Effect of bilateral ablation of the vestibular cerebellum on visual-vestibular interaction

Vertical Purkinje cells of the monkey floccular lobe: simple-spike activity during pursuit and passive whole body rotation

To understand how the simian floccular lobe is involved in vertical smooth pursuit eye movements and the vertical vestibuloocular reflex (VOR), we examined simple-spike activity of 70 Purkinje (P) cells during pursuit eye movements and passive whole body rotation. Fifty-eight cells responded during vertical and 12 during horizontal pursuit. We classified P cells as vertical gaze velocity (VG) if their modulation occurred for movements of both the eye (during vertical pursuit) and head (during pitch VOR suppression) with the modulation during one less than twice that of the other and was less during the target-fixed-in-space condition (pitch VOR X1) than during pitch VOR suppression. VG P cells constituted only a minority of vertical P cells (19%). Other vertical P cells that responded during pitch VOR suppression were classified as vertical eye and head velocity (V/) P cells (48%), regardless of the synergy of their response direction during smooth pursuit and VOR suppression. Vertical P cells that did not respond during pitch VOR suppression but did respond during rotation in vertical planes other than pitch were classified as off-pitch V/ P cells (33%). The mean eye-velocity and eye-position sensitivities of the three types of vertical P cells were similar. One-third (2/7 VG, 2/11 V/, 6/13 off-pitch V/), in addition, showed eye position sensitivity during saccade-free fixations. Maximal vestibular activation directions (MADs) were examined during VOR suppression by applying vertical whole body rotation with the monkeys oriented in different vertical planes. The MADs for VG P cells and V/ P cells with eye and vestibular sensitivity in the same direction were distributed near the pitch plane, suggesting convergence of bilateral anterior canal inputs. In contrast, MADs of off-pitch V/ P cells and V/ P cells with oppositely directed eye and vestibular sensitivity were shifted toward the roll plane, suggesting convergence of anterior and posterior canal inputs of the same side. Unlike horizontal G P cells, the modulation of many VG and V/ P cells when the target was fixed in space (pitch VOR X1) was not well predicted by the linear addition of their modulations during vertical pursuit and pitch VOR suppression. These results indicate that the populations of vertical and horizontal eye-movement P cells in the floccular lobe have markedly different discharge properties and therefore may be involved in different kinds of processing of vestibular-oculomotor interactions.

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.