Vestibulo-ocular reflex, optokinetic response and their interactions in the cerebellectomized cat

The effects of total ablation of the cerebellum on eye movements were studied in alert adult cats. The normal cat could easily hold a steady eye position after a saccadic movement in the dark. The cerebellectomized animal could not: after a saccade the eye position shifted towards a more central position. Vision reduced this 'post-saccadic drift'. The sinusoidal vestibulo-ocular reflex (v.o.r.) was strongly affected by total cerebellectomy. In darkness the v.o.r. gain remained stable at high frequencies (0.5 and 1 Hz) but decreased markedly at lower frequencies to as low as 0.18 at 0.05 Hz. A phase advance (up to 65 degrees at 0.05 Hz) paralleled this gain depression. Velocity characteristics of optokinetic nystagmus (o.k.n.) and optokinetic after-nystagmus (o.k.a.n.) induced by constant-velocity full-field rotation of 60 deg/s amplitude and 60 s duration were studied. The features of o.k.n. (initial velocity, maximal velocity and time constant) were only mildly affected by cerebellectomy. On cessation of visual stimulation when the animal was plunged into darkness, the velocity of the eyes decreased progressively (o.k.a.n.). The time constant of o.k.a.n. was 12.5 s in the normal cat and 4.2 s in the cerebellectomized cat. Furthermore cerebellectomy abolished the secondary o.k.a.n. Optokinetic response was also tested by a set of sinusoidal (0.05-1 Hz; 3-20 degrees) full-field stimuli. The o.k.n. was not abolished but dramatically decreased, especially at higher frequencies. No response could be detected above 0.15 Hz. Visual suppression of inappropriate vestibulo-ocular reflex was still possible but was mildly impaired after cerebellectomy. Visual suppression could only be detected with stimuli below 0.25 Hz. Visual suppression of caloric nystagmus was studied in the normal cat. A clear dependence of the effectiveness of visual suppression on the velocity of the nystagmus was demonstrated. In the cerebellectomized cat, the visual suppression of caloric nystagmus was lost when tested on nystagmus velocities above 20 deg/s but remained when tested on nystagmus velocities below 20 deg/s. The relationship between cerebellectomy and the loss of visual suppression of caloric nystagmus was found to be at least partially indirect: cerebellectomy increased the velocity of caloric nystagmus, and visual suppression was usually less effective at higher velocities.