The vestibulo-ocular reflex (VOR) or oculovestibular reflex 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, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read using print, because they cannot stabilize the eyes during small head tremors. The VOR does not depend on visual input and works even in total darkness or when the eyes are closed.
The main neural circuit for the horizontal VOR is fairly simple. It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses via the vestibular nerve (cranial nerve VIII) through Scarpa's ganglion and end in the vestibular nuclei in the brainstem. From this nuclei fibers cross to the contralateral cranial nerve VI nucleus (abducens nucleus). There they synapse with 2 additional pathways. One pathway projects directly to the lateral rectus of eye via the abducens nerve. Another nerve tract projects from the abducens nucleus by the abducens internuclear interneurons or abducens interneurons to the oculomotor nuclei, which contain motorneurons that drive eye muscle activity, specifically activating the medial rectus muscles of the eye through the oculomotor nerve.
Another pathway (not in picture) directly projects from the vestibular nucleus through the ascending tract of Dieters to the ipsilateral medial rectus motoneurons. In addition there are inhibitory vestibular pathways to the ipsilateral abducens nucleus. However no direct vestibular neuron to medial rectus motoneuron pathway exists.
Furthermore, some neurons from the right vestibular nucleus directly stimulate the right medial rectus motoneurons, and inhibits the right abducens nucleus.
Ethanol consumption can disrupt the VOR, reducing dynamic visual acuity.
Another way of testing the VOR response is a caloric reflex test, which is an attempt to induce nystagmus (compensatory eye movements in the absence of head motion) by pouring cold or warm water into the ear.
Currently, vestibulo-ocular reflexes can only be comprehensively tested in specially equipped laboratories. The tests sometimes provide valuable diagnostic information; but the laboratory setting is unnatural, the tests are time-consuming, and the people being tested are often asymptomatic while in the lab. A device capable of tracking eye movement outside the laboratory would be very useful to clinicians. Steven Rauch, of the Massachusetts Eye and Ear Infirmary, is in the process of developing an ambulatory vestibular monitoring device.
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Diazepam Tolerance Effects on Vestibular Function Testing, Part II: Vestibulo-ocular Reflex Parameters During Rotational Testing
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