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Vestibulo-ocular reflex
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Vestibulo-ocular reflex

The vestibulo-ocular reflex, or VOR, is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement.

The VOR elicits eye movements in response to both horizontal and vertical head rotations, as well as head translations. Although the function of the VOR is to support clear vision, the VOR is measured in the dark to isolate the eye movements driven by vestibular stimuli from eye movements driven by visual stimuli.

The performance of the VOR is characterized by the gain, which is defined as the ratio between eye and head velocities. If the gain of the VOR is poorly calibrated, then head movements result in image motion on the retina, resulting in blurred vision. Under such conditions, motor learning adjusts the gain of the VOR to produce more accurate eye motion. Such adjustments are needed throughout life, as neurons and muscles develop, weaken, and die--or for humans, when a new pair of eyeglasses changes the magnification of the visual field.

In the laboratory, motor learning in the VOR is induced by pairing image motion with head motion. Depending on the relative direction of head motion and image motion, the gain of the VOR can be adaptively increased or decreased. An increase in VOR gain is induced by image motion in the direction opposite that of the head (gain-up stimulus), and a decrease in VOR gain is induced by image motion in the same direction as the head (gain-down stimulus).

The main neural circuit for the VOR is simple. Vestibular nuclei in the brainstem receive signals related to head movement from the vestibular nerve and project to oculomotor nuclei, which contain motoneurons that drive eye muscle activity. The flocculus and ventral paraflocculus of the cerebellum form an inhibitory side loop in the VOR circuit. Mossy fibers provide vestibular input to the cerebellum, and Purkinje cells inhibit VOR interneurons in the vestibular nucleus. For the VOR, the vestibular nuclei serve the role that the deep cerebellar nuclei serve for other cerebellum-dependent behaviors like eyeblink conditioning. Thus, when we refer to the contribution of the cerebellum to the VOR, we mean the cerebellar cortex.