Unit activity in the cerebellar nuclei related to arm reaching movements

Single units in the fastigial, interpositus and dentate nuclei of two stump-tail macaque monkeys were studied in relation to a right arm, visually guided reaching task. Of 638 recorded cells, 149 showed activity changes correlated to the task, including 24 in the contralateral fastigial and interpositus. Reach-related discharge patterns fell into two broad categories, tonic and phasic. Tonic responses were maintained throughout the reach with no observable relation to kinematic parameters. Most of the task-related activity occurred during the upward lift of the arm toward the target button, with a drop-off as the arm was lowered toward the rest plate. Phasic response cells fired bursts (or suppressed discharge) at specific points in the arm trajectory, most commonly during the lift phase. Many had a sharp drop in discharge when the shoulder flexion torque was transiently reversed to decelerate the arm. For either type, restricted directional specificity was rarely seen in any nucleus, and correlations with recorded EMGs were weak. Visual responses to target button illumination were observed in both the fastigial and dentate nuclei, but did not necessarily correspond with the button giving the best movement-related response. Task-related activity changes started earliest in the fastigial nuclei and latest in the interpositus nuclei. The data suggested that cerebellar output facilitates motor centers in a rather general manner, but at precisely determined times.