Vertical optokinetic nystagmus and after-responses during backward tilt

Correia MJ, Kolev OI, Rupert AH, Guedry FE
Aviat Space Environ Med 1997; 68:289-95

Abstract

Method: Vertical optokinetic nystagmus (VOKN) and after-responses were detected in nine subjects using the corneo-retinal potential (CRP) technique and an infrared video camera detection apparatus (ISCAN) simultaneously. The ISCAN method produced a much smaller inter-subject variability, a higher linear regression coefficient (0.94) when vertical eye position was regressed against vertical target position (±30°, 5° increments). Detected by ISCAN, VOKN responses were measured at 3 angles of pitch head (and body) tilt: upright (0°), supine (90°), and declined 45° below horizontal (135°). Two stripe velocities (40° · s-1 and 60° · s-1) were used. Results: In six of the nine subjects (67%) and in 40/54 conditions (74%) for all subjects, upward (slow phase velocity up) VOKN gain was greater than downward VOKN gain for both stripe velocities at all tilt angles. The gain for both upward and downward VOKN decreased as stripe velocity increased from 40-60° · s-1, which suggests that both upward and downward VOKN systems were starting to saturate. Across subjects, a mean up-down asymmetry index, l, increased monotonically as the subjects declined. The slope of the monotonic function was greater for 60° · s-1 stripe velocity than for 40° · s-1 stripe velocity. The mean of all subjects' individual asymmetry ratios (ASYM) also increased as tilt increased but the slope of the best fitted regression line was not statistically significantly different from zero (p > 0.05). Conclusion: Optokinetic after-responses observed in the present study were of two types: 1) resetting of the eye by a drift, with nystagmus superimposed, from a beating field (eye position) that occurred during optokinetic stimulation; and 2) resetting of the eye without nystagmus superimposed. Upward VOKN produced the greatest number of after- responses. The beating field of VOKN was not correlated with pitch tilt.

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Table of Contents for Volume 68, Number 4 of the ASEM journal.