Individual Differences in Binocular Coordination Are Uncovered by Directly Comparing Monocular and Binocular Reading Conditions

Abstract

We evaluated systematically binocular coordination during a reading task by comparing binocular and monocular reading, and considering the potential effects of individual heterophoria and eye dominance. A total of 13 participants (aged 19-29 years, refractive errors -0.5 to 0.125 diopters [D]) read single sentences in a haploscope while eye movements were measured with an EyeLinkII eyetracker. When reading monocularly, saccade amplitudes increased by 0.04 degrees and first fixation durations became longer by approximately 10 ms. Furthermore, saccade disconjugacies increased, and compensatory vergence drifts during fixation turned into a divergent drift relative to the viewing distance. The vergence angle adjusted for the actual viewing distance became less convergent during monocular reading by 0.5 degrees. Moreover, in participants who were almost orthophoric, only the first fixation duration became longer (by 20 ms) when the reading conditions changed from binocular to monocular. For exophoric participants, all parameters of binocular coordination changed, and first fixation duration decreased by 20 ms. When reading monocularly, no differences between the dominant right eye and the nondominant left eye were found. Because of obvious differences in binocular coordination between monocular and binocular reading, some vergence adjustments are driven actively by fusional processes. Furthermore, higher demands on these binocular fusional processes can be uncovered only by a detailed evaluation of monocular reading conditions.