An asymmetry in the optokinetic nystagmus response in normal individuals.

Optokinetic nystagmus (OKN) was induced by using a modified projector system to provide a moving pattern of irregular spots of light. A systematic electro-oculographic study of horizontal and vertical OKN for a series of stimulus velocities from 10 to 100 degrees s-1 was conducted on 10 healthy young subjects. It was found that the vertical responses, in particular to upward-moving stimuli, were less well sustained at velocities above 30 degrees s-1 than the horizontal responses. In view of these observations, care should be taken in the interpretation of OKN results clinically, and only when the response is absent in one direction or an extreme asymmetry is present should clinical significance be attached. This applies particularly for the vertical OKN responses. Clearly, the use of stimulus velocities below 40 degrees s-1 is advised and the standard suggested by Holm -Jensen and Peitersen [Acta oto-lar. 88, 110-116 (1968)] of a target moving at 20 degrees s-1 seems a reasonable choice.