Influence of target direction, luminance and velocity on monocular horizontal optokinetic nystagmus.

The present investigation demonstrates that the slow phase of human monocular horizontal optokinetic nystagmus (OKN) stimulated during scotopic vision does not depend on target direction but does depend on target luminance and velocity. OKN was elicited by a horizontally moving vertical grating projected inside a modified Goldman's perimeter hemisphere. Each of six target velocities (20, 40, 60, 80, 100 and 120 deg/s) were tested on each subject with step-wise increased target luminance. Eye movement was recorded by DC EOG. Both right and left target directions were tested. It was found that there is no directional preference in OKN gain (slow phase velocity/stimulus velocity). For the target velocities 20-80 deg/s, OKN gain increases with increasing target luminance with an exponential decline. The exponential luminance constant increases as target velocity increases. The maximum OKN gain decreases as target velocity increases. The threshold luminance needed to initiate OKN increases with increasing target velocity. A linear relationship was found between OKN gain and target luminance for the target velocities 100 deg/s and 120 deg/s.