Modification of the macaque's vestibulo-ocular reflex after ablation of the cerebellar vermis

No Gain No Pain: Relations Between Vestibulo-Ocular Reflexes and Motion Sickness in Mice

Motion sickness occurs when the vestibular system is subjected to conflicting sensory information or overstimulation. Despite the lack of knowledge about the actual underlying mechanisms, several drugs, among which scopolamine, are known to prevent or alleviate the symptoms. Here, we aim at better understanding how motion sickness affects the vestibular system, as well as how scopolamine prevents motion sickness at the behavioral and cellular levels. We induced motion sickness in adult mice and tested the vestibulo-ocular responses to specific stimulations of the semi-circular canals and of the otoliths, with or without scopolamine, as well as the effects of scopolamine and muscarine on central vestibular neurons recorded on brainstem slices. We found that both motion sickness and scopolamine decrease the efficacy of the vestibulo-ocular reflexes and propose that this decrease in efficacy might be a protective mechanism to prevent later occurrences of motion sickness. To test this hypothesis, we used a behavioral paradigm based on visuo-vestibular interactions which reduces the efficacy of the vestibulo-ocular reflexes. This paradigm also offers protection against motion sickness, without requiring any drug. At the cellular level, we find that depending on the neuron, scopolamine can have opposite effects on the polarization level and firing frequency, indicating the presence of at least two types of muscarinic receptors in the medial vestibular nucleus. The present results set the basis for future studies of motion sickness counter-measures in the mouse model and offers translational perspectives for improving the treatment of affected patients.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Vestibulo-oculomotor characteristics in surgically treated patients with cerebellar haemangioblastoma (Lindau tumor)

In a survey of 18 patients that were operated on because of a cerebellar haemangioblastoma (Lindau tumor) 94% showed characteristic cerebellar oculomotor deficits when DC EOG recordings were analyzed. Only 22.1% showed also clinically cerebellar deficits. We studied the vestibular ocular reflex (VOR) with and without visual interaction, and also saccades and pursuit eye movements in our patients with lesions involving one cerebellar hemisphere (n = 14) or the vestibulocerebellum (n = 4). Gain of dysmetric saccades ipsilateral to the side of the lesion was significantly increased; gain of pursuit eye movements, that were often saccadic, was decreased contralaterally. Our patients showed highly increased responses for all three conditions of visual-VOR-interaction (sinusoidal or steplike rotation in the dark, in the light with fixed surround, or in the dark with head fixed fixation point) ipsilateral to the lesion, together with a remarkable inability to modulate their VOR with vision on this side. With movements contralateral to the side of the lesion they showed a larger variability of test responses than normals, but only for fixation suppression of the VOR a significantly increased gain. The very similar time constants of slow phase decay after a step change in chair velocity indicated a similar poor modulation of visual-VOR-interaction under these 3 conditions for stimulations ipsilateral to the lesion. Contrariwise, optokinetic step stimulation demonstrated a generally impaired response that was significantly lowered towards the side contralateral to the lesion. Our results indicate the critical function of the cerebellum for voluntary fast and slow eye movements as well as for visual-vestibular interaction in humans.