Vertical optokinetic nystagmus in normal individuals.

In this study, 20 subjects (5 women and 15 men, 22-28 years old) were tested for vertical optokinetic nystagmus (OKN) to establish the asymmetry between up and down OKN responses in humans. The subjects were exposed to both upward and downward OKN stimulus at constant velocities of 30, 40, 50, 60, 70, 80, 90 degrees/s in the upright position. Eye movements were recorded precisely using the magnetic search coil system in order to eliminate any methodological problem associated with the eye-lid artifact. Slow phase velocity during upward OKN stimulus was higher than that during downward OKN stimulus for each OKN velocity. It is likely that otolithic stimuli cause up and down asymmetry. Statistical significance in the gain between upward and downward OKN was noted for stimulus velocities of 30-60 degrees/s (p < 0.01), but not for higher velocities, because vertical OKN saturated around 40-50 degrees/s.