To obtain a fine resolution of visual field during motion, either self–motion (translational or rotational) or object motion, precise control of eye movements is required. These oculomotor movements are controlled at various levels from end organ (retinal and extraretinal sources) to supranuclear foci (motor planning stage).
Movements of the head during routine activities include angular rotation and linear acceleration. The oculomotor system maintains gaze fixed in space during these head movements by using vestibulo–ocular reflex (by semi-circular canals) and ocular counter-rolling reflex (by otoliths).
Vestibulo–ocular reflex is an involuntary reflex which stabilizes the visual field and retinal image during head motion by producing eye movements in a counter direction.
Reflex arc involved in the maintenance of stable visual field includes:
The vestibular system consists of semi-circular canals which transduce angular motion as during rotation of head and otoliths (utricle and saccule) which transduce linear motion of head as during head tilt and roll.
Angular acceleration of head stimulates the hair cells of semi-circular canals and results in eye rotations that are roughly equal and opposite the head's motion; this stabilization reflex has a brief latency of 7 to 15 msec and is accurate for head turns at velocities over 300 deg/sec. Head rotations about the horizontal, vertical and nasal–occipital axes produce vestibulo–ocular reflexes with horizontal, vertical and torsional counter-rotations of the eye, seen as the slow phase of the nystagmus.
These reflex eye movements must maintain a stable retinal image to be effective. However, the axis of rotation of the head is the neck and not the center of the eye; hence, the eye rotates and translates as well. This exacerbates during the near vision, hence the gain of the vestibulo-ocular reflex increases with convergence, leading to more eye movement than head movement.
However, the vestibulo–ocular reflexes are far from perfect, and yet the objects appear stable during head rotation without oscillopsia, which indicates that supranuclear foci of the visual system anticipate residual retinal image motion occurring due to the inaccuracy of the compensatory eye movements during head rotation and corrects for the same.
Head rotation also produces retinal image motion of the visual field, which stimulates reflex eye movements with a slow phase following the moving field interrupted by resetting saccades. This reflex is known as optokinetic nystagmus, and this complements the vestibulo–ocular reflex during low velocity sustained head movements such as walking.
Neuro-control of stabilization reflexes
The three semi-circular canals are the end organs converting the head motion signals into a neural stimulus driving vestibulo–ocular reflex. The hair cells in the horizontal canals undergo depolarization when the endolymph moves towards ampulla and vice versa in vertical canals. The anterior semi-circular canal of one side pairs with the posterior semi-circular canal of the other side. They act as opponent pairs such that, stimulation of one canal causes inhibition of opponent canal. The three semi-circular canals lie in the same plane as the extraocular muscles. Thus the horizontal semicircular canals lie in the plane of the lateral and medial recti; the left anterior semi-circular canal and right posterior semi-circular canal are parallel to the muscle planes of the left eye vertical recti and the right eye obliques and vice versa. Each canal excites a pair of muscles and inhibits a pair of muscles in its plane; its partner excites the muscles it inhibits and vice versa.
For example, impulses from the left medial vestibular nucleus pass via the right abducens nucleus causing abduction of the right eye and to left eye medial rectus via oculomotor nucleus (via interneuron connecting abducens and oculomotor nucleus) causing left eye adduction leading to conjugate eye movements.
Since there is fine involuntary head motion even when the person is still, there is a requirement of a reflex eye movement to maintain the stable visual field for maximizing visual potential, and this is achieved using various involuntary reflexes such as vestibulo–ocular reflex and optokinetic nystagmus. This accommodation is of particular importance in high demand situations such as sports-related activities.
Disruption of vestibulo–ocular reflex results in symptoms such as nausea, head tilt, imbalance during walking, and other daily life activities, dizziness, oscillopsia, and blurred vision during motion. The vestibulo–ocular reflex can be tested using various tests, including head impulse testing, rotational chair testing, velocity step test, impulse angular acceleration, and caloric reflex test. In the caloric reflex test, the examiner irrigates the external auditory canal with 20 to 40 ml of ice water leading to slow movements of the eyes towards the ear irrigated and corrective horizontal nystagmus towards the contralateral ear under the normal physiological conditions. This reflex, however, is damaged in brain stem injury.
Any patient presenting with dizziness or imbalance should be evaluated looking at both systemic conditions such as hypoglycemia, transient ischemic attack or end organ damage such as vestibular damage or trauma to limbs, etc. Simple bedside test such as caloric reflex test along with auditory symptoms such as tinnitus or hearing defects acts as a pointer towards a defect in vestibulo-ocular reflex. Vestibular rehabilitation with or without medication can help the patient depending upon the underlying condition.
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