Vestibulo-ocular reflex

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.

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Circuit

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. ^[1]

Excitatory example

For instance, if the head is turned clockwise as seen from above, then excitatory impulses are sent from the semicircular canal on the right side via the vestibular nerve (cranial nerve VIII) through Scarpa's ganglion and end in the right vestibular nuclei in the brainstem. From this nuclei excitatory fibers cross to the left abducens nucleus. There they project and stimulate the lateral rectus of the left eye via the abducens nerve. In addition, by the abducens internuclear interneurons and oculomotor nuclei, they activate the medial rectus muscles on the right eye. As a result, both eyes will turn counterclockwise.

Furthermore, some neurons from the right vestibular nucleus directly stimulate the right medial rectus motoneurons, and inhibits the right abducens nucleus.

Speed

The vestibulo-ocular reflex needs to be fast: if we want clear vision, head movements need to be compensated almost immediately. Otherwise our vision corresponds to a photograph taken with a shaky hand. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles. For instance, the connection involves only three neurons, and is correspondingly called Three-neuron-arc. Using these direct connections, eye movements lag the head movements by less than 10 ms, one of the fastest reflexes in the human body.

Gain

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)—for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses—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.

Ethanol consumption can disrupt the VOR, reducing dynamic visual acuity.^[2]

Testing

This reflex can be tested by the Rapid head impulse test or Halmagyi-Curthoys-test, in which the head is rapidly moved to the side with force, and is controlled if the eyes succeed to remain looking in the same direction. When the function of the right balance system is reduced, by a disease or by an accident, quick head movements to the right cannot be sensed properly any more. As a consequence, no compensatory eye movements are generated, and the patient cannot fixate a point in space during this rapid head movement.

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.

Role of cerebellum

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.

See also

• Caloric reflex test
• Semicircular canals
• Vestibular system
• Pursuit movement

References