timing and specificity of gating of somatosensory evoked potentials during active movement in man.

Short latency somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded at different times simultaneously and after an order to move the ipsilateral thumb (0, 100, 200, 300, 400 and 780 ms). Brain potentials were derived from surface electrodes over the scalp at contralateral postcentral and precentral sites and muscle potentials were derived from surface electrodes over the thenar eminence. Ipsilateral thumb movement did not affect the early lemniscal P14 or the postcentral cortical N20 but was accompanied by attenuation of the cortical postcentral P27 and cortical precentral P22, N30 and P45. These results suggest that the gating induced by voluntary movement in man occurs at thalamocortical level and that the differential effect on postcentral N20 and precentral P22 may represent the activity of different generators. Gating of SEPs during voluntary movement begins approximately 80-100 ms before EMG onset, is maximal at the time the EMG is maximal, and returns to control values after the EMG is concluded. These results suggest that the gating that precedes the onset of EMG activity is related to premotor events in the cerebral cortex and not to centripetal influences. Gating of SEPs to median nerve stimulation was localized to movements of areas innervated by the median nerve, that is, the ipsilateral thumb, index or middle finger, but not to movements of ipsilateral little finger or contralateral thumb.