PROPRIOCEPTIVE ACTIVITY IN PRIMATE PRIMARY SOMATOSENSORY CORTEX DURING ACTIVE ARM REACHING MOVEMENTS

1. We studied the activity of 254 cells in the primary somatosensory c ortex (SI) responding to inputs from peripheral proprioceptors in a va riety of tasks requiring active reaching movements of the contralatera l arm. 2. The majority of cells with receptive fields on the proximal arm (shoulder and elbow) were broadly and unimodally tuned for movemen t direction, often with approximately sinusoidal tuning curves similar to those seen in motor and parietal cortex. 3. The predominant tempor al response profiles were directionally tuned phasic bursts during mov ement and tonic activity that varied with different arm postures. 4. M ost cells showed both phasic and tonic response components to differin g degrees, and the population formed a continuum from purely phasic to purely tonic cells with no evidence of separate distinct phasic and t onic populations. This indicates that the initial cortical neuronal co rrelates of the introspectively distinguishable sensations of movement and position are represented in an overlapping or distributed manner in SI. 5. The directional tuning of the phasic and tonic response comp onents of most cells was generally similar, although rarely identical. 6. We tested 62 cells during similar active and passive arm movements . Many cells showed large differences in their responses in the two co nditions, presumably due to changes in peripheral receptor discharge d uring active muscle contractions. 7. We tested 86 cells in a convergen t movement task in which monkeys made reaching movements to a single c entral target from eight peripheral starting positions. A majority of the cells (46 of 86, 53.5%) showed a movement direction-related hyster esis in which their tonic activity after movement to the central targe t varied with the direction by which the arm moved to the target. The directionality of this hysteresis was coupled with the movement-relate d directional tuning of the cells. 8. We recorded the discharge of 93 cells as the monkeys performed the task while compensating for loads i n different directions. The large majority of cells showed a statistic ally significant modulation of activity as a function of load directio n, which was qualitatively similar to that seen in motor cortex under similar task conditions. Quantitatively, however, the sensitivity of S I proprioceptive cells to loads was less than that seen in motor corte x but greater than in parietal cortex. 9. We interpret these results i n terms of their implications for the central representation of the sp atiotemporal form (''kinematics'') of arm movements and postures. Most importantly, the results emphasize the important influence of muscle contractile activity on the central proprioceptive representation of a ctive movements.