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C.M. Knapp, R.J. McLean, F.A. Proudlock, I. Gottlob; The Effect of Contrast and Target Velocity on Vertical Optokinetic Nystagmus . Invest. Ophthalmol. Vis. Sci. 2004;45(13):2520.
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Purpose:Horizontal optokinetic nystagmus (OKN) has been widely investigated, with the general agreement that in the normal adult visual system, there is no horizontal asymmetry. There is however, controversy concerning vertical OKN with some authors describing a higher slow phase gain for upward moving stimuli, some for downwards moving stimuli and others describe no asymmetry at all. Various recording methods and stimuli have been used. We have investigated vertical OKN generated viewing gratings of varying contrast at linear velocities of 20°/s and 40°/s. Methods:The right eye of fourteen healthy volunteers, were tested, using a 2–alternative forced choice preferential looking technique, to determine a 70% correct contrast threshold.The subjects viewed a sinusoidal contrast grating at 33cm covering a visual field of ±20o. Eye movements were recorded using a high–resolution infrared pupil tracker (EyeLink, Sensomotoric Instruments). Four different target contrasts were presented in all directions at target velocities of 20°/s and 40°/s; threshold contrast (70% correct detection), supra–threshold contrast (2 x 70% correct detection), medium contrast and high contrast. Slow phase OKN vertical gains were estimated. Results: Vertical OKN slow phase gains were asymmetric with a downward preference for medium and high contrast gratings moving at 40°/s (P<0.05). However, there were no vertical asymmetries for threshold or supra–threshold gratings at 40°/s or at any contrast for targets moving at 20°/s. There were no horizontal asymmetries for any contrast level at either 20°/s or 40°/s. Conclusions: Our data stands in contrast to several recent reports suggesting that normal subjects have an asymmetry with a preference for upward movement. Possible causes of these differences could be the size and shape of the target stimulus or possibly the close proximity of the observer to the stimulus.
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