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Robert Dowd, Antoine Barbot, Geunyoung Yoon, Krystel R Huxlin, Duje Tadin; Neural Binocular Summation Altered by Abnormal Interocular Disparities in Normally Developed Visual Systems. Invest. Ophthalmol. Vis. Sci. 2016;57(12):No Pagination Specified.
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© ARVO (1962-2015); The Authors (2016-present)
Keratoconus (KC) and monovision (MV) are two examples of large and progressive inter-ocular disparities due to higher order aberrations and defocus that arise after normal development of the visual system. This study serves to investigate the ensuing changes in binocular function associated with these two models and to test the hypothesis that larger interocular disparities result in lower binocular summation.
A large stroke binocular adaptive optics system was used to correct ocular aberrations for 9 human subjects in real time. Subjects were grouped as normal (n=4), MV (n=2) or KC (n=3) and tested for both monocular and binocular contrast sensitivity (CS). AO correction eliminated the contribution of ocular aberrations to binocular function, allowing for the assessment of neural function. A contrast sensitivity function was measured in a ±45 degree orientation discrimination forced-choice task. Subjects were presented with a Gabor patch stimulus (2 degrees in diameter), which varied in contrast and spatial frequency (0.25-30 c/deg). For each subject, binocular summation ratios (BSR) were computed by dividing binocular CS by monocular CS of the best eye.
Under full AO correction (average residual wavefront RMS<0.05μm), subjects with large interocular differences exhibited reduced binocular summation, relative to normal eyes. KC subjects exhibited a large impairment in binocular summation (BS), with an average BSR of 1.09 ± 0.28 SEM, a 41% decrease from the average normal BSR of 1.87 ± 0.27 SEM. The interocular difference in Keratometry value for KC subjects ranged from 1.5-28.5D. The KC subject with the greatest interocular disparity (28.5D) displayed strong binocular inhibition, with a BSR of 0.63 ± 0.055 SEM. MV subjects with approximately 1.5D anisometropia also showed reduced binocular summation, with an average BSR of 0.98 ± 0.36 SEM. The MV subject with the most long-standing monovision exhibited an average BSR of 0.62 ± 0.046 SEM, indicating strong binocular inhibition as well.
The results of this study indicate that considerable inter-ocular differences in optical quality detrimentally affect binocular neural function, as illustrated by the lowered binocular summation in KC and MV subjects. This finding suggests that long-term visual experience with abnormal binocular visual input alters binocular function of normally developed visual systems.
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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