Cerebellar contribution to the spatial encoding of orienting gaze shifts in the head-free cat.

1. Cerebellar saccadic dysmetria may result from a disturbance in the processes that ensure correct execution of gaze displacement. Alternatively, an impairment in the preparatory processes that lead to the specification of the movement goal may also produce this deficit. 2. We report here on a pharmacologically induced dysmetria that suggests a cerebellar contribution to the neural processes encoding the location of the goal for orienting gaze shifts. 3. Shifts of gaze (eye-in-space) were recorded in the head-free cat after the GABA agonist muscimol was unilaterally injected into the caudal part of the fastigial nucleus. 4. Gaze saccades towards the inactivated side were hypermetric. These ipsiversive movements overshot the target by a constant error, regardless of target eccentricity and initial gaze position. 5. Gaze saccades directed away from the inactivated side undershot the target. The degree of hypometria increased when the amplitude of the required movement increased. 6. These results suggest a different contribution of the caudal fastigial nucleus to the accuracy of visually triggered gaze shifts, depending on the direction of the impending saccade. The systematic error of ipsiversive movements and the inappropriate movements evoked by presenting a target at the same physical location as gaze reveal that fastigial inactivation interfered with the processes that encode the location of a visual target.