Primary motor cortical responses to perturbations of prehension in the monkey.

1. Two monkeys were trained to grasp, lift, and hold an object within a vertical position window. A downward force-pulse perturbation was delivered during stationary object holding to simulate slip of the object due to gravity. The responses evoked by the perturbation were studied in 189 neurons of the hand area of the primary motor cortex. In addition, the slip-evoked responses were compared with the modulation of neural discharge with textures and weights described in the previous paper. 2. The perturbation evoked responses with sharp onsets in the majority of motor cortical neurons (115/189, 61%) active during the task. The majority of the responses were of sufficiently short latency (43.17 +/- 17.24 ms, mean +/- SD) to have participated in the reflex grip force increase that followed at latencies from 50 to 100 ms. 3. Although a similar proportion of neurons with cutaneous (43/70) or proprioceptive (35/59) receptive fields (RFs) were responsive to the perturbation, the cutaneous afferents provided a stronger excitation of motor cortical cells than the feedback originating from proprioceptive receptors. 4. The covariation of the neural discharge related to the surface texture of the grasped object and the responsiveness to object slip was studied in 89 cells tested with the perturbation and with more than one surface texture on unperturbed trials. Within this population, motor cortical cells with cutaneous RFs were more sensitive to the perturbation (25/31) than neurons receiving proprioceptive input (8/16). Furthermore, all (17/17) neurons with cutaneous RFs that were more active with the smooth than with the rough surface textures showed a vigorous response to the perturbation. 5. A detectable downward displacement of the object was not always necessary to excite neurons with cutaneous RFs and whose activity increased with the smooth textures. Their sensitivity to the perturbation was consistent with the hypothesis that the cutaneous afferent activity generated by object slips or shear forces on the skin contributed to the increased discharge when lifting objects of slippery surface textures. The activity of these slip- or shear-sensitive cells may have contributed to the reflex grip force increases and to the greater sustained muscular activity needed to lift smooth objects. 6. Ten cells that were excited by stroking of the glabrous skin of the hand decreased their discharge frequency during the task even though their RFs were in direct contact with the object. Most of these neurons (7/10) did not respond to the object slip, and three cells had very weak responses to the perturbation.(ABSTRACT TRUNCATED AT 400 WORDS)