Centrifugal and centripetal mechanisms involved in the 'gating' of cortical SEPs during movement.

Pre- and post-central cortical SEPs were of reduced amplitude and altered wave form when the median nerve stimulus was delivered during active movement of the thumb. Peripheral, cervical and scalp far-field potentials were not significantly affected, apart from the positive wave following N13 at upper cervical level. The algebraic contribution of movement-related potentials was eliminated by wave form subtraction. Qualitatively similar effects were seen when the thumb was moved passively, suggesting that sensory input associated with the movement exerts a 'gating' effect which is at least as significant as that due to centrifugal influences. The effect was in some respects similar to that associated with interfering cutaneous stimulation, although an additional component was identifiable in the difference wave forms associated with active and passive movement. In contrast to passive movement, SEP gating during active movement was equally marked when the stimulus was delivered instantaneously with the first detectable displacement of the thumb. Thus the effect cannot have been entirely due to movement-related activity in distal cutaneous, muscle and joint receptors, since input from the latter could not have arrived at the CNS before the median nerve volley. Other centripetal mechanisms are not excluded, however, since arrival of the volley would be preceded by proprioceptive activity from proximal muscle receptors.