Effects of Prism Eyeglasses on Objective and Subjective Fixation Disparity

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.

Individual Objective and Subjective Fixation Disparity in Near Vision

“…There are two types of fixation disparity—subjective and objective—that differ in the measurement paradigm, the physiological meaning, and the area of applications. In a recent paper, Schroth et al [ 4 ] have given an extended research overview of the two types of fixation disparity which is summarized here regarding the present study on horizontal fixation disparity in near vision.…”

“…The geometrically expected vergence angle V 0 = 2 arc tan (p/2D) depends on the viewing distance disparity and the inter-pupillary distance p so that objective fixation disparity is the resulting vergence error oFD = V − V 0 . Subjective fixation disparity sFD = arc tan (d/D) is given by the angular amount of the nonius offset (Figure adopted from Schroth et al [ 4 ]). In this case, the visual axes cross in front of the target which is referred to as eso fixation disparity with a positive sign.…”

“…However, in conditions of fusion, the psychophysical result of dichoptic nonius lines (subjective fixation disparity) does not agree with the result of eye tracker recordings (objective fixation disparity). This nonius artifact was revealed in different experimental paradigms: (a) dichoptic nonius lines gave different results than the psychophysical phenomenon of the bandwidth of border enhancement that varies with retinal eccentricity, but does not depend on retinal correspondence [ 8 – 10 ]; this was measured as a function of static forced vergence, i. e., changes in vergence that were induced by absolute disparity at a fixed viewing distance, thus at a fixed accommodative stimulus; (b) dichoptic nonius lines gave also different results as eye tracker recordings in static forced vergence [ 11 – 14 ]; (c) dynamic forced vergence changes were not indicated by a monocular line [ 15 ]; (d) the disparity offset of two stereoscopic depth planes was not indicated by two pairs of nonius lines [ 16 ]; (e) the nonius artifact increased the closer the nonius test was located to the binocular fusion target [ 16 – 20 ]; (f) when forced vergence was not applied, individual measures can still be about 10 times larger for objective fixation disparity than for subjective fixation disparity [ 4 , 21 ]. For the interpretation of these effects, partly different terminology was used: changed relation between retinal location and perceived visual direction, shifts in retinal correspondence, remapping of visual directions, capture of visual direction.…”

“…The subjective fixation disparity seems to be affected by two processes: the oculomotor adjustment of the visual axes (objective fixation disparity) and the remapping of visual directions by sensory fusion [ 9 , 22 ]. This may explain why subjective and objective fixation disparity were correlated and each measure was correlated with heterophoria [ 4 , 21 ].…”

Hoya Eyegenius®: New Method for Measuring and Correcting Fixation Disparity

“…This research phase was done in the cooperation with Dr. W. Jaschinski from the IfADo Leibniz Research Centre for Working Environment and Human Factors in Dortmund, Germany, and the research group Individual Visual Performance, who have a lot of experience and familiarity with longstanding research on subjective and objective FD. Significant effects on subjective and objective FD after several weeks of continuous prism wearing could be demonstrated for the first time (Schroth et al, 2015). Even small prism values were effective and the outcomes of this study show the evidence that the vergence system does not fully adapt to prisms.…”