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Deepika Sridhar, Harold E. Bedell; Effect Of Vergence Velocity On Perceived Egocentric Visual Direction. Invest. Ophthalmol. Vis. Sci. 2011;52(14):5713.
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Hering’s laws of visual direction predict that complete information about the position of each eye contributes to perceived egocentric visual direction (EVD) during both binocular and monocular viewing. Previous studies confirmed this prediction for binocular asymmetric vergence, but suggest that the deviation of one eye during monocular viewing (phoria) generates a smaller-than-expected influence on perceived EVD. We investigated whether the different contributions of eye-position information to perceived EVD during binocular and monocular viewing can be explained by an under-registration of eye-position information during slow-velocity phoric eye movements, compared to the faster asymmetric vergence.
Seven subjects binocularly viewed a bright ‘cross’ on a black background at a distance of 50 cm in a dark room. The cross was presented with asymmetric vergence demands of 0, +/-6 or 12 PD, which were introduced either instantaneously to stimulate step vergence or by 0.75 or 1.5 deg/s ramp motion, to approximate the velocity of phoric eye movements. Subjects pointed at the cross to indicate its perceived location. Vergence accuracy was confirmed using dichoptic Nonius lines. The contribution of each eye’s position information to perceived EVD was estimated in each viewing condition from the slope of straight line fit to the pointing errors as a function of the vergence demand.
Pointing errors were in the direction of the eye deviation in all viewing conditions. However, the change in the pointing error with an increase in the asymmetric vergence demand did not differ significantly for the step- and ramp-vergence wave forms. In 4 subjects with phorias ≥ 2.5 PD, pointing errors measured during monocular viewing were mostly in the direction of the deviated eye and generally smaller than in the asymmetric vergence conditions.
The reduced contribution to perceived EVD of a change in one eye’s position during monocular compared to binocular viewing can not be accounted for by a difference in the vergence eye velocity.
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