IN-vision

Under some circumstances, nystagmus is a normal behaviour of the oculomotor system.

Optokinetic nystagmus (OKN)

OKN is the normal response of the visual system to a moving visual scene (a common example is looking out of a moving train’s window). We can fixate an object outside the window as it drifts past. When the eyes reach the limit of their movement in the direction that the object is moving, a saccade is made in the opposite direction in order to refixate on a new object. This repeated pattern of drifting and refixation allows the visual scene to be partially stabilised so that more visual information can be gained than if we were to hold our eyes in a fixed position (which would cause the perception of a blurred image).

Vestibulo-ocular reflex (VOR) nystagmus

VOR is the visual reflex which allows stabilisation of the visual scene during rotational head movements (for example, when spinning on a chair or carousel). Rotation of the head causes movement of the fluid within the semi-circular canals of the inner ear. This results in a nystagmus with slow-phases (slow eye movements) in the same direction as the head rotation and quick-phases (fast eye movements) in the opposite direction. If rotation is stopped then the direction of the nystagmus will reverse with slow-phases opposing the previous direction of rotation and the quick-phases in the same direction as the preceding rotation. This after-effect occurs because the vestibular system is sensitive to changes in acceleration; ceasing rotation causes the perception of acceleration in the opposite direction.

End point nystagmus

End point nystagmus occurs when the eyes are forced to move to the extreme far point of their lateral (sideways) movement. This results in an oscillation of the eyes which will disappear if the extremity of the gaze is reduced. End-point nystagmus is chiefly caused by the shortcomings of the human ‘neural integrator’. Top of page