VESTIBULAR-OCULAR REFLEX (VOR)


OVERVIEW

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 in order to preserve the target image on the center of the visual field, or fovea. The VOR is effective up to a head rotation speed of 50 ͦ/sec. This allows us to comfortably adjust to everyday head movements and to see clearly when moving our head. For example, when you move your head to the right your eyes move to the left to compensate for the initial head rotation and focus the image on the fovea of your eye. Since humans are constantly moving their heads, the VOR is very important for stabilizing visions. People who have VOR dysfunction find it difficult to focus their eyes while they move their heads. Reading print and looking out a car window becomes almost impossible to them because they cannot stabilize their eyes during small and slight head tremors. The VOR does not depend on visual input, however, only on the vestibular system detecting head rotation. Because of this it works even in total
darkness or when eyes are closed.
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There are two different types of head movements that the vestibular system detects: rotational and translational movement. Both rotational and translational movement stimulate the vestibule-ocular reflex. With a rotational movement, the head moves relative to the body. Examples of this include turning the head back and forth, nodding, and bringing the ear in contact with the shoulder. Translational movements occur when the entire body (including the head) is moved in tandem. Thus, rotational vestibuloocular reflex (r-VOR) responds to angular motion of the head and results from stimulation of the semicircular canals, whereas translational vestibuloocular reflex (t-VOR) responds to linear motion of the head and results from stimulation of the otolithic organs. Some head movements may involve a combination of both translational VOR and rotational VOR.

In this wikispace we will discuss rotational VOR which responds to angular motion of the head and results from stimulation of the semicircular canals.


FUNCTIONAL ANATOMY


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The VOR works on all three muscle pairs in the eye. However, we will focus only on rotational (r-VOR) which includes the medial-lateral rectus pair, and the horizontal semicircular canals.


The diagram to the left shows the six eye muscles responsible for eye movement. These six muscles work in pairs, producing 3 muscle pairs that work together to allow for specific eye movement. In rotational VOR (r-VOR), the lateral and medial rectus work together to respond and compensate for left and right head rotation and keep the eye focused on an image.


Innervations of the Extraocular Muscles:
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The eyes are rotated by the action of six extraocular muscles, which act as three agonist/antagonist pairs allowing rotations in horizontal, vertical and torsional directions. The six extraocular muscles are controlled by three cranial nerves:
  1. 1. Oculomotor nerve (III)
  2. 2. Trochlear nerve (IV)
  3. 3. Abducens nerve (VI).



The Oculomotor nerve (III) innervates the superior and inferior recti, the inferior oblique, and the medial rectus.
The Trochlear nerve (IV) innervates the superior oblique.
The Abducens nerve (VI) innervates the lateral rectus.
The Oculomotor and Trochlear nerves originate from the midbrain. The Abducens nerve originates from the pons.


This diagram shows the medial and lateral recti and the semicircular canals. The semicircular canals are responsible for detecting movement of the endolymph
and decoding the direction of head rotation ultimately activating or inhibiting the medial-lateral recti to produce side to side eye movement and thus compensate for the initial head rotation.
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The horizontal canals in each ear are responsible for detecting side to side rotational movement of the head. Depending on the direction of the head rotation (either to the left or to the right), the horizontal canals in each ear will either be activated or inhibited. This will ultimately send signals to the medial and lateral eye muscles to compensate for the initial head movement and keep eyes fixed on the object (pathway is discussed in a later section). The lateral rectus muscle will pull the eye lateral towards the ear, and the medial rectus will pull the eye medially towards the nose, both in the horizontal plane.
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The semicircular canals:
There are 3 canals, corresponding to the three dimensions in which you move, so that each canal detects motion in a single plane. Each canal is a loop filled with a gelatinous liquid called endolymph. At the base of each canal are clusters of hair cells that sit in a small swelling called the ampula. The hair cells are arranged as a single clump that pertrudes up into the cupula.










OVERVIEW of PATHWAY
The pathway begins as a result of the initial head rotation. The head moves, either to the left or to the right, and this brings about a series of responses and signals in the vestibular system due to the movement of the endolymph in the horizontal canals.
When you turn your head, the inertia of the endolymph causes it to slosh against the cupula, deflecting the hair cells. Now, if you were to keep turning in circles, eventually the fluid would catch up with the movement of the head, and there would be no more pressure on the cupula. And the endolymph would stop deflecting the hair cells. If all of a sudden you stopped spinning, the moving fluid would slosh up against a suddenly still cupula, and you would feel as though you were turning in the other direction. Thus sudden change in head rotation will affect the endolymph and cause it to slosh against the hair cells, deflecting them and activating them. This initiates the signals that encode for head rotation.
Naturally, you have the same but mirrored arrangement on both sides of the hea, in each ear. Hair cells in the cupula will be polarized depending on which way you deflect them. If you push them one way, they will be excited, but if you push them the other way, they will be inhibited.
This means that the canals on either side of the head will generally be operating in a push-pull rhythm; when one side is excited, the other side is inhibited (see diagram below).
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It is important that both sides agree as to what the head is doing. If there is disagreement and both sides are activated or inhibited at the same time, then you will feel debilitating vertigo and nausea. This is the reason that infections of the endolymph or damage to the inner ear can cause vertigo. However, if one vestibular nerve is cut, the brain will gradually get used to only listening to one side - this can actually be a treatment for intractable vertigo.

GENERAL PATHWAY
Rotational vestibuloocular reflex

1. During rotational movements of the head, the endolymphatic fluid within the semicircular canals shifts because of its inertia, which deflects the hair cells on the cupula

  • Endolymphatic flow toward the ampulla (called ampullopetal) is excitatory in the horizontal canals and increases firing
  • Endolymphatic flow away the ampulla is inhibitory (called ampullofugal) is inhibitory in the horizonatal canals and decreases firing

2. Afferent nerves from the ampulla carry both excitatory and inhibitory signals to the 4 major vestibular nuclei: medial vestibular nucleus, lateral vestibular nucleus, inferior or descending vestibular nucleus, and superior vestibular nucleus
3. Different regions within each of the nuclei project to the oculomotor nuclei and abducens nuclei
4. Efferent signals from these nuclei then result in contraction and relaxation of the appropriate ocular muscles.

Excitation of one lateral canal, let’s say in the left ear, results in contraction of the ipsilateral medial rectus and contralateral lateral rectus muscles and relaxation of the contralateral medial rectus and ipsilateral lateral rectus muscles. This results in a horizontal eye movement toward the opposite ear, the right ear.
The vestibulocerebellum compares input from visual and vestibular sensors and mediates necessary changes in the vestibuloocular reflex.


SPECIFIC EXAMPLE
Left Head Rotation
If you move your head to the left, you will excite the left horizontal canal, inhibiting the right.
In order to keep your eyes fixed on a stationary point, you need to fire the right lateral rectus and the left medial rectus, to contract these muscle and move the eyes to the right.


The pathway is as follows:picture_8.gif

1. Head rotation to left causes endolymph to flow in the opposite direction (right), this stimulates the hair cells in the cupula, exciting the left horizontal canal.
2. The vestibular nerve picks up the signals coming from the hair cells, enters the brainstem and synapses in the vestibular nucleus.
3. Fibers from the vestibular nucleus cross over to the contralateral side and project to the abducens nucleus (VI) on the right side, to stimulate the
contralateral (right) lateral rectus.
4. They also project to the occulomotor nucleus (III) on the ipsilateral side, to stimulate the ipsilateral (left) medial rectus.

The same vestibular cells also inhibit the opposing muscles (in this case, the right medial rectus, and the left lateral rectus).

On the other side, the right horizontal canal is wired to the complementary set of muscles. Since it is inhibited, it will not excite its target muscles (the right medial
rectus and the left lateral rectus), nor will it inhibit the muscles you want to use (the right lateral rectus and the left medial rectus).


NORMAL VOR

-Neurological Exam: Normal Cranial Nerves and VOR response
Normal VOR Video
Video shows a neurological exam showing a normal VOR. In a neurological exam the VOR is obtained by having the patient visually fixate on an object straight ahead, then rapidly turning the patient’s head from side to side and up and down. In a healthy and normal VOR, the eyes should stay fixed on the object and turn in the opposite direction of the head movements.

-University of Utah Videos of Normal and Abnormal Neurological Exams
Normal and Abnormal Neurological Exam Videos
Website lists a good number of videos showing abnormal neurological exams, and abnormal eye movement control including abnormal VOR, nystagmus and other vestibular related dysfunctions.

-Normal VOR
The top picture shows a normal VOR response to head rotation to the right and left. When head is rotated to the left (picture on the left), eyes turn in the opposite direction, towards the right. When head rotates to the right (picture on the right), eyes turn to the left to compensate for head movements and keep the
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DYSFUNCTION
The importance of the VOR mechanism is vital for everyday life. A recent study discussed the importance of this mechanism to everyday life and tasks. In this article, the VOR and its necessity were described by a physician whose inner ear had been severely damaged by excessive streptomycin therapy.He could read in bed only by bracing his head against the headboard; otherwise the printed page jumped with each heartbeat. When walking he was unable to recognize faces or read signs unless he stood still. We experience what it must be like for those afflicted in this way when watching a movie shot from a vehicle being jolted by a bad road; viewers of the movie, unlike those doing the shooting, are not assisted by their VORs so that their gaze is constantly having to jump about to recapture the target with a succession of saccades.

Abnormalities in Comatose Patients:
Once it has been determined that the cervical spine is not damaged, a test of the VOR can be performed in comatose patients. This is done by turning the head to one side and observing the reactive movements of the eyes in response to the head movement. If the brainstem is intact, the eyes will move conjugately away from the direction of the head movement (as if they were still looking at the examiner rather than the fixed object straight ahead). This is called “Doll’s Eyes”, simply because it is how a doll’s eyes would move.

Having “Doll’s Eyes” is thus a sign that a comatose patient’s brainstem is still intact.

Abnormal VOR Video
Table A. Manifestations of Vestibuloocular Reflex Dysfunction Dysfunction


VOR Testing

Rapid head impulse test or Halmagyi-Curthoys-test
Head is rapidly moved to the side with force. If eyes succeed to remain looking in the same direction and fixate on one object in front of them then VOR is normal. When the function of the VOR system is damaged, by a disease or by an accident, quick head movement can no longer be sensed properly any more. When no reactive eye movement is generated to compensate for the head movement, and the patient cannot fixate a point in space during this rapid head movement then VOR is abnormal.

Caloric Reflex test
The Caloric Reflex test is an attempt to induce nystagmus by pouring cold or warm water into the ear. Nystagmus produced by water irrigation is then observed and compared to normal VOR response


REHABILITATION
Acute VOR dysfunction is disturbing and constant to a patient, present and noticed every time the head is moved. With time and as patients learn to compensate, the dysfunction becomes less debilitating and noticeable. Patients can actively promote compensation through a series of rehabilitation exercises.

VOR disturbances are treated with different forms of rehabilitation exercises. For example, patients are asked to hold an index card 1 foot from their eyes and to focus on a word or object on the index card as they rotate their head.

Other types of rehabilitation are mostly to make the patients more comfortable and make adjustments to their everyday lives and tasks to allow them to carry out a normal life, as much as possible.


SUMMARY
A large role of the semicircular canal system is to keep your eyes still in space while your head moves around them. The reason is that the semicircular canals exert direct control over the eyes, so they can directly compensate for head movements. The three pairs of muscles that control eye movements are: the medial and lateral rectus, the superior and inferior rectus, and the inferior and superior oblique. These muscles respond to the signals sent from the semicircular canals and produce compensatory reflex to keep the eyes fixed on an image during head movements. This is called the vestibulo-ocular reflex (VOR).

The human brain is one of the most organized and complex structure in the universe. The system as a whole with its subsystems, allows us to function. In this discussion, we focused on the brain and vestibular system and in their direct control over eye muscles to compensate for head rotation, by use of the vestibule-ocular reflex

Lesions, diseases and old age can cause a gradual breakdown of the vestibular system and disrupt the VOR. This causes us to begin to lose control over our compensatory eye movements and become unable to focus an image. Dizziness, nystagmus and vertigo become potential symptoms, making it impossible to see clearly. It is usually not until something goes wrong and VOR becomes abnormal, that we start to notice it. It is not until we notice something wrong with our balance, vision and perception of motion that we begin to admire the complexity of the elaborate systems that are constantly working and the importance of the vestibule-ocular reflex.


GLOSSARY
Vestibular system: a complex sensory system located in the inner ear consisting of the saccule, utricle, and semicircular canals that contributes to the sense of balance, sense of spatial orientation and allows for coordinated movement.

Conjugate eye movements: the eyes moving in unison together

Vestibulo-occular reflex (VOR): controls eye movements to stabilize images on the fovea during head movements

Semicircular canals: 3 canals (horizontal, posterior and anterior) corresponding to the three dimensions in which you move, detect angular acceleration.

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Ampulla: a flask-like dilatation of a tubular structure
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Cupula: located within the ampullae of each of the three semicircular canals. As fluid rushes by the cupula, hair cells within it sense rotational acceleration and transmit the corresponding signal to the brain the vestibulocochlear nerve (CN VIII)

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Occulomotor nerve: CN III, controls ipsilateral medial recti of the eyes
Abducens nerve: CN VI, controls contralateral lateral recti of the eyes
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Caloric reflex test: in medicine, is a test of the vestibule-ocular reflex that involves irrigating cold or warm water or air into the external auditory canal. It is done to produce nystagmus and see if the body’s response to the test is normal.

Nystagmus: abnormal eye movements to the left and the right when vestibular system is damaged; compensatory eye movement in the absence of head motion

QUIZ

1. TRUE/FALSE: The horizontal semicircular canals are involved in directing rotational VOR.
2. TRUE/FALSE: When the head rotates to the right, the endolymph in the horizontal canal flows in the same direction.
3. TRUE/FALSE: The occulomotor nerve innervates the ipsilateral medial rectus.
4. TRUE/FALSE: The six extraocular muscles are controlled by three nerves: III, IV, V.
5. TRUE/FALSE: The oculomotor and abducens nerves are involved in lateral eye movement to compensate for head rotation.
6. TRUE/FALSE: The abducens nerve innervates the contralateral lateral rectus.

7. All of the follow cranial nerves are involved in controlling the six extraocular muscles of the eye, EXCEPT:
A) VI
B) V
C) IV
D) III

8. The oculomotor nerve (III) innervates all of the following muscles, EXCEPT:
A) Superior recti
B) Medial recti
C) Inferior recti
D) Inferior oblique
E) Lateral recti

9. What region of the cereballum compares input from visual and vestibular sensors and mediates necessary changes in the vestibuloocular reflex?
A) Vestibulocerebellum
B) Spinocerebellum
C) Cerebrocerebellum
D) Thalamus

Short Answer Questions:
10. Define the terms ampullopetal and ampullofugal and describe how the direction of the flow of the endolymph affects neural firing.
11. Describe some symptoms or signs of VOR dysfunction.
12.How are comatose patients tested for VOR function? Describe the term "Doll's eyes".

Essay:
13. Describe and draw the pathway in right head rotation.

LINKS/SUGGESTED READINGS

-Article: Neural Learning Rules for the Vestibulo-Ocular Reflex
http://www.jneurosci.org/content/18/21/9112.full

-Article: Vestibulo-ocular Function during Coordinated Head and Eye Movements to Acquire Visual Targets
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1281486/
Article discusses VOR function during head and eye movements to focus visual targets on the fovea and produce
clear images.

-University of Utah Video of Neurological Exam
http://library.med.utah.edu/neurologicexam/html/cranialnerve_abnormal.html#21
Website lists a good number of videos showing abnormal neurological exams, and abnormal eye movement
control including abnormal VOR, nystagmus and other vestibular related dysfunctions.

-Caloric Test Video
http://www.youtube.com/watch?v=Vjk1f99N13M&feature=related
Video shows the VOR response after ear is irrigated with water during a caloric test.
-Video on Anatomy of Vestibular System
http://www.youtube.com/watch?v=BbKU0AbbARg&feature=related
Video discusses and describes the vestibular system.


ANSWERS
1. T
2. F
3. T
4. F
5. T
6. T
7. B
8. E
9. A


REFERENCES
1. Gurney, Peter. Our Eye Movement and Their Control: Part 2. [Online]. Answersingenesis.org. http://www.answersingenesis.org/articles/tj/v17/n1/eye. [April 1, 2003]

2. Lincoln Gray, Ph.D. Chapter 10: Vestibular System: Structure and Function. [Online]. Department of Communication Sciences and Disorders, James Madison University. http://neuroscience.uth.tmc.edu/s2/chapter10.html. [1997-Present].

3. Manali, Amin. "Vestibuloocular Reflex Testing: EMedicine Clinical Procedures." EMedicine-Medical Reference. Medscape, 2010. Web. 01 May 2010. <http://emedicine.medscape.com/article/1836134-overview>.

4. "Neural Learning Rules for the Vestibulo-Ocular Reflex -- Raymond and Lisberger 18 (21): 9112 -- Journal of Neuroscience." The Journal of Neuroscience Online. Web. 5 May 2010. <http://www.jneurosci.org/cgi/content/full/18/21/9112>.

5. Weedman, Dianna, Ph.D. Auditory and Vestibular Pathways. [Online]. Neuroscience Tutorial, Washington University School of Medicine. http://thalamus.wustl.edu/course/audvest.html. [1997].

6. Vestibulo-Ocular Reflex. [Online]. Wikipedia. http://en.wikipedia.org/wiki/Vestibulo-ocular_reflex. [October 13, 2011].