Abstract
In optometry of binocular vision, the question may arise whether prisms should be included in eyeglasses to compensate an oculomotor and/or sensory imbalance between the two eyes. The corresponding measures of objective and subjective fixation disparity may be reduced by the prisms, or the adaptability of the binocular vergence system may diminish effects of the prisms over time. This study investigates effects of wearing prisms constantly for about 5 weeks in daily life. Two groups of 12 participants received eyeglasses with prisms having either a base-in direction or a base-out direction with an amount up to 8 prism diopters. Prisms were prescribed based on clinical fixation disparity test plates at 6 m. Two dependent variables were used: (1) subjective fixation disparity was indicated by a perceived offset of dichoptic nonius lines that were superimposed on the fusion stimuli and (2) objective fixation disparity was measured with a video based eye tracker relative to monocular calibration. Stimuli were presented at 6 m and included either central or more peripheral fusion stimuli. Repeated measurements were made without the prisms and with the prisms after about 5 weeks of wearing these prisms. Objective and subjective fixation disparity were correlated, but the type of fusion stimulus and the direction of the required prism may play a role. The prisms did not reduce the fixation disparity to zero, but induced significant changes in fixation disparity with large effect sizes. Participants receiving base-out prisms showed hypothesized effects, which were concurrent in both types of fixation disparity. In participants receiving base-in prisms, the individual effects of subjective and objective effects were negatively correlated: the larger the subjective (sensory) effect, the smaller the objective (motor) effect. This response pattern was related to the vergence adaptability, i.e. the individual fusional vergence reserves.
Highlights
Using two eyes has advantages for vision with respect to binocular summation and depth perception, but requires that the two eyes are properly coordinated on the oculomotor and sensory level
Relations between sensory and motor effects are analysed by correlations between individual prism effect sizes in subjective versus objective fixation disparity
The individual response patterns are compared with the fusional vergence reserve to test influences of vergence adaptation
Summary
Using two eyes has advantages for vision with respect to binocular summation and depth perception, but requires that the two eyes are properly coordinated on the oculomotor and sensory level. Effects of Prism Eyeglasses on Fixation Disparity travel grants (including accommodation) from HOYA. VS and WJ contributed to two patents applied by Hoya Corporation, Amsterdam [(1) World Organizational Property Organization (Patent name: Software procedure for the optometric determination of prisms, WIPO/PCT WO 2014/112626 A1) and (2) US Patent Office (Patent name: Prism Prescription value acquisition system, acquisition method, acquisition apparatus and program for correcting fixation disparity, No 13P36J19US)]. If the patent should be practically applied, VS and WJ will not benefit from a potential marketing of a potential product. There are no further patents, products in development, or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials
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