Specific gating of the early somatosensory evoked potentials during active movement.

The gating effect of self-paced rapid flexion movements of the fingers on the early somatosensory evoked potentials following electrical stimulation of the median nerve at the wrist was studied in normal volunteers. Triggering of the median nerve stimulation by the EMG signals with a delay of 100 msec showed that the slow positive wave of the movement-associated potential was not directly responsible for the SEP amplitude variations observed. The nerve action potential at Erb's point as well as far-field components P9 and P11 were unchanged by the active movements. Far-field components P13-P14, which are presumably generated in the medial lemniscus, were not significantly modified. An enhancing effect on the widespread N18 component was found, which is in favour of a subcortical gating process. The parietal component N20 was unchanged by active movement interference whereas the frontal P22 component showed a marked suppression. A fronto-parietal dissociation was thus disclosed which could be in favour of separate cortical generators in the debate on the origin of SEP components. An important gating effect was observed on parietal P27 and frontal N30 components, the latter being considerably reduced in amplitude. The parietal P45 component showed no significant alteration. Each component of the early SEPs was thus distinctly influenced by the gating process during active movement interference.