HIGH-FREQUENCY VIBRATORY SENSITIVE NEURONS IN MONKEY PRIMARY SOMATOSENSORY CORTEX - ENTRAINED AND NONENTRAINED RESPONSES TO VIBRATION DURING THE PERFORMANCE OF VIBRATORY-CUED HAND MOVEMENTS

The activity of high-frequency vibratory sensitive (HFVS) neurons was recorded in monkey primary somatosensory cortex (SI) while animals per formed wrist flexions and extensions in response to 57-Hz or 127-Hz pa lmar vibration. HFVS neurons were distinguished by their exquisite res ponsiveness to the higher frequency vibration (127 Hz). These neurons probably received input from Pacinian afferents. Systematic selection of HFVS neurons was made using K-means cluster analysis of neuronal fi ring rates during stimulating at 127 Hz and 57 Hz. HFVS neurons consti tuted similar to 4% of all recorded cells and more frequently were fou nd in areas 3b, 1, and 2 (similar to 5% of total in each area) than in area 3a (similar to 1%). Using circular-statistics analyses for nonun iformity of discharges over the vibratory cycle, HFVS neurons were spl it into two groups of vibration-entrained neurons (E1 and E2 neurons) and one group of nonentrained neurons (NE neurons). E1 neurons were en trained to vibration at both 127 Hz and 57 Hz, whereas E2 neurons were entrained only at one of these vibratory frequencies. Vibration-entra ined neurons often exhibited multimodal distributions of interspike in tervals (ISIs), with the modes at multiples of the period of vibration . In addition, for these neurons, ISI clusters in joint interval plots commonly had diagonal orientations that were indicative of negative s erial correlations of the ISIs, a feature of extrinsically driven rhyt hmic activity. HFVS neurons located in areas 3a, 3b, and 1 responded t o vibration onset at shorter latencies (16.5+/-1.6, 19.8+/-5.9, and 21 .4+/-6.4 ms, respectively, during 127-Hz stimulation) than those locat ed in area 2 (35.6+/-13.8 ms). These observations are consistent with a scheme in which HFVS area 2 neurons receive their inputs from more a nterior areas of SI. Moreover, entrained neurons exhibited shorter res ponse latencies than nonentrained neurons. During 127-Hz stimulation, response latencies were 17.3+/-3.0, 17.5+/-2.6, and 25.7+/-6.4 ms for E1, E2, and NE neurons, respectively, located in areas 3a, 3b, and 1. Thus, entrained and nonentrained HFVS neurons may belong to different hierarchical stages of information processing.