Vestibulo-ocular reflex
487910
223350539
2008-07-03T18:19:41Z
80.226.0.1
/* References */ +de
[[Image:Simple_vestibulo-ocular_reflex.PNG|thumb|300px|right|The vestibulo-ocular reflex. A rotation of the head is detected, which triggers an inhibitory signal to the [[extraocular muscles]] on one side and an excitatory signal to the muscles on the other side. The result is a compensatory movement of the eyes.]]
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.
==Circuit==
[[Image:Vestibulo-ocular reflex EN.svg|thumb|500px]]
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. <ref>{{cite journal |author=Straka H, Dieringer N |title=Basic organization principles of the VOR: lessons from frogs |journal=Prog. Neurobiol. |volume=73 |issue=4 |pages=259–309 |year=2004 |pmid=15261395 |doi=10.1016/j.pneurobio.2004.05.003}}</ref>
===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.<ref>{{cite web|url=http://cat.inist.fr/?aModele=afficheN&cpsidt=15155766|title=Effect of Ethanol on visual-vestibular interactions during vertical linear body acceleration}}</ref>
==Testing== <!--Rapid head impulse test and Halmagyi-Curthoys redirect here-->
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.
===Testing Complications===
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.
==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]]
== External links ==
* [http://edboyden.org/03.09.boyden.html Motor Learning in the VOR in Mice] at edboyden.org
* [http://www.jhu.edu/strucfunc/Archives/2004_files/2004_11_30.pdf Review on VOR adaptation via slides] at [[Johns Hopkins University]]
* {{MeshName|Vestibulo-Ocular+Reflex}}
* [http://www.cimit.org Center for Integration of Medicine and Innovative Technology - Testing device development]
* {{eMedicine|ent|482}} - "Vestibuloocular Reflex Testing"
==References==
{{Reflist}}
{{Reflex}}
{{Visual system}}
[[Category:Reflexes]]
[[Category:Vision]]
[[de:Vestibulookulärer Reflex]]
[[nl:Vestibulo-oculaire reflex]]
[[pl:Odruch przedsionkowo-oczny]]
[[pt:Reflexo vestíbulo-ocular]]