cerebellar output pathway involved in saccadic gain adaptation

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1) Effects of inactivating the fastigial oculomotor region (FOR) in the monkey Unilateral inactivation of the FOR is known to result in marked hypermetria of ipsiversive saccades. It remains an open question whether this is due to a uniform gain change over saccades of different amplitudes, i.e., a parametric gain change, or due, instead, to an overshoot of a constant size regardless of movement amplitudes, as suggested by a previous study in the cat. We analyzed dysmetria following unilateral FOR inactivation in the monkey and concluded that the resultant overshoots were due to a uniform gain increase for saccades of all sizes.2) Changes in FOR activity during saccadic adaptation The results of the above and other FOR inactivation studies suggest that the FOR exerts a suppressive action on ipsiversive saccades. If the FOR is involved in adaptive modification of saccade amplitude, its saccade-related activity is expected to increase as the saccade size in decreased. We recorded from single FOR neurons in the monkey as the gain of ipsiversive saccades was decreased. The FOR activity was found to increase as the gain was decreased. During the course of adaptation, the neuronal activity and gain were altered with a similar course but in the opposite directions. The results are thus consistent with the hypothesized suppressive action of FOR activity on ipsiversive saccades and provide support on a single neuron basis for the cerebellar involvement in saccadic adaptation.