The vestibulo-ocular reflex refers to the detection of the rotation of the head.
This triggers an inhibitory signal to the extraocular muscles on one side and an excitatory signal to the muscles on the other side:resulting is a compensatory movement of the eyes.
The vestibulo-ocular reflex (VOR) stabilizes gaze during head movement, with eye movement due to activation of the vestibular system.
The VOR stabilizes images on the retinas of the eye during head movement.
VOR holds gaze steady on a location, by producing eye movements in the direction opposite to head movement.
As the head moves to the right, the eyes move to the left, so that the image a person sees stays the same even though the head has turned.
VOR is necessary for stabilizing vision.
Impaired VOR makes reading difficult
because the eyes do not stabilize during small head tremors, and also because damage to reflex can cause nystagmus.
The VOR can be activated by hot or cold stimulation of the inner ear, the site of the vestibular system.
The VOR works in total darkness or when the eyes are closed.
VOR in the presence of light adds a fixational reflex.
Humans have semicircular canals, neck muscle stretch receptors, and the utricle.
The semicircular canals cause most of the reflexes which are responsive to acceleration.
The maintenance of balance is mediated by the stretch of neck muscles and the pull of gravity on the utricle of the inner ear.
VOR has both rotational and translational aspects, so that when head rotates about any axis (horizontal, vertical, or torsional) visual images are stabilized by rotating the eyes about the same axis, but in the opposite direction.
During walking, the visual fixation point is maintained by rotating gaze direction in the opposite direction.
The VOR is driven by signals arising from the vestibular system of the inner ear.
The semicircular canals detect head rotation and provide the rotational component of VOR.
The otoliths detect head translation and drive the translational component.
The horizontal rotational signal component travels via the vestibular nerve through the vestibular ganglion and ends in the vestibular nuclei in the brainstem.
From these vestibular nuclei in the brainstem fibers cross to the abducens nucleus of the opposite side of the brain.
From the abducens additional pathways result: One pathway projects directly to the lateral rectus muscle of the eye via the abducens nerve.
A nerve tract from the abducens nucleus via the medial longitudinal fasciculus to the oculomotor nucleus of the opposite side contains motor neurons that drive eye muscle activity, specifically activating the medial rectus muscle of the eye through the oculomotor nerve:
These abovebdirect pathways drive the velocity of eye rotation.
An indirect pathway exists that builds up the position signal needed to prevent the eye from rolling back to center when the head stops moving.
The neural integrator for horizontal eye position is found in the nucleus prepositus hypoglossi of the medulla, and the neural integrator for vertical and torsional eye positions is found in the interstitial nucleus of Cajal in the midbrain.
Neural integrators also generate eye position for other conjugate eye movements such as saccades and smooth pursuit.
When the head is turned clockwise excitatory impulses are sent from the semicircular canal on the right side via the vestibular nerve through Scarpa’s ganglion and end in the right vestibular nuclei in the brainstem.
From the right vestibular nuclei excitatory fibers cross to the left abducens nucleus.
These fibers stimulate the lateral rectus of the left eye via the abducens nerve.
The medial longitudinal fasciculus and oculomotor nuclei activate the medial rectus muscles on the right eye.
As a result, both eyes will turn counter-clockwise.
Some neurons from the right vestibular nucleus directly stimulate the right medial rectus motor neurons, and inhibits the right abducens nucleus.
The vestibulo-ocular reflex to provide clear vision, head movements must be compensated almost immediately;
Semicircular canal signals useonly three neurons: three neuron arc.
The rapid vestibule- ocular reflex results in eye movements that lag head movement by less than 10 ms, and is one of the fastest reflexes in the human body.
The VOR can disrupted by the alocohol ingestion reducing dynamic visual acuity.
This VOR rcan be tested by the rapid head impulse test: 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.
If the right balance system function is reduced, a quick head movement to the right, the patient cannot fixate a point in space during this rapid head movement.
The head impulse test can be done as a screening tool for problems with the vestibular system.
A video-head impose test (VHIT): highly sensitive goggles that detect rapid changes in eye movement providing site-specific information on vestibular system and its function.
In the VOR response to a caloric reflex test, which attempts to induce nystagmus by pouring cold or warm water into the ear.
The vestibulo-ocular reflex can be tested and plays an important part in confirming diagnosis of brainstem death.
The cervico-ocular reflex, works with VOR to achieve stabilization of a visual target, and image on the retina, through adjustments of gaze impacted by neck and, or head movements or rotations.