Vestibulo-Ocular Reflex

The vestibulo-ocular reflex (VOR) is a reflex, where activation of the vestibular system causes eye motion. This reflex functions to stabilize images on the retinas (in yoked vision) during head motion by producing eye motions in the instructions opposite to head movement, hence maintaining the image on the center of the visual field(s). For instance, when the head moves to the right, the eyes relocate to the left, and vice versa. Since slight head movement is present all the time, the VOR is crucial for stabilizing vision: patients whose VOR suffers find it hard to read using print, since they can not support the eyes during little head tremors, and also because damage to the VOR can cause vestibular nystagmus.

What Is Vestibulo-Ocular Reflex?

The VOR does not depend on visual input. It can be elicited by calorie (hot or cold) stimulation of the inner ear, and works even in total darkness or when the eyes are closed. However, in the presence of light, the fixation reflex is likewise contributed to the motion.

In other animals, the gravity organs and eyes are strictly linked. A fish, for example, moves its eyes by reflex when its tail is moved. Human beings have semicircular canals, neck muscle “stretch” receptors, and the utricle (gravity organ). Though the semicircular canals cause the majority of the reflexes which are responsive to velocity, the keeping of balance is mediated by the stretch of neck muscles and the pull of gravity on the utricle (otolith organ) of the inner ear.

Vestibulo–ocular reflex

Vestibulo-Ocular Reflex has both rotational and translational elements. When the head turns about any axis (horizontal, vertical, or torsional) remote visual images are supported by turning the eyes about the exact same axis, however in the opposite instructions. When the head translates, for example during walking, the visual fixation point is preserved by turning look instructions in the opposite direction, by a quantity that depends on distance.

How Does Vestibulo-Ocular Reflex Work

The VOR is eventually driven by signals from the vestibular device in the inner ear. The semicircular canals spot head rotation and drive the rotational VOR, whereas the otoliths spot head translation and drive the translational VOR. The main “direct path” neural circuit for the horizontal rotational VOR is relatively easy. It starts in the vestibular system, where semicircular canals get activated by head rotation and send their impulses through the vestibular nerve (cranial nerve VIII) through Scarpa’s 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 tasks straight to the lateral rectus of eye via the abducens nerve. Another nerve tract tasks from the abducens nucleus by the medial longitudinal fasciculus to the contralateral oculomotor nucleus, which contains motorneurons that drive eye muscle activity, specifically triggering the medial rectus muscle of the eye through the oculomotor nerve.

Another pathway directly jobs from the vestibular nucleus through the ascending tract of Dieters to the ipsilateral median rectus motoneuron. In addition there are repressive vestibular pathways to the ipsilateral abducens nucleus. However no direct vestibular neuron to medial rectus motoneuron pathway exists.

Similar pathways exist for the vertical and torsional parts of the VOR.

In addition to these direct paths, which own the velocity of eye rotation, there is an indirect path that develops the position signal had to prevent the eye from rolling back to center when the head stops moving. This pathway is especially important when the head is moving slowly, due to the fact that here position signals dominate over velocity signals. David A. Robinson found that the eye muscles need this double velocity-position drive, and also proposed that it must emerge in the brain by mathematically integrating the velocity signal then sending out the resulting position signal to the motoneurons. Robinson was appropriate: the ‘neural integrator’ for horizontal eye position was discovered in the nucleus prepositus hypoglossi in the medulla, and the neural integrator for vertical and torsional eye positions was discovered in the interstitial nucleus of Cajal in the midbrain. The exact same neural integrators also generate eye position for other conjugate eye movements such as saccades and smooth pursuit.

Excitatory example

For example, if the head is turned clockwise as seen from above, then excitatory impulses are sent out from the semicircular canal on the right side by means of 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 forecast and promote the lateral rectus of the left eye through the abducens nerve. In addition, by the median longitudinal fasciculus and oculomotor nuclei, they trigger the medial rectus muscles on the right eye. As a result, both eyes will turn counterclockwise.

Additionally, some nerve cells from the right vestibular nucleus straight promote the right medial rectus motoneurons, and prevents the right abducens nucleus.

Speed of Vestibulo-Ocular Reflex

The vestibulo-ocular reflex needs to be quick: for clear vision, head movement must be compensated nearly instantly; otherwise, vision corresponds to a photograph taken with an unsteady hand. To achieve clear vision, signals from the semicircular canals are sent as directly as possible to the eye muscles: the connection involves just 3 neurons, and is similarly called the 3 nerve cell arc. Using these direct connections, eye motions lag the head movements by less than 10 ms, and hence the vestibulo-ocular reflex is among 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 modification in the head angle during the head turn. Preferably the gain of the rotational VOR is 1.0. The gain of the horizontal and vertical VOR is usually near to 1.0, but the gain of the torsional VOR (rotation around the line of sight) is normally low. The gain of the translational VOR needs to be changed for range, due to the fact that of the geometry of motion parallax. When the head equates, the angular instructions of near targets changes much faster than the angular direction of far targets.

If the gain of the VOR is wrong (different from 1) – for instance, if eye muscles are weak, or if a person places on a new set of spectacles – then head motion leads to image movement on the retina, leading to blurred vision. Under such conditions, motor learning changes the gain of the VOR to produce more accurate eye movement. This is what is described as VOR adaptation.

Ethanol consumption can disrupt the VOR, lowering dynamic visual acuity.

Vestibulo-Ocular Reflex Test

This reflex can be evaluated by the quick head impulse test or Halmagyi-Curthoys test, in which the head is quickly moved to the side with force, and is managed if the eyes succeed to remain looking in the exact same instructions. When the function of the right balance system is lowered, by a disease or by a mishap, quick head movement to the right can not be picked up properly anymore. As an effect, no countervailing eye motion is created, and the patient can not focus a point in space during this quick head motion.

Another way of testing the VOR action is a caloric reflex test, which is an attempt to induce nystagmus (offsetting eye motion in the absence of head movement) by pouring cold or warm water into the ear. Also readily available is bi-thermal air calorie irrigations, in which warm and cool air is administered into the ear.

Comatose patients

In comatose patients, when it has been determined that the cervical spinal column is intact, a test of the vestibulo-ocular reflex can be carried out by turning the head to one side. If the brainstem is undamaged, the eyes will move conjugately far from the direction of turning (as if still taking a look at the examiner rather than repaired directly ahead). Unfavorable “doll’s eyes” would remain fixed midorbit, and having unfavorable “doll’s eyes” is therefore a sign that a comatose patient’s brainstem is functionally not intact.

Testing complications

Presently, vestibulo-ocular reflexes can only be adequately tested in specifically equipped laboratories. The tests often supply important diagnostic information; nevertheless, the tests can be lengthy and pricey to administer [citation needed] The scleral search coil can be used to examine the vestibulo-ocular reflex.

Role of cerebellum

The cerebellum is essential for motor discovering how to remedy the VOR in order to make sure accurate eye movement. Motor discovering in the VOR is in lots of ways analogous to classical eyeblink conditioning, because the circuits are homologous and the molecular systems are comparable.

Reyus Mammadli (Eyexan Team Leader) / author of the article
Bachelor in biomedical and electrical apparatus and systems. For more than 20 years he has been studying methods to improve health using affordable and safe methods. Collaborates with eye care charity organization of the CCP. Specialization is a vision correction by laser surgery, including LASIK.
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