Effect of Pitch Tilt on Vertical Optokinetic Nystagmus

Vertical optokinetic nystagmus (VOKN) and VOKN after-responses were measured simultaneously in nine subjects using the corneo-retinal potential (CRP) technique and an infrared video-camera detection apparatus (ISCAN). The ISCAN method produced a much smaller intersubject variability, a higher linear regression coefficient (0.94) when vertical eye position was regressed against vertical target position (6 subjects; ±30°, 5° increments), and VOKN gains comparable to the scleral search coil method. Detected by ISCAN, VOKN responses were measured at three angles of pitch head (and body) tilt: upright (0°), supine (90°), and declined 45° below horizontal (135°). Two stripe velocities (407s and 607s) were used. In six of the subjects, upward (slow-phase velocity up) VOKN gain (eye velocity/stripe velocity) was greater than downward (slow-phase down) VOKN gain for both stripe velocities at all tilt angles. The gain for both upward and downward VOKN decreased as stripe velocity increased from 40 to 607s, which suggests that both upward and downward VOKN systems were starting to saturate. Across subjects, a mean up-down asymmetry index (I) increased monotonically as the tilt angle increased. The slope of the monotonic function was greater for 607s stripe velocity than for 407s stripe velocity. The mean of all subjects' individual asymmetry ratios (ASYM), also increased as tilt increased. Optokinetic after-responses observed in the present study were of two types: 1) resetting of the eye from a beating field (eye position) that occurred during optokinetic stimulation with nystagmus superimposed, and 2) resetting of the eye without nystagmus superimposed. Upward VOKN produced the greatest number of after-responses. The beating field of VOKN during optokinetic stimulation was not correlated with pitch tilt.