The importance of hand use to discharge of interpositus neurones of the monkey.

1. Monkey interpositus neurones show large discharge modulations during reaching to grasp, however, the same neurones show little or no modulation during operation of devices that exercise individual forelimb joints. We tested the hypothesis that grasping during the reach-to-grasp is necessary for eliciting high discharge modulation. 2. Three monkeys (Macaca mulatta) moved an articulated lever between low and high target zones. While in the lower zone the monkey's hand was at its waist, in the upper zone its hand was in a position that required forelimb extension at right-angles to the body axis. Small drawers adjacent to the target zones contained raisins, and the drawers could be remotely opened. Thus, we could elicit two types of reaches having similar trajectories: one reach involved limb transport while holding the lever handle, and the other involved limb transport while forming the hand to grasp a raisin. 3. Eighty-one neurones from two monkeys, mostly from interpositus with some from adjacent regions of dentate, were tested during device use and reaching to grasp: 93% of the neurones discharged at high rates during at least one of the tasks. Of these, about half increased discharge rate solely during reaching to grasp; the other half showed some increase during device use but only discharged strongly during reaching to grasp. Overall, discharge modulations during the reach-to-grasp averaged twice as high as during the corresponding device movement (112 versus 56 impulses s-1). 4. Individual neurones consistently discharged with characteristic patterns during the reach-to-grasp with rates often exceeding 300 impulses s-1. Discharge during the reach-to-grasp was independent of reach trajectory: discharge patterns and amplitudes were similar when reaching from either the lower or upper target zone to the upper raisin drawer as when reaching from the upper target zone to the upper raisin drawer. Reach direction also made little difference: reaches from the upper target zone to the lower drawer typically elicited similar discharge modulation as those from the lower target zone to the upper drawer. 5. High discharge rates associated with grasping were independent of the item being grasped: typically, grasping the device handle elicited as high discharge rates as grasping a raisin. 6. The hypothesis was confirmed that grasping is critical for eliciting high discharge modulation in interpositus during reaching to grasp. Discharge pattern and modulation do not vary with reach direction or amplitude of the reach and, therefore, it is unlikely that intermediate cerebellum controls these features of the reach-to-grasp.(ABSTRACT TRUNCATED AT 400 WORDS)