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Stereoscopic acuity as a function of induced monocular defocus measured with an adaptive optics simulator


Purpose : Different binocular approaches are used to correct for presbyopia. Many of them are based in extending binocular depth of focus by adding a relative defocus in one eye. Although it is well known that this reduces stereoscopic acuity, the details of this process are not completely quantified. In this study, we measured stereoscopic acuity using a binocular adaptive optics instrument for controlled amounts of induced defocus.

Methods : The binocular adaptive optics visual simulator allows for the simultaneous measurement and manipulation of the optics in the two eyes of a subject. The apparatus incorporates two programmable modulators using liquid crystal on silicon technology for wavefront shaping and to produce the artificial pupil respectively. The prototype was also equipped with a stimulus generator based on a pair of micro-projectors to perform visual testing through the modified optics. Each image was independently projected over its corresponding eye simultaneously, creating retinal disparity. Stereoscopic acuity was measured using a three-needle test. Subjects underwent a forced choice test, discerning if the central wire was in front or behind the two other wires for different interocular disparities. Measurements were performed in three normal young subjects when natural astigmatism was corrected, with paralyzed accommodation, for a 3.5 mm artificial pupil diameter, in green (pseudo-monochromatic) light and photopic conditions. The measurements were obtained for one eye at best focus while in the fellow eye controlled amounts of defocus were added.

Results : Stereoscopic acuity was best when both eyes were at best focus (mean value 6.3 ± 3.9 arcsec) and became worse with increasing defocus in one of the eyes. We found a reduction of stereoscopic acuity at a rate around 55 arcsec/diopter (95% confidence intervals: [40,60] arcsec/diopter) in the range up to 1D. For larger values of defocus, stereoscopic acuity was too poor to be measured with the instrument.

Conclusions : A binocular adaptive optics simulator was used to study the impact of unilateral defocus on stereovision. There is a linear reduction between induced defocus and stereoscopic acuity for defocus values up to 1 D. Even small values of defocus, such as 0.25 D, will reduce stereoscopic acuity by a factor of 2 as a result of different blur in the retinal images of both eyes.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.

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