Somatosensory Evoked Potentials to Mechanical Disturbances of Positioning Movements in Man: Gating of Middle-range Components

Somatosensory evoked potentials (SEPs) and EMG responses to mechanical disturbances of positioning movements of the index finger were recorded from normal subjects who performed smooth flexions against the constant load of a torque motor in order to reach a specified target zone. According to a random series 60% of the movements were disturbed by step increases or decreases of the load which subjects had to compensate for (movement condition, MC). As control procedures (a) the same stimuli were applied while the subject had to maintain a constant finger position against the same load (hold condition, HC), and (b) comparable mechanical stimuli were administered to the stationary relaxed finger (resting condition, RC). In MC and HC, the EMG responses consisted of a long-latency (53-61 msec) reflex component, and a 'voluntary' component (98-134 msec). In all 3 conditions, the sequential SEP deflections measured over the sensorimotor cortex included peaks P85 (ipsilateral), N150 (bilateral) and P220 (bilateral). A contralateral N90 component (onset latency 55-60 msec), present in HC and RC, was markedly reduced or absent in MC. Neither in the preceding nor in the consecutive SEP components were there any significant condition-dependent differences, though the N150 showed a trend to diminished amplitude in MC as well. The suppression of SEP components in MC is discussed in terms of efferent and afferent 'gating' actions exerted on the somatosensory input during movement.