The influence of motor preparation on the response of cerebellar neurons to limb displacements.

This study examined the influence of "motor set" on the response of neurons in the deep cerebellar nuclei to peripheral afferent inputs. Two monkeys were trained to perform arm movements which were triggered by imposed displacements of their forelimb. Imposed displacements were generated by a torque motor which was coupled to a handle held by the monkey. The direction of the triggered movements depended on a prior instruction presented to the animal before the onset of the imposed displacement. Single neuron recordings were made from 105 interpositus and 191 dentate neurons whose activity was related to the performance of the task. Forty-three of the interpositus and 46 of the dentate neurons displayed a change in activity within 70 msec of the onset of the imposed displacement. None of these neurons displayed a neural response following the onset of the instruction. The short latency response of most interpositus neurons was related to the parameters of the afferent input generated by the imposed displacements. For more than 60% of the interpositus neurons this response was strongly influenced by the direction of the imposed displacement. None of these responses were markedly altered by the prior instructions. In contrast, the short latency response of 67% of the dentate neurons, although triggered by the imposed displacement, was markedly altered by the "motor set" of the animal. For most of the dentate neurons whose activity was influenced by the animal's "motor set," the neural response was contingent on two factors: (1) the prior instruction given to the animal and (2) the direction of the imposed displacement. In a second set of dentate neurons (13% of those influenced by motor preparation) the characteristics of the short latency response were determined solely by the nature of the prior instruction. For these neurons the imposed displacement served as a trigger for possible changes in the activity. Thus, dentate neuron activity can behave like a motor command signal which is triggered at short latency by a peripheral event but depends on the intent of the animal.