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C.M. Knapp, F.A. Proudlock, M. Surendran, R.J. McLean, I. Gottlob; The Effect of Target Proximity and Accommodation on Vertical OKN Symmetry . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2496.
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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.
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).
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).
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.
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