Changes in a motor pattern following cerebellar and olivary lesions in the squirrel monkey.

Squirrel monkeys were trained to perform ballistically initiated flexion movements of the forearm. It is shown that the motor output to agonist and antagonist subsequent to the onset of the movement depends continuously on the movement's parameters, primarily the velocity, as in man. The contribution of cerebellar activities to maintain this relationship was investigated by means of discrete lesions of the cerebellum and inferior olive. After lesions of the cerebellar nuclei, EMG activity of agonist and antagonist is no longer precisely structured. This result implies that activity of the cerebellar nuclei is necessary to maintain the gain and timing of the relationship between sensory inputs and motor outputs. Participation of the inferior olive is also required, since lesions of this structure mimic the effect of destruction of the cerebellar nuclei. Lesions of the cerebellar cortex, instead, affect mainly the timing of the motor output.