Responses of cerebellar interpositus neurons to predictable perturbations applied to an object held in a precision grip

Two monkeys were trained to lift and hold an instrumented object at a fixed height for 2.5 s using a precision grip. The device was equipped with load cells to measure both the grip and lifting or load forces. On selected blocks of 20 - 30 trials, a downward force-pulse perturbation was applied to the object after 1.5 s of stationary holding. The animals were required to resist the perturbation to obtain a fruit juice reward. The perturbations invariably elicited a reflex-like, time-locked increase in grip force at latencies between 50 and 100 ms. In this study, we searched for single cells in the interpositus and dentate nuclei with activity related to grasping and lifting, and we tested 127/150 task-related cells for their responses to the perturbation. Of the 127 cells, reflex-like increases or decreases in discharge frequency occurred in 75 cells (59%) at a mean latency of 36 ms. Preparatory increases in grip force preceding the perturbation appeared gradually and increased in strength with repetition in 39/127 (31%) cells. These preparatory increases did not immediately disappear when the perturbations were withdrawn but decreased progressively over repeated trials. Although a few cells showed anticipatory activity without a reflex-like response (15/127 or 12%), the majority of these cells (24/39) displayed both anticipatory and reflex-like responses. From an examination of the histological sections, cells with both anticipatory and reflex-like responses appeared to be confined to the dorsal anterior interpositus, adjacent to, but not within, the dentate nucleus. These results confirm and extend the suggestion by Dugas and Smith that the cerebellum plays a major role in organizing anticipatory responses to predictable perturbations in a manner that medial and lateral premotor areas of the cerebral cortex do not.