VIBRATION-ENTRAINED AND PREMOVEMENT ACTIVITY IN MONKEY PRIMARY SOMATOSENSORY CORTEX

1. Primary somatosensory cortical(SI) neurons exhibit characteristic a ctivity before the initiation of movements. This premovement activity (PMA) may result from centrally generated as well as from peripheral i nputs. We examined PMA for 55 SI neurons (10, 13, 28, and 4 in areas 3 a, 3b, 1, and 2, respectively) with activity that was entrained to vib rotactile stimulation (i.e., was temporally correlated with the stimul us). We sought to determine whether the temporal characteristics of vi bration-entrained discharges would change throughout the reaction time period, and, if they did, whether these changes might be accounted fo r by central inputs. 2. Monkeys made wrist flexions and extensions in response to sinusoidal vibration (27, 57, or 127 Hz) of their palms. V ibration remained on until the animal moved at least 5 degrees from th e initial hold position. Mean firing rate (MFR), a measure of the leve l of activity, was derived from the number of spikes per vibratory cyc le. The correlation between the vibration and the neuronal firing was described by the mean phase (MP) of the vibratory cycle at which spike s occurred. The degree of entrainment was quantified as synchronicity (Synch), a statistical parameter that could change from 0 for no entra inment to 1 for responses at a constant phase. 3. Premovement MFR incr eases (activation) and decreases (suppression) were observed. Moreover , two changes in MFR often were observed for the same neuron (2-event PMA). Many MFR shifts, especially the first in the two-event PMA, prec eded electromyographic (EMG) onset. The pre-EMG MFR shifts more often had the same sign both for flexion and extension movements rather than having opposite signs. However, with equal frequency, post-EMG PMA ev ents had the same or opposite sign for different movement directions. We suggest that the pre-EMG PMA has an origin different from movement- related peripheral reafference. 4. Premovement activation was accompan ied by shifts of MP corresponding to earlier responses to the ongoing vibratory stimulus and by decreases of response Synch. Premovement sup pression was not associated with consistent shifts of MP and Synch. We suggest that during premovement activation, asynchronous (uncorrelate d with vibration) signals are integrated with the vibratory input. The se asynchronous signals may make neurons more likely to discharge and to do so earlier with respect to the vibratory stimulus. The asynchron ous component may also disrupt the vibration-entrained activity patter n. 5. From these data we conclude that the activity of SI neurons that most faithfully represent the sensory periphery is modulated before v oluntary movements. We suggest that inputs of central origin may contr ibute to this premovement modulation. Presumably, the role of the cent ral inputs may be to prepare SI for changes in sensory activity that r esult from voluntary movement. Premovement asynchronous signals, both of central and peripheral origin, disrupt the fidelity of coding of th e vibratory stimulus by SI neurons. This deterioration of the quality of representation of sensory inputs by SI neurons may be related to th e phenomena of premovement elevation of the tactile threshold of perce ption and of premovement decrease of somatosensory-evoked potentials.