Spontaneous Saccades and Gaze-Holding Ability in the Pigmented Rat. II. Effects of Localized Cerebellar Lesions.

Abstract

We have studied the effects of the ablation of the cerebellar vermal area corresponding to lobules VI - VIII and of the flocculus - paraflocculus of both sides on the spontaneous eye movements performed in the light and in the dark in head-restrained pigmented rats. These effects have been compared with those already described for the inferior olive lesion. The cerebellar lesions were performed 1 week to 6 months in advance. Eye movements were recorded through a phase detection search coil apparatus. Following vermal topectomy, the main characteristics of the spontaneous saccades are unmodified. Following the ablation of the flocculus - paraflocculus there is no change in the saccadic main sequence. However, the spontaneous saccades in the dark present a postsaccadic drift made up of two components with different time courses, the first one being fast and the second one slow. The former is due in part to a mismatch between the phasic (the pulse) and the tonic (the step) components of the eye movements; the latter to the leakage of the neural integrator. In light only the first component is present and the eye maintains a steady position. The time constant of the neural integrator is considerably reduced to approximately 600 - 900 ms from a value of approximately 1600 - 4000 ms in the intact rats. The amplitude of the postsaccadic drift in the light depends on both the mismatch between the pulse and the step of innervation of the extraocular muscles and the increased leakiness of the neural integrator. The gain of the pulse to step transformation is reduced to approximately 0.79 at all saccadic amplitudes and eccentricities and such a reduction is due to a decreased step amplitude, while the pulse amplitude remains unchanged. The contribution of the leakage of the neural integrator to the postsaccadic drift in the light is a function of the eccentricity with a slope of 0.23. The deficits described after flocculus - paraflocculus ablation are also very similar to those described following inferior olive lesion from a quantitative point of view. The possible mechanisms of the visually activated olivocerebellar system in the control of saccadic performance and in maintaining its calibration are discussed.