Optokinetic after-nystagmus in man after loss or reduction of labyrinthine function - a preliminary report.

Horizontal optokinetic after-nystagmus (OKAN) was studied in normal volunteers and in patients with either unilateral or bilateral loss of labyrinthine function. Optokinetic stimulation in normal volunteers produced OKAN in the same direction as preceding optokinetic nystagmus (OKN) with a duration of 45-90 seconds and a slowly decreasing slow phase eye velocity. Similar stimulation in the bilateral labyrinthine loss group produced markedly diminished or no OKAN. In the unilateral group when the optokinetic stimulus produced OKN with fast phase to the affected ear, OKAN was markedly diminished or absent and replaced by a nystagmus which was reversed in direction to the preceding OKN. The opposite stimulus produced OKAN similar to the volunteer group. The above results support the observation that in human beings, the production of normal OKAN depends on an intact bilateral peripheral vestibular input and further demonstrates that the technique has lateralizing value in unilateral vestibular disease. These results also suggest that separate storage mechanisms are responsible for producing OKAN in the two directions, and that these storage mechanisms are decoupled from each other.