Vestibulo-ocular reflex.

The vestibulo-ocular reflex (VOR) ensures best vision during head motion by moving the eyes contrary to the head to stabilize the line of sight in space. The VOR has three main components: the peripheral sensory apparatus (a set of motion sensors: the semicircular canals, SCCs, and the otolith organs), a central processing mechanism, and the motor output (the eye muscles). The SCCs sense angular acceleration to detect head rotation; the otolith organs sense linear acceleration to detect both head translation and the position of the head relative to gravity. The SCCs are arranged in a push-pull configuration with two coplanar canals on each side (like the left and right horizontal canals) working together. During angular head movements, if one part is excited the other is inhibited and vice versa. While the head is at rest, the primary vestibular afferents have a tonic discharge which is exactly balanced between corresponding canals. During rotation, the head velocity corresponds to the difference in the firing rate between SCC pairs. Knowledge of the geometrical arrangement of the SCCs within the head and of the functional properties of the otolith organs allows to localize and interpret certain patterns of nystagmus and ocular misalignment. This is based on the experimental observation that stimulation of a single SCC leads v ia the VOR to slowphase eye movements that rotate the globe in a plane parallel to that of the stimulated canal. Furthermore, knowledge of the mechanisms that underlie compensation for vestibular disorders is essential for correctly diagnosing and effectively managing patients with vestibular disturbances.