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).
Highlights
Seeing with two eyes is an important aspect of human vision
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
Inter-individual regression analyses are made of the observers’ objective versus subjective fixation disparity. 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
Summary
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. There are two PLOS ONE | DOI:10.1371/journal.pone.0170190. There are two PLOS ONE | DOI:10.1371/journal.pone.0170190 January 30, 2017
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