The impact of binocular vision dysfunction and simulated refractive imbalance on reading performance in the digital age

Abstract

Normal binocular vision is thought to be helpful for reading tasks at short distances, as visual signals from each eye can combine to improve vision (e.g. binocular summation). In binocular vision (BV) dysfunction, such as in refractive imbalance or convergence insufficiency (CI), there may be reduced vision clarity, abnormalities in eye movements or positioning which may impact fusion, which may result in changes in the activity of neurons involved in vision and reading: These may be associated with reading difficulty symptoms, and decrements in reading performance and comprehension.
Therefore, this thesis aims to investigate how simulated refractive imbalance and habitual binocular dysfunction, specifically CI, affects reading performance (reading rate and comprehension) and symptomatology related to reading difficulty. To address the aim, this thesis includes seven chapters, staring with the introduction with hypothesis (Chapter I), followed by a literature review (Chapter II), four research papers (Chapters III-VI) and a discussion (Chapter VII). The research papers present four studies that address the aims and hypothesis of this thesis: Study 1 investigated the effect of refractive imbalance (up to ±2.00 D over the dominant eye), as a model of binocular vision dysfunction, on binocular vision status, reading performance and vision-related reading difficulty symptomatology. Study 2 investigated the effect of one kind of habitual binocular vision dysfunction, specifically CI, on reading performance and vision-related reading symptoms using the developed model. Next, two studies were designed to assess potential mechanisms for findings in study 1 and 2 by investigating the binocular vision parameters, the binocular and monocular accommodative response, and neural processing using visually evoked potentials (EP) at Oz for vision (overlying V1) and P3 (overlying the left angular gyrus) for reading under conditions of simulated refractive imbalance (Study 3) and convergence insufficiency (Study 4). Participants were expert readers aged 18-40 years with tertiary education experience.
The main findings in the current thesis were that reading performance (reading rate and comprehension) was unaffected by simulated refractive imbalance or CI. However, simulated refractive imbalance and CI resulted in worse vision-related reading symptomatology in comparison with normal binocular vision. Additionally, simulated refractive imbalance resulted in strong monocular blur at distance, slight binocular blur at near, abnormal binocular vision status, iso-accommodation that followed the dominant eye and increased accommodative variability. During refractive imbalance, Oz amplitude was decreased in the dominant eye, but binocular amplitude and binocular summation were unaffected. In P3, monocular and binocular amplitudes, and binocular summation findings were not affected. In the CI group, reading was not associated with a change in binocular vision parameters, visual acuities (VA) (monocular, binocular) and near VA binocular summation. In comparison with participants with normal BV status, the CI group had lower lag of accommodation during reading, but there was no difference in accommodative variability during reading, unlike the refractive imbalance conditions. In CI groups, poorer vergence parameters were associated with greater reading symptomatology. All cortical processing measures, using visual evoked potentials (VEP) at Oz and P3, showed no difference in amplitude and binocular summation ratio between the CI and participants with normal BV.
Overall, the findings of this thesis are that abnormal binocular vision status, such as due to refractive imbalance and convergence insufficiency, was found to have a significant impact on ability to read comfortably and for long periods of time but did not affect reading rate and comprehension, in the expert readers assessed. Overall, from the potential mechanisms investigated, the negative effects of refractive imbalance were associated with heightened activity in variability of the accommodative system associated with conflicting monocular blur signals, maintenance of the binocular summation response and neural effort to maintain binocular near clarity. In contrast, the negative effects of CI were associated with reduced vergence facility and increased accommodative effort to assist convergence to maintain binocular fusion during reading.

Date of Award	2026
Original language	English
Supervisor	Mei Ying BOON (Supervisor), Jeroen van Boxtel (Supervisor) & Myra LEUNG (Supervisor)