Ups and downs of optokinetic nystagmus

Optokinetic nystagmus (OKN) is an oculomotor reflex closely linked to the vestibular system which contributes to the stabilisation of retinal images. During combined vestibular and optokinetic stimulation, which occurs during natural situations of self rotation, the optokinetic input takes over as the vestibular drive declines.1 When a continuously moving stimulus is viewed, a characteristic eye movement pattern consisting of a slow phase in the direction of the stimulus and a fast phase in the opposite direction is elicited. In primates, OKN represents the responses of both the smooth pursuit and optokinetic system. The response to a full field moving visual stimulus has two phases. The first response, reflecting mainly smooth pursuit, promptly generates the nystagmus within 1–2 seconds of stimulus onset. The slow phase velocity approximates stimulus velocity. The second response corresponds to a slower buildup of stored neuronal activity. The neural pathway controlling OKN involves the cortex, brainstem, and cerebellum.2-5 Anatomical pathways of the OKN are known from animal studies, lesions in humans and, recently, by functional imaging. A recent study6 using functional magnetic resonance imaging (fMRI) …