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Sowmya Ravikumar, Bjorn Vlaskamp, Martin Banks; Effect of inter-ocular differences in blur on spatial and stereo-resolution. Invest. Ophthalmol. Vis. Sci. 2013;54(15):2659.
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© ARVO (1962-2015); The Authors (2016-present)
Monovision—setting the refraction of one eye to a different value than the other—is a procedure for dealing with presbyopia. The difference in refractions for the two eyes should be the appropriate value for maximizing visual performance, but there is little research on what the appropriate value is. We know that both visual acuity and stereoacuity worsen as blur increases. However, visual-acuity tasks can in principle be performed when one eye is suppressed, but stereo tasks cannot. Thus, visual acuity might be limited by image quality in the better eye while stereoacuity might be limited by the poorer eye. Some previous observations are consistent with these expectations. Indeed, blur in one eye can be more detrimental to stereoacuity than the same blur in both eyes (blur paradox).
We compared visual acuity and stereoacuity in the same observers with the same blur manipulations. Stimuli were displayed binocularly using a custom stereoscope. Accommodation was paralyzed with cyclopentolate. An artificial pupil set the effective aperture to 4mm. Blur was introduced with ophthalmic lenses. We measured visual acuity with a letter-identification task. We measured stereoacuity using a random-dot stereogram that created sinusoidal corrugations in depth oriented +/- 20 degrees about the horizontal; subjects indicated which of two corrugation orientations was presented.
The results were consistent with our expectations. For varying amounts of blur in the two eyes, visual acuity closely conformed to the better eye’s performance while stereoacuity was largely limited by the worse eye’s blur. Monocular blur had almost no effect on visual acuity, with binocular visual acuity remaining close to 20/20 with up to 1.5D blur in one eye. For the same amount of blur, stereoacuity decreased by a factor of almost two. When the blur was equal in the two eyes, stereoacuity was slightly better than when the same blur was present in one eye only (e.g., In comparing 1.5D in both eyes to 1.5D of blur in one eye only, in one subject, stereo-acuity was 1.98 cpd compared to 1.13 cpd and in another subject, stereoacuity was 1.54 cpd compared to 1.37 cpd).
The results will help us design monovision protocols in a way that is the best compromise between visual acuity and stereoacuity for various viewing distances.
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