Research Article: Individual Objective and Subjective Fixation Disparity in Near Vision

Date Published: January 30, 2017

Publisher: Public Library of Science

Author(s): Wolfgang Jaschinski, Susana Martinez-Conde.

http://doi.org/10.1371/journal.pone.0170190

Abstract

Binocular vision refers to the integration of images in the two eyes for improved visual performance and depth perception. One aspect of binocular vision is the fixation disparity, which is a suboptimal condition in individuals with respect to binocular eye movement control and subsequent neural processing. The objective fixation disparity refers to the vergence angle between the visual axes, which is measured with eye trackers. Subjective fixation disparity is tested with two monocular nonius lines which indicate the physical nonius separation required for perceived alignment. Subjective and objective fixation disparity represent the different physiological mechanisms of motor and sensory fusion, but the precise relation between these two is still unclear. This study measures both types of fixation disparity at viewing distances of 40, 30, and 24 cm while observers fixated a central stationary fusion target. 20 young adult subjects with normal binocular vision were tested repeatedly to investigate individual differences. For heterophoria and subjective fixation disparity, this study replicated that the binocular system does not properly adjust to near targets: outward (exo) deviations typically increase as the viewing distance is shortened. This exo proximity effect—however—was not found for objective fixation disparity, which–on the average–was zero. But individuals can have reliable outward (exo) or inward (eso) vergence errors. Cases with eso objective fixation disparity tend to have less exo states of subjective fixation disparity and heterophoria. In summary, the two types of fixation disparity seem to respond in a different way when the viewing distance is shortened. Motor and sensory fusion–as reflected by objective and subjective fixation disparity–exhibit complex interactions that may differ between individuals (eso versus exo) and vary with viewing distance (far versus near vision).

Partial Text

Seeing with two eyes is an important aspect of human vision. Binocular vision refers to the integration of the images in the two eyes for improved visual performance and depth perception. A comprehensive review of binocularity is provided in the seminal books of Howard [1], Howard & Rogers [2], Howard [3]. One aspect of binocular vision is the fixation disparity, which refers–broadly speaking–to a suboptimal condition in individuals with respect to binocular eye movement control and subsequent neural processing [1], p. 488–492. 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.

Measurements of objective fixation disparity in near vision at viewing distances of 40, 30, and 24 cm pose considerable methodological requirements on display technology, eye movement recordings and data analyses. Important features of the present approach are (1) the use of a purpose-made high resolution OLED-display to present sharp images even at the 24 cm viewing distance and to operate shutter glasses for diochoptic nonius lines, (2) elaborate procedures for precise recording binocular eye movements with the video eye tracker EyeLink II that detects the position of the pupil centre, (3) correction for the artifact that the pupil center shifts nasally if the pupil shrinks when the viewing distance is shortened from 40 to 24 cm and when vision changes from monocular calibration to binocular recording. These methodological issues and particularly the correction of the pupil artifact were described by the author [46] in a parallel paper on the same dataset; the present methodological description is confined to the general structure and design of this experiment.

The results comprise three steps. First, the averages in the group are presented as a function of viewing distance. Second, mean effects and individual differences are tested by linear mixed-effects models. Third, the data of each participant are described by the individual mean fixation disparity and the individual change in fixation disparity with viewing distance; the reliability of these data between repeated sessions is reported. Fourth, inter-individual regression analyses are made of the observers’ objective versus subjective fixation disparity. Finally, it was tested whether the inter-individual variance in the two types of fixation disparity can be explained by the observers’ heterophoria or accommodative performance.

The discussion comprises several steps. First, the results concerning subjective and objective fixation disparity are discussed separately in comparison with previous research. Second, the difference between subjective and objective fixation disparity and the relation between the two are interpreted. Third, based on these results, an outlook to further research is provided.

For heterophoria and subjective fixation disparity, this study replicated that the binocular system does not properly adjust to near targets: outward (exo) deviations typically increase as the viewing distance is shortened. This exo proximity effect—however—was not found for objective fixation disparity, which–on the average–was zero. But individuals can have reliable outward (exo) or inward (eso) vergence errors. Cases with eso objective fixation disparity tend to have less exo states of subjective fixation disparity and heterophoria.

 

Source:

http://doi.org/10.1371/journal.pone.0170190

 

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