Effect of induced monocular blur on monocular and binocular visual functions

Abstract

Background and purpose: Anisometropia and surgical monovision is likely to have a detrimental impact on binocular visual functions. This study evaluates the effect of induced monocular blur in the form of a refractive defocus on visual functions and binocularity. Methods: An experimental study was conducted on 30 young emmetropic adults. Monocular myopia and hypermetropia was induced using 6 plus and minus spherical lenses in steps of 0.5 Dioptre ranging from +/-0.5DS to +/-3.00DS. Visual Acuity (LogMAR), contrast sensitivity (Pelli Robson), binocularity (Bagolini striated glasses) and stereoacuity (near Randot, distance Randot and Frisby davis Distance) were evaluated at baseline and each level of refractive defocus. Results: Induced monocular myopic shift resulted in reduction of visual acuity to 0.94 log MAR while hyperopic shift reduced visual acuity to 0.1 log MAR. Contrast sensitivity did not show a significant reduction with optical blur. Hyperopic shift did not hamper gross binocular visiona unlike myopic shift where distance binocular vision was elicited only till a +2D defocus and near binocular vision till a +3D defocus. Near stereopsis deteriorated with both a hyperopic and myopic blur but was not completely lost and could be elicited even with the maximal blur. Distance randot stereopsis was lost on inducing a +2.5D blur while Frisby Davis distance stereopsis was lost at +3D blur. Induced hyperopic shift did not lead to an absolute loss of distance stereopsis. Conclusion: Binocular visual functions were significantly affected by induced monocular blur through a spherical refractive defocus. Induced myopic shift caused significantly more stereo acuity loss than induced hyperopia. Optical blur impacts distance stereoacuity to a greater extent than near. Keywords: Binocularity, Defocus, Hyperopia, Optical blur, Myopia, Stereopsis, Stereo-acuity