The brain does not need the cerebellum to make movements. Rather, it needs the cerebellum to make accurate movements. The cerebellum endows the organism with the ability to internally monitor and correct ongoing motor commands. This monitoring requires the cerebellum to be able to predict errors that are about to happen, and correct for them before they occur. That is, the brain relies on the cerebellum to have accurate internal models that learn to predict sensory consequences of motor commands. However, it has been difficult to decipher how the cerebellum represents internal models: for many forms of behavior, including saccadic eye movements. By examining the relationship between simple spikes of Purkinje cells (P-cells) and behavior, this project will advance our understanding of computations in the cerebellum. Results of these studies could provide new avenues of rehabilitation for patients with cerebellar damage.
The new idea in this proposal is that the basic unit of computation in the cerebellum may not be a single P-cell or a randomly selected population of P-cells, but rather a specific group of P-cells wherein all the P-cells share the same preference for prediction error. Using this idea, the collaborative team of investigators from Johns Hopkins University and University of Washington has found that during saccades, the simple spikes of P-cells predict with exquisite accuracy future behavior of the eyes. The aim of this project is to understand how the cerebellum learns to make such accurate predictions. The investigators present a new paradigm, one in which sensory errors are perpendicular to the direction of motion of the eyes. This paradigm is interesting because behavior shows considerable richness: motor commands that arrive early in the movement appear to change little following error, but those that come late show both high learning and rapid forgetting. Using a combination of experiments and computational modelling, the investigators will test ideas that behavioral changes are due to the neural changes in the P-cells: micro-clusters that do not prefer the error express their learning in the acceleration phase of the movement, whereas those that prefer the error express their learning in the deceleration phase.
