Human optokinetic after-nystagmus: variability in serial test results.

Test-retest variability of values for directional asymmetry in primary and secondary horizontal optokinetic after-nystagmus (OKAN I and OKAN II, respectively) was studied in 16 apparently healthy subjects. OKAN was induced by 60 s of whole-field optokinetic stimulation at speeds of 60 degrees/s and 90 degrees/s in either direction (left and right), each subject being tested on the same respective weekday once a week for 4 consecutive weeks. Values for directional asymmetry were calculated as the relative side-difference between response to drum rotation toward the right and toward the left. The subjects manifested considerable variation in values for directional asymmetry in OKAN I. This suggests prediction of a given individual's true value for directional asymmetry in OKAN I to require several measurements. On the other hand, 15/16 subjects manifested no asymmetry in OKAN II (the 16th subject was a further investigation found to have significant asymmetric caloric responses). As both OKAN I and OKAN II are known to reflect asymmetric vestibular function it is suggested that studying OKAN II may require fewer measurements of directional asymmetry, compared with studying OKAN I, when assessing the course of peripheral vestibular asymmetry.