Abstract
Attenuation of somatosensory evoked potentials (SEPS) recorded from the scalp during voluntary movement occurs for specific combinations of the finger moved and the peripheral nerve stimulated. The cerebral potential component occurring at a latency of 27 msec (P27) evoked either by stimulation of median nerve at the wrist or by stimulation of 1st and 2nd digit nerves in the fingers were selectively attenuated during movement of 1st digit but were not altered during movement of 5th digit. By contrast, the cerebral P27 component evoked by stimulation of ulnar nerve at the wrist or by stimulation of 5th digital nerve were attenuated during movement of that digit but were not altered during movement of 1st digit. Gating of somatosensory activity is a selective phenomenon occuring when movement involves the areas being stimulated.
Highlights
The definition of specific topographical effects of motor activity on sensory processes as defined in this report may have a bearing on the study of 'plasticity' of sensory and motor functions m humans following lesions of both peripheral and central pathways For Instance, Merzenlch et al (1984) have shown that the distribution of afferents from the d~g~ts m the cerebral cortex m monkey changes as a functmn of amputauon of individual >ts The measurement of somatosensory evoked potentials in man both at rest and during movement, following naturally o~.curring damage to peripheral nerves, may provide comparattve evidence of such changes in human cerebral cortex
Attenuation of somatosensory evoked potentmls (SEPS) recorded from the scalp during voluntary movement occurs for specific combinations of the finger moved and the peripheral nerve stimulated The cerebral potential component occumng at a latency of 27 msec (P27) evoked either by stimulation of median nerve at the wrist or by stimulation of 1st and 2nd dlgtt nerves in the fingers were selectively attenuated dunng movement of 1st digit but were not altered dunng movement of 5th digit By contrast, the cerebral P27 component evoked by stimulation of ulnar nerve at the wrist or by stimulation of 5th digital nerve were attenuated during movement of that digit but were not altered during movement of 1st digit Gating of somatosensory activity is a selective phenomenon occurring when movement involves the areas being stimulated
Summary
Low lntensaty (10-25 dB sensation level) chcks produced by activating TDH-39 earphones with a 100 p,sec duration pulse at a rate of 0 7/sec were the signal to move The mean latency of the EMG from the thenar muscles indicating the onset of the reacuon time, measured in three of the subjects, was 193 8 + 42 0 msec (223 6 + 64 0, 144 8 ± 36 7 and 213 ± 72 7 msec) All subjects performed with an accuracy of 100% We dad not record indwldual trials so that measures of the relationship between reaction tame speeds and SEP amplitudes were not performed Peripheral nerves were stimulated 250 msec after each chck when movement had begun and the EMG was of high amphtude (Starr and Cohen 1985) The same rate of stimulation of the peripheral nerve was used in the control sessions when the subjects were instructed to be still We stud~ed the effects on SEP of two different movements mediated by muscles innervated by different peripheral nerves First, opposition of the thumb (digit 1) towards digit 5 (primarily a me&an nerve innervated movement) and secondly, abduction of digit 5 (primarily an ulnar nerve Innervated movement) The opposition of digit 1 is mainly mediated by the opponens polllcas muscle, innervated by the median nerve, whale the abduction of digit 5 is mainly mediated by the abductor dlgiU nnmrm innervated by the ulnar nerve. Tlcular digits Electrical sttmuh were applied through surface electrodes to either mixed nerves at the wrist or to cutaneous nerves in the dints The amphtudes of SEP to stimulation of digital nerves were smaller than to stimulation of mixed nerves at the wrist, requlnng a larger number of mals in the former (up to 1500) to obtmn reproducible averages The averaged potentials evoked by these stimuli were recorded from surface electrodes placed over the peripheral nerve (PN) and the scalp m a quiet resting state or during movement Peripheral nerve (PN) potentials to stimulation of the nerves at the wrist were recorded from a surface electrode over Erb's point referenced to (Fpz) during median and ulnar nerve stimulation When stimulating digital nerves the peripheral nerve activity was recorded from two surface electrodes placed over the median or ulnar nerves at the v,nst Peripheral nerve potentials were momtored and did not change dunng the movements required in the experimental session Somatosensory potentials were recorded from a scalp electrode located post-centrally 3 cm behind the vertex (Cz) and 7 cm from the midhne contralateral to the limb stimulated, referenced to the lpsdateral earlobe (A1) The potentials were amplified with a gmn of 500,000 using a bandpass of 30-3000 Hz (6 dB/octave slope) The amphtudes of the SEPs were measured between baseline and positive (P) or negative (N) peaks whale peripheral nerve potentials (PN) were measured between basehne and the negative peak The number of sttmuh used to elicit SEPs varied from 150 to 500 for the different subjects and depended on achieving a 20% or less amplitude difference of the N20
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