Vestibulo-ocular reflex

The vestibulo-ocular reflex (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, 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 cannot read, because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed.

The "gain" of the VOR is defined as the change in the eye angle divided by the change in the head angle during the head turn. If the gain of the VOR is wrong (different than 1)&mdash;for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses&mdash;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. This is what is referred to as VOR adaptation.

The main neural circuit for the VOR is fairly simple. Vestibular nuclei in the brainstem receive signals related to head movement from the Scarpa's ganglion located on CN VIII, or the vestibular nerve From this Vestibular nuclei excitatory fibers cross to the contralateral CN VI nerve nucleus. There they synapse with 2 additional pathways. One projects directly to the lateral rectus of eye. Another nerve tract projects from the CN VI nucleus to the oculomotor nuclei, which contain motorneurons that drive eye muscle activity, specifically activating the medial rectus muscles of the eye.

The cerebellum is essential for motor learning to correct the VOR in order to ensure accurate eye movements. Motor learning in the VOR is in many ways analogous to classical eyeblink conditioning, since the circuits are homologous and the molecular mechanisms are similar.