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S.J. Farooq, F.A. Proudlock, I. Gottlob; The Effect of Central and Peripheral Stimulation on the Gain of Torsional Optokinetic Nystagmus . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2737.
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Background: The effect of central and peripheral stimulation on torsional OKN (tOKN) has been investigated in only one previous study. This study concluded that central field stimulation was important in evoking the tOKN response.The horizontal OKN response has been shown not to be significantly affected in patients with central scotomas and this has been partially attributed to a 'filling-in' response occurring. Purpose: To investigate the effect of varying degrees of central and peripheral stimulation on tOKN gain and the influence of filling-in on tOKN. Methods: Torsional OKN was elicited by a sinusoidal grating pattern that subtended a maximum of 50.8° in diameter. The stimulus was rotated in the clockwise direction at constant velocities of 40°/s and 400°/s. To investigate the effects of central stimulation, the entire field of stimulation was varied from 2.86° to 50.8°. To assess varying degrees of peripheral stimulation on the tOKN response, an artificial central scotoma (black round spot) was incorporated into the largest size stimulus to occlude its central portion. The black spot was varied in size from 2.86° to 43.2° in diameter. A response box was used by the subjects to indicate when they ‘filled-in’ the missing central portion of the stimulus. Eight subjects were used in each experiment and torsional eye movements were recorded using a pupil and iris tracking video oculography technique. Results: The smallest field size rotating at 40°/s evoked only 10% (0.0054) of the gain elicited by the full field display (0.054). At 400°/s, the gain elicited to the smallest field size was 29% (0.0015) of the gain obtained by the largest stimulus (0.0052). When the largest central black spot occluded the centre of the full field display, at 40°/s tOKN gain was maintained at 50% (0.03) of the gain evoked when no central occluder was used (0.06). At 400 °/s, the gain was 17.5% (0.0014) of the gain evoked without any central occlusion (0.0080). There was no difference in tOKN gain when the subjects ‘filled- in’ the area of the artificial scotomas with the peripheral stimulus to when they did not. Conclusions: Peripheral field stimulation contributes to a large proportion of the tOKN response and this is probably because the degree of retinal slip of the tOKN stimulus is greatest the further the stimulus is presented away from the central retina. Filling-in of the peripheral stimulus did not have any recognisable effect on the eye movements generated although there maybe other visual areas which are active while this occurs.
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