Velocity-dependent suppression of somatosensory evoked potentials during movement.

Sensory transmission is known to be impaired during movement of the stimulated body part. This phenomenon is manifested by a decrease of perceptual acuity and a correlated decrease in the size of somatosensory evoked potentials (SEPs). The goal of the present study was to define the relationship between SEP amplitude and speed of movement. SEPs were elicited by brief (25 msec) loading of the wrist flexor muscles. Stimuli were applied while the wrist joint was stationary or moving voluntarily at one of several velocities. In all subjects, SEP amplitude was approximately inversely related to speed of movement at the time of stimulation. The findings refine and extend studies suggesting the velocity dependence of sensory suppression during movement.

Decrement of somatosensory evoked potentials during repetitive stimulation.

In normal subjects, cerebral potentials were evoked by brief, passive extension of the wrist joint at various interstimulus intervals (ISIs). The resulting somatosensory evoked potentials (SEPs) were found to decrease during repetitive stimulation. The greatest decrement occurred between the first and second responses of each series. After cessation of stimuli, the SEP amplitude returned to control values over a prolonged, exponential time course. The authors postulate that the observed response decrement may be a form of habituation, which provides a model for studying the neuronal substrates of behavior.

Kinematic and EMG reactions to imposed interlimb phase alterations during bipedal cycling.

Strong 'neural constraints' synchronizing the step cycles of bilaterally paired limbs in decerebrate cats have been revealed by forcing those limbs to step at different speeds. To determine if humans operate under similar restrictions, we studied subjects pedaling a bicycle ergometer whose coupled cranks imposed continuous change upon interlimb phasing. Unlike the cat, the EMG timing patterns of each leg proved to vary little as a function of phase. Phase change did induce fluctuations in average EMG signal energy, but these were highly idiosyncratic to each muscle. Nevertheless, leg kinematic trajectories remained confined to narrow bounds well within biomechanical limitations. We thus suggest that the trajectory of each leg is actively regulated during pedaling, although the interlimb neural constraints apparent in decerebrate stepping are absent.

Cerebral evoked potentials and somatosensory perception.

In normal subjects, somatosensory evoked potentials (SEPs) were produced by increases or decreases of the load on the biceps muscle during voluntary contraction. The stimuli lasted only 20 msec and caused less than 2 degrees of elbow flexion or extension. When the stimulus was applied during voluntary movement of the elbow, the SEP was attenuated, and the subject was less able to discriminate between loading and unloading pulses. In each of eight subjects, there was a positive correlation between the percentage of correct responses and the size of the SEP. The measurement of both SEPs and perceptual accuracy under various test conditions provides a refined technique for studying the relations between electrical events and sensory processes.