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Vestibulo–ocular reflex: Difference between revisions - Wikipedia
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}}[[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.]]▼
{{Refimprove
| date = October 2016
▲}}[[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''') is a [[reflex]], whereacting activationto ofstabilize thegaze during head movement, with [[vestibulareye systemmovement]] ofdue theto inneractivation earof causesthe [[eyevestibular movement (sensory)|eye movementsystem]]. ThisThe reflex functionsacts to [[image stabilization|stabilize images]] on the [[retina]]s (whenof the [[eye]] during head movement, holding gaze is held steadysteadily on a location) during head movement, by producing eye movements in the direction opposite to head movement, thus preserving the image on the center of the visual field(s).{{Citation needed|date=October 2016}} For example, when the head moves to the right, the eyes move to the left, andmeaning the image a person sees stays the same even though the head vicehas versaturned. Since slight head movement is present all the time, VOR is necessary 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, and also because damage to the VOR can cause vestibular [[nystagmus]].<ref>{{cite web|url=http://www.dizziness-and-balance.com/practice/nystagmus/vestibular.html|title=Vestibular nystagmus|website=www.dizziness-and-balance.com}}</ref>
 
The VOR does not depend on visualwhat inputis seen. It can also be elicitedactivated by caloric (hot or cold) stimulation of the [[inner ear]], where the vestibular system sits, and works even in total darkness or when the eyes are closed.{{Citation needed|date=October 2016}} However, in the presence of light, the [[fixation reflex]] is also added to the movement.<ref name="eb">"Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 179 Encyclopædia Britannica, 1987</ref>
 
In animals other animalsthan humans, the organs that coordinate balance and motormovement coordination doare not operate independentlyindependent from theeye organs that control the eyesmovement. A fish, for instance, moves its eyes by reflex when its tail is moved. Humans have [[semicircular canals]], neck muscle "stretch" receptors, and the [[Utricle (ear)|utricle]] (gravity organ). Though the semicircular canals cause most of the reflexes which are responsive to acceleration, the maintaining of balance is mediated by the stretch of neck muscles and the pull of gravity on the utricle (otolith organ) of the inner ear.<ref name="eb"/>
 
The VOR has both rotational and translational aspects. When the head rotates about any axis (horizontal, vertical, or torsional) distant visual images are stabilized by rotating the eyes about the same axis, but in the opposite direction.<ref name="Crawford1991">{{cite journal | vauthors = Crawford JD, Vilis T | title = Axes of eye rotation and Listing's law during rotations of the head | journal = Journal of Neurophysiology | volume = 65 | issue = 3 | pages = 407–23 | date = March 1991 | pmid = 2051188 | doi = 10.1152/jn.1991.65.3.407 }}</ref> When the head translates, for example during walking, the visual fixation point is maintained by rotating [[Gaze (physiology)|gaze]] direction in the opposite direction<ref>{{Cite web|url=https://collections.lib.utah.edu/details?id=187678|title=VOR (Slow and Fast) {{!}} NOVEL - Daniel Gold Collection|website=collections.lib.utah.edu|language=en|access-date=2019-10-03}}</ref>, by an amount that depends on distance.<ref name="Angelaki2004">{{cite journal | vauthors = Angelaki DE | title = Eyes on target: what neurons must do for the vestibuloocular reflex during linear motion | journal = Journal of Neurophysiology | volume = 92 | issue = 1 | pages = 20–35 | date = July 2004 | pmid = 15212435 | doi = 10.1152/jn.00047.2004 }}</ref>
 
==CircuitFunction==
[[Image:Vestibulo-ocular reflex EN.svg|thumb|300px]]
 
The mainvestibulo-ocular "directreflex path"is neuraldriven circuitby forsignals arising from the horizontalvestibular rotationalsystem VORof isthe fairlyinner simpleear. ItThe starts[[semicircular incanals]] detect head rotation and provide the rotational component, whereas the [[vestibular systemotolith]],s wheredetect semicircularhead canalstranslation getand activateddrive bythe headtranslational rotationcomponent. andThe sendsignal theirfor impulsesthe horizontal rotational component travels via the [[vestibular nerve]] (cranial nerve VIII) through the [[vestibular ganglion]] and end in the [[vestibular nuclei]] in the [[brainstem]]. From these nuclei, fibers cross to the contralateral cranial nerve VI nucleus ([[abducens nucleus]]) of the opposite side of the brain. ThereHere, theyfibres synapse with 2 additional pathways. One pathway projects directly to the [[lateral rectus|lateral rectus muscle]] of the eye via the [[abducens nerve]]. Another nerve tract projects from the abducens nucleus by the [[medial longitudinal fasciculus]] to the contralateral [[oculomotor nucleus]] of the opposite side, which contains [[motor neuron|motorneuronsmotor neurons]] that drive eye [[muscle]] activity, specifically activating the [[medial rectus|medial rectus muscle]] muscle of the eye through the [[oculomotor nerve]] (cranial nerve III).▼
The VOR is ultimately driven by signals from the vestibular apparatus in the [[inner ear]]. The [[semicircular canals]] detect head rotation and drive the rotational VOR, whereas the [[otolith]]s detect head translation and drive the translational VOR.
▲The main "direct path" neural circuit for the horizontal rotational 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 the [[vestibular ganglion]] and end in the [[vestibular nuclei]] in the [[brainstem]]. From these 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 the eye via the [[abducens nerve]]. Another nerve tract projects from the abducens nucleus by the [[medial longitudinal fasciculus]] to the contralateral [[oculomotor nucleus]], which contains [[motor neuron|motorneurons]] that drive eye [[muscle]] activity, specifically activating the [[medial rectus]] muscle of the eye through the [[oculomotor nerve]] (cranial nerve III).
 
Another pathway (not in picture) directly projects from the vestibular nucleus through the [[ascending tract of Dieters]] to the ipsilateral [[medial rectus|medial rectus muscle]] [[motor neuron|motoneuron]] of the same side. 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 | vauthors = Straka H, Dieringer N | title = Basic organization principles of the VOR: lessons from frogs | journal = Progress in Neurobiology | volume = 73 | issue = 4 | pages = 259–309 | date = July 2004 | pmid = 15261395 | doi = 10.1016/j.pneurobio.2004.05.003 }}</ref>
 
Similar pathways exist for the vertical and torsional components of the VOR.
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In addition to these direct pathways, which drive the velocity of eye rotation, there is an indirect pathway that builds up the position signal needed to prevent the eye from rolling back to center when the head stops moving. This pathway is particularly important when the head is moving slowly because here position signals dominate over velocity signals. David A. Robinson discovered that the eye muscles require this dual velocity-position drive, and also proposed that it must arise in the brain by mathematically integrating the velocity signal and then sending the resulting position signal to the motoneurons. Robinson was correct: the 'neural integrator' for horizontal eye position was found in the nucleus prepositus hypoglossi<ref name="Cannon1987">{{cite journal | vauthors = Cannon SC, Robinson DA | title = Loss of the neural integrator of the oculomotor system from brain stem lesions in monkey | journal = Journal of Neurophysiology | volume = 57 | issue = 5 | pages = 1383–409 | date = May 1987 | pmid = 3585473 | doi = 10.1152/jn.1987.57.5.1383 }}</ref> in the medulla, and the neural integrator for vertical and torsional eye positions was found in the interstitial nucleus of Cajal<ref name="Crawford1991#2">{{cite journal | vauthors = Crawford JD, Cadera W, Vilis T | title = Generation of torsional and vertical eye position signals by the interstitial nucleus of Cajal | journal = Science | volume = 252 | issue = 5012 | pages = 1551–3 | date = June 1991 | pmid = 2047862 | doi = 10.1126/science.2047862 | bibcode = 1991Sci...252.1551C }}</ref> in the midbrain. The same neural integrators also generate eye position for other conjugate eye movements such as saccades and smooth pursuit.
 
===Excitatory exampleExample===
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 fibres 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 [[medial longitudinal fasciculus]] and [[oculomotor nuclei]], they activate the [[medial rectus]] muscles on the right eye. As a result, both eyes will turn counter-clockwise.
 
Furthermore, some neurons from the right vestibular nucleus directly stimulate the right [[medial rectus]] motoneuronsmotor neurons, and inhibits the right abducens nucleus.
 
===Speed===
The vestibulo-ocular reflex needs to be fast: for clear vision, head movement must be compensated almost immediately; otherwise, vision corresponds to a photograph taken with a shaky hand. ToSignals achieveare clear vision, signalssent from the semicircular canals are sent as directly as possible to the eye muscles: the connection involvesusing only three neurons, and is correspondingly called the ''three neuron arc''.{{Citation Usingneeded|date=May these2020}} directThis connections,results in eye movements that lag the head movementsmovement by less than 10 ms,.<ref>{{cite journal | vauthors = Aw ST, Halmagyi GM, Haslwanter T, Curthoys IS, Yavor RA, Todd MJ | title = Three-dimensional vector analysis of the human vestibuloocular reflex in response to high-acceleration head rotations. II. responses in subjects with unilateral vestibular loss and selective semicircular canal occlusion | journal = Journal of Neurophysiology | volume = 76 | issue = 6 | pages = 4021–30 | date = December 1996 | pmid = 8985897 | doi = 10.1152/jn.1996.76.6.4021 }}</ref> and thus theThe vestibulo-ocular reflex is one of the fastest reflexes in the human body.{{Citation needed|date=May 2020}}
 
=== VOR suppression ===
During head-free pursuit of moving targets{{Clarify|reason=|date=May 2020}}, the VOR is counterproductive to the goal of reducing retinal offset. Research indicates that there exists mechanisms to suppress VOR using active visual feedback.<ref>{{Cite web|url=http://psycnet.apa.org/record/1980-24636-001|title=PsycNET|website=psycnet.apa.org|language=en|access-date=2018-05-15}}</ref> In the absence of visual feedback, such as during occlusions, we use anticipatory (extra-retinal) signals to supplement our pursuit movements by VOR suppression.<ref>{{cite journal | vauthors = Ackerley R, Barnes GR | title = The interaction of visual, vestibular and extra-retinal mechanisms in the control of head and gaze during head-free pursuit | journal = The Journal of Physiology | volume = 589 | issue = Pt 7 | pages = 1627–42 | date = April 2011 | pmid = 21300755 | pmc = 3099020 | doi = 10.1113/jphysiol.2010.199471 }}</ref>
 
===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. Ideally the gain of the rotational VOR is 1.0. The gain of the horizontal and vertical VOR is usually close to 1.0, but the gain of the torsional VOR (rotation around the line of sight) is generally low.<ref name="Crawford1991"/> The gain of the translational VOR has to be adjusted for distance, because of the geometry of motion parallax. When the head translates, the angular direction of near targets changes faster than the angular direction of far targets.<ref name="Angelaki2004"/>
 
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[[Ethanol]] consumption can disrupt the VOR, reducing dynamic visual acuity.<ref>{{cite journal | vauthors = Schmäl F, Thiede O, Stoll W | title = Effect of ethanol on visual-vestibular interactions during vertical linear body acceleration | journal = Alcoholism, Clinical and Experimental Research | volume = 27 | issue = 9 | pages = 1520–6 | date = September 2003 | pmid = 14506414 | doi = 10.1097/01.ALC.0000087085.98504.8C | url = http://cat.inist.fr/?aModele=afficheN&cpsidt=15155766 }}</ref>
 
==Clinical significance==
==Testing== <!--Rapid head impulse test and Halmagyi-Curthoys redirect here-->▼
 
▲=== 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 to look in the same direction. When the function of the right balance system is reduced, by a disease or by an accident, a quick head movement to the right cannot be sensed properly anymore. As a consequence, no compensatory eye movement is generated, and the patient cannot fixate a point in space during this rapid head movement.
 
The head impulse test can be done at the bed side and used as a screening tool for problems with a person's vestibular system.<ref>{{Cite web|url=https://collections.lib.utah.edu/ark:/87278/s63b97tz|title=VOR (Slow and Fast)|last=Gold|first=Daniel|date=|website=Neuro-Ophthalmology Virtual Education Library (NOVEL): Daniel Gold Collection. Spencer S. Eccles Health Sciences Library.|url-status=live|archive-url=|archive-date=|access-date=20 November 2019}}</ref> It can also be diagnostically tested by doing a video-head impose test (VHIT). In this diagnostic test, a person wears highly sensitive goggles that detect rapid changes in eye movement. This test can provide site-specific information on vestibular system and its function.<ref>{{cite journal | vauthors = McGarvie LA, MacDougall HG, Halmagyi GM, Burgess AM, Weber KP, Curthoys IS | title = The Video Head Impulse Test (vHIT) of Semicircular Canal Function - Age-Dependent Normative Values of VOR Gain in Healthy Subjects | journal = Frontiers in Neurology | volume = 6 | pages = 154 | date = 2015-07-08 | pmid = 26217301 | pmc = 4495346 | doi = 10.3389/fneur.2015.00154 }}</ref>
 
Another way of testing the VOR response is a [[caloric reflex test]], which is an attempt to induce [[physiologic nystagmus|nystagmus]] (compensatory eye movement in the absence of head motion) by pouring cold or warm water into the ear. Also available is bi-thermal air caloric irrigations, in which warm and cool air is administered into the ear.{{Citation needed|date=May 2020}}
 
== Role in diagnosing brainstem death ==
The vestibulo-ocular reflex can be tested by the aforementioned [[caloric reflex test]]; this plays an important part in confirming diagnosis of brainstem death. A code of practice must be followed in this process, namely that of the Academy of Medical Royal Colleges.<ref>{{Cite journal|last=Oram|first=John|last2=Murphy|first2=Paul|date=2011-06-01|title=Diagnosis of death|url=https://academic.oup.com/bjaed/article/11/3/77/257231|journal=Continuing Education in Anaesthesia Critical Care & Pain|language=en|volume=11|issue=3|pages=77–81|doi=10.1093/bjaceaccp/mkr008|issn=1743-1816}}</ref>
 
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== See also ==
*[[Caloric reflex test]]
*[[Image stabilization]]
*[[Pursuit movement]]
*[[Semicircular canals]]
*[[Vestibular system]]
*[[Vestibulocerebellar syndrome]]
 
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* [https://web.archive.org/web/20160304073046/http://headimpulse.com/ (Video) Head Impulse Testing site] (vHIT) Site with thorough information about vHIT
* [https://web.archive.org/web/20160303232050/http://edboyden.org/03.09.boyden.html Motor Learning in the VOR in Mice] at edboyden.org
* {{MeshName|Vestibulo-Ocular+Reflex}}▼
* [http://www.jhu.edu/strucfunc/Archives/2004_files/2004_11_30.pdf Review on VOR adaptation via slides] {{Dead link|date=April 2015}} 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"
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