The role of the cerebellum in the control of saccades as a window into neural mechanisms of movement optimization

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Our earlier work on the oculomotor vermis (OMV), a distinct part of the cerebellum, could establish that information processing by the neuronal machinery of the OMV is the major substrate of the almost instantaneous adjustment (short-term oculomotor learning) of the kinematics of the two complementary types of goal-directed eye movements, saccades and smooth pursuit, demanded by performance errors. Our experiments in the first funding period demonstrate that the OMV deploys the one and the same output neurons (Purkinje cells) to control both types of eye movements without jeopardizing the need to warrant that learning-based changes are eye movement type specific. The deployment of hybrid Purkinje cells is surprising in view of the very different kinematic profiles of saccades and smooth pursuit. These findings have implications for the integration of saccade and smooth pursuit-related signals by Purkinje-cell dendritic trees and in general constrain the wiring of the various neuronal elements involved. A second line of experiments on the transfer of short-term learning between saccades and smooth-pursuit lends further support to the notion that the major biological function of short-term learning of goal-directed eye movements is the compensation of cognitive fatigue, a term we use in order to capture changes of eye movement kinematics due to changes of the subjective value of movement goals without necessarily excluding other factors such as attention. For the second funding period we suggest experiments that will try to critically test the hypothesis that the OMV plays a central role in compensating cognitive fatigue by translating information on the subjective value of movements and movement goals into corrective control signals for saccades.