Combination, complementarity and automatic control: a role for the cerebellum in learning movement coordination.

We have examined several different paradigms of adaptation and of 'acquisition of skill'--skill defined as a movement specialized to meet a certain goal and gained through practice. In each paradigm, change occurs through trial-and-error performance. In some of the tasks, damage of cerebellar cortex impairs adaptation and not performance. The deficits in performance cannot explain the deficits in adaptation. In some of the tasks, the discharge of Purkinje cells and, by inference, the discharge of inferior olive cells and mossy fibres has behaved in a manner consistent with the Marr-Albus theory of motor learning. We extend the theory to show how parallel fibres could implement both the coordination of complex movements and the learning of new movements. The size of the response combinations would be proportionate to the length of parallel fibres. The mechanism proposed here would permit optimized complex movement behaviours to respond to specific behavioural contexts rapidly, stereotypically and automatically. The mechanism would permit storage of many context-response couplings, and many complex responses. The mechanism would permit privacy, individuality and a large number of behavioural responses.