Cerebellar limb ataxia: abnormal control of self-generated and external forces

Our work has been focused on understanding the mechanism of movement abnormalities associated with cerebellar ataxia. The hypothesis tested is that the cerebellum acts to modulate muscle activity across multiple joints in anticipation of the mechanical interaction torques generated by one's own movement and by external forces. Individuals with cerebellar damage were studied in two sets of experiments. In the first experiment, we studied how cerebellar subjects' movement changed when interaction torques were present, and then reduced via mechanically constraining movement to a single joint. Consistent with the hypothesis, it was found that cerebellar endpoint errors were greatly improved when interaction torques were reduced. We also found that cerebellar deficits in the unconstrained condition were not explained by a general failure of torque timing or magnitude scaling. This supports the idea that the cerebellum plays a specific and important role in adjusting for the dynamics of one's own body movements. In the second experiment, we studied how well cerebellar subjects could adapt arm movements to external loads. Cerebellar subjects were tested as they adapted a catching movement to balls of different weight. It was found that they were slow or unable to adapt through practice and did not show evidence of storage of the adaptation. This suggests that the cerebellum is needed for rapid adaptations for loads in movement. Given these findings, we think that the cerebellum is important in anticipating and adjusting for the mechanical demands of movement though trial-and-error practice.