Abstract
Purpose: :
Vertical OKN asymmetry is not clearly defined. Recent published literature suggests an upward preference, whilst older papers suggest no vertical OKN asymmetry or even a downward asymmetry. Studies in our laboratory suggest normal subjects have no vertical OKN asymmetry when viewing a target at 33cm. We investigate the effect of target proximity and accommodation on the symmetry of vertical look and stare OKN.
Methods: :
16 normal healthy volunteers viewed two different sinusoidal OKN gratings (5 cycles/second) on a translucent tangent screen at 33cm, 1m and 3m. The smaller stimuli subtended 6 x 8°, whilst the larger subtended 60 x 80° at each distance. The stimuli were matched for contrast and luminance. Both targets were viewed at 33cm and 1m, however, only the 6 x 8° stimulus was viewed at 3m. To test the effect of accommodation, the 33 cm target was viewed with a full distance spectacle correction and a 3 dioptre reading addition. Eye movement recordings were made using an infrared pupil tracker (250Hz).
Results: :
There were no statistically significant horizontal or vertical OKN asymmetries, however, responses to small targets tended to show a greater downward preference than large targets (P=0.02). Accommodation and target proximity appeared to have no effect on OKN asymmetry. Although the variance in vertical asymmetries was wide, individuals asymmetries were consistent regardless of the stimuli applied (P=0.001).
Conclusions: :
Target proximity and accommodation have little effect on OKN asymmetry, however, individuals vertical OKN asymmetries are consistent regardless of target distance and size or accommodation.
Keywords: eye movements • nystagmus