Optokinetic Nystagmus (OKN), Optokinetic Response
Optokinetic nystagmus (OKN) is familiar to anyone who has watched a railway passenger observing passing telegraph poles from the window
of a moving train: OKN is an involuntary rhythmic eye movement induced by observing moving stimuli. In clinical practice a striped drum serves to test both visual pursuit and saccades. Rotation of the stripe to the left produces leftward pursuit, followed by a compensatory saccade to the right, followed by pursuit to the left of the next stripe, with another compensatory saccade, and so on. Hence, OKN is a physiological nystagmus.
Parietal hemisphere lesions (vascular or neoplastic) typically impair OKN. Testing for OKN may be useful in patients with suspected hysterical visual loss, since OKN cannot occur unless visual function is present; the response is lost in blindness. An internuclear ophthalmoplegia may be made more evident by testing OKN.