Dexterity in cerebellar agenesis.

Given the well established role of the cerebellum in motor control, deficient motor performance during life time appears to be common in cerebellar agenesis. However, behavioural data on motor performance in living subjects with cerebellar agenesis are scarce. Dexterity during object manipulation was comparatively investigated in a 63-year old female with cerebellar agenesis and three healthy age- and gender-matched control subjects. Participants performed a transport task with an instrumented object and caught a weight that was dropped into a hand-held receptacle either expectedly from the opposite hand or unexpectedly from the experimenter's hand. Compared to healthy subjects, the subject with cerebellar agenesis generated greater grasping forces. For the transport task the patient showed a clear impairment of the predictive adjustment of grasping forces to the differential loading requirements of movement direction. For the weight-catching task, the patient established an accurate reactive mode of control when the weight was dropped unexpectedly. In case the weight was dropped expectedly from the opposite hand, predictive control mechanisms were severely disturbed in the subject with cerebellar agenesis. These data highlight the role of the cerebellum for predictive force control and are interpreted within the concept of internal models reflecting the causal relationship between actions and their consequences.