Vestibulo–ocular reflex: Difference between revisions

}}[[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]], wherethat activationacts ofto thestabilize [[vestibularGaze system(physiology)|gaze]] ofduring thehead innermovement, ear causeswith [[eye movement]] (sensory)|eyedue movementto activation of the [[vestibular system]], it is also known as the Cervico-ocular reflex. ThisThe reflex functionsacts to [[image stabilization|stabilize images]] on the [[retina]]s (whenof gazethe [[eye]] during head movement. Gaze is held steadysteadily on a location) during head movement by producing eye movements in the direction opposite tothat of 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: patientspeople whosewith VOR isan impaired reflex find it difficult to read using print, because theythe cannoteyes stabilizedo thenot eyesstabilise during small head tremors, and also because damage to the VORreflex 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 otherlower animals, 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>{{Citecite 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 | s2cid = 15755814 }}</ref>

==CircuitFunction==

Another pathway (not in picture) directly projects from the vestibular nucleus through the [[ascending tract of DietersDeiter's]] 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 | s2cid = 38651254 }}</ref>

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 | s2cid = 15724175 }}</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''. UsingThis theseresults direct connections,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.

=== VOR suppression ===

DuringWhen head-freea pursuitperson tracks the movement of movingsomething with both their eyes and head targetstogether, the VOR is counterproductive to the goal of reducingkeeping retinalthe gaze and head angle offsetaligned. Research indicates that there exists mechanisms in the brain to suppress the VOR using the active visual (retinal) feedback obtained by watching the object in motion.<ref>{{Citecite 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 duringwhen the object passes behind an opaque occlusionsbarrier, wehumans usecan continue to visually track the apparent position of the object using anticipatory (extra-retinal) signalssystems towithin supplementthe ourbrain, pursuitand movementsthe byVOR is also suppressed during this activity. The VOR suppressioncan even be cognitively suppressed, such as when following an imagined target with the eyes and head together, although the effect tends to be less dramatic than with visual feedback.<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===

If the gain of the VOR is wrong (different from 1)&mdash;for—for example, if eye muscles are weak, or if a person puts on a new pair of eyeglasses&mdash;theneyeglasses—then head movement results 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 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>

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>{{Citecite 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 imposeimpulse 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 | doi-access = free }}</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}}

</ref> and image on the retina, through adjustments of gaze impacted by neck and, or head movements or rotations. The process works in conjunction with the vestibulo-ocular reflex (VOR).<ref>Kelders, W P A ; Kleinrensink; G J , van der Geest, J N ; Feenstra, L ; de Zeeuw, C I ; Frens, M. (November 2003). Compensatory increase of the cervico-ocular reflex with age in healthy humans. [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2343479/]</ref> It is conspicuous in certain animals that cannot move their eyes much, such as owls.<ref>{{cite journal |last1=Money |first1=K.E |last2=Correia |first2=M.J |date=June 1972 |title=The vestibular system of the owl |url=https://linkinghub.elsevier.com/retrieve/pii/0300962972901168 |journal=Comparative Biochemistry and Physiology Part A: Physiology |language=en |volume=42 |issue=2 |pages=353–358 |doi=10.1016/0300-9629(72)90116-8|pmid=4404369 }}</ref>