The cerebellum and the adaptive coordination of movement.

Based on a review of cerebellar anatomy, neural discharge in relation to behavior, and focal ablation syndromes, we propose a model of cerebellar function that we believe is both comprehensive as to the available information (at these levels) and unique in several respects. The unique features are the inclusion of new information on (a) cerebellar output--its replicative representation of body maps in each of the deep nuclei, each coding a different type and context of movement, and each appearing to control movement of multiple body parts more than of single body parts; and (b) the newly assessed long length of the parallel fiber. The parallel fiber, by virtue of its connection through Purkinje cells to the deep nuclei, appears optimally designed to combine the actions at several joints and to link the modes of adjacent nuclei into more complex coordinated acts. We review the old question of whether the cerebellum is responsible for the coordination of body parts as opposed to the tuning of downstream executive centers, and conclude that it is both, through mechanisms that have been described in the cerebellar cortex. We argue that such a mechanism would require an adaptive capacity, and support the evidence and interpretation that it has one. We point out that many parts of the motor system may be involved in different types of motor learning for different purposes, and that the presence of the many does not exclude an existence of the one in the cerebellar cortex. The adaptive role of the cerebellar cortex would appear to be specialized for combining simpler elements of movement into more complex synergies, and also in enabling simple, stereotyped reflex apparatus to respond differently, specifically, and appropriately under different task conditions. Speed of learning and magnitude of memory for both novel synergies and task-specific performance modifications are other attributes of the cerebellar cortex.