Activity of interpositus neurons during a visually guided reach.

Neurons in the cerebellar interpositus nucleus greatly increase their discharge rates when a monkey reaches out to grasp an object. However, when the monkey is required to track a target on a screen by moving a manipulandum, the increase in discharge rate is relatively small or nonexistent. Moving the hand directly to a target is a visuomotor task that may be fundamentally different from a remote tracking task. We hypothesize that the interpositus nucleus is specialized for direct visual guidance of the limb or, alternatively, interpositus is specialized for controlling hand movements required to grasp an object. A monkey was trained to hold a sensor and move it directly over a visual target to obtain water reward. Small drawers were mounted next to two of the targets; on some trials a drawer would open so that the monkey would reach out and retrieve a raisin that had been placed in it. Interpositus neurons discharged strongly during reach to grasp the raisin but not when the monkey was positioning the sensor over the target. For individual cells, discharge pattern and amplitude were largely independent of the size and direction of the reach to grasp, suggesting that interpositus does not control direction or amplitude of the reach. The results are consistent with the hypothesis that neurons in forelimb regions of interpositus participate in the control of hand movements used in grasping, but they are not consistent with the hypothesis that interpositus neurons participate in direct visual guidance of the limb.