RHYTHMICALLY FIRING (20-50 HZ) NEURONS IN MONKEY PRIMARY SOMATOSENSORY CORTEX - ACTIVITY PATTERNS DURING INITIATION OF VIBRATORY-CUED HAND MOVEMENTS

The activity patterns of rhythmically firing neurons in monkey primary somatosensory cortex (SI) were studied during trained wrist movements that were performed in response to palmar vibration. Of 1,222 neurons extracellularly recorded in SI, 129 cells (similar to 11%) discharged rhythmically (at similar to 30 Hz) during maintained wrist position. During the initiation of vibratory-cued movements, neuronal activity u sually decreased at similar to 25 ms after vibration onset followed by an additional decrease in activity at similar to 60 ms prior to movem ent onset. Rhythmically firing neurons are not likely to be integrate- and-fire neurons because, during activity changes, their rhythmic firi ng pattern was disrupted rather than modulated. The activity pattern o f rhythmically firing neurons was complimentary to that of quickly ada pting SI neurons recorded during the performance of this task (Nelson et al., 1991). Moreover, disruptions of rhythmic activity of individua l SI neurons were similar tb those reported previously for local field potential (LFP) oscillations in sensorimotor cortex during trained mo vements (Sanes and Donoghue, 1993). However, rhythmic activity of SI n eurons did not wax and wane like LFP oscillations (Murthy and Fetz, 19 92; Sanes and Donoghue, 1993). It has been suggested that fast (20-50 Hz) cortical oscillations may be initiated by inhibitory interneurons (Cowan and Wilson, 1994; Llinas et al., 1991; Stern and Wilson, 1994). We suggest that rhythmically firing neurons may tonically inhibit qui ckly adapting neurons and release them from the inhibition at go-cue o nsets and prior to voluntary movements. It is possible that rhythmical ly active neurons may evoke intermittent oscillations in other cortica l neurons and thus regulate cortical population oscillations.