A NEUROMAGNETIC STUDY OF MOVEMENT-RELATED SOMATOSENSORY GATING IN THEHUMAN BRAIN

Neuromagnetic fields from the left cerebral hemisphere of five healthy , right-handed subjects were investigated under three different experi mental conditions: (1) electrical stimulation of the right index finge r (task S); (2) voluntary movement of the same finger (M); (3) M+S con dition, consisting of voluntary movements of the right index finger tr iggering the electrical stimulus at the very beginning of the electrom yogram. The three conditions were administered in random order every 5 -8 s. In addition, the task somatosensory evoked fields (task SEFs) ga thered during condition (I) were compared with control SEFs recorded a t the beginning of the experiment during rest. In all subjects the ove rlay of somatosensory stimulation on movement provoked a decrement in brain responsiveness (gating) as determined by the amplitude of gated SEFs. The latter was found as the difference between the neuromagnetic fields during M+S condition (overlaying of movement and sensory stimu lation) minus neuromagnetic fields under M condition (M only). The gat ing effect was found to begin approximately 30 ms after movement onset , and to last for the whole period of the ongoing movement. The theore tical locus of gating was estimated by dipole localisation of the diff erence between task SEFs and gated SEFS using a moving dipole model. T he site of the ''early'' gating effect (<40 ms) was found to be more a nteriorly located than the ''later'' (>40 ms) gating effect. The task SEFs were found to be larger (significant after 30 ms) than the contro l SEFs elicited under the basal condition. The results are discussed w ith respect to timing, mechanism (centrifugal and centripetal), locus and selectivity of gating. In addition, the results are discussed with regard to clinical application (measuring attentional deficits in pat ients with impairments of higher mental functions and measuring gating deficits in patients with disturbed sensorimotor integration).