Investigations of binocularity and reading performance in healthy subjects and patients with mild traumatic brain injury

Abstract

Background: It is known that problems with binocular vision can cause issues for reading, less well known is to what extent binocular vision improves reading performance. A key aim for this project was to estimate the contribution of binocularity in reading in healthy subjects, and in patients with mild traumatic brain injury (MTBI). A second aim was to evaluate the role of eye dominance in reading and the occurrence of graded eye dominance under true binocular viewing conditions. The third and final aim was to evaluate the effect of spectacle treatment on visual function, symptoms and reading performance in patients with MTBI.

Subjects: Papers I and II each included 18 healthy subjects and paper III 32 healthy subjects, all with normal vision. Paper IV included eight patients with a documented history of MTBI who had been referred due to persisting symptoms and vision based issues.

Methods: Reading was done with left, right and both eyes in a balanced repeated measures study design (paper I-II). Continuous text, controlled for readability, was presented at high or reduced contrast. Graded eye dominance was assessed with a binocular sighting test and the Variable Angle Mirror Test (paper III). In paper IV visual symptoms, visual function and reading performance were assessed before and after spectacle treatment.

Results: A marginal binocular advantage occurred at reading of high contrast text. At reduced contrast levels, however, binocular reading speed was significantly faster than monocular. An interaction effect was found between monocular reading and low contrast levels leading to prolonged mean fixation duration. The binocular eye dominance tests showed a weighted balance between the eyes for a majority of subjects. The strength of the weighting (towards either eye) was correlated to the amount of monocular blur required to alter the balance. Spectacle treatment led to symptom reduction and minor improved visual function in some of the patients. The relation between improved reading performance and symptom reduction was inconsistent. Monocular reading resulted in worse reading efficiency and comprehension.

Conclusion: The findings in reading performance parallel the literature where the binocular advantage is small for complex stimuli of high contrast, but increases with reduced contrast levels. The results suggest that binocularity contributes to the robustness of reading performance. Monocular reading performance was generally equal in subjects with normal vision with no clear relation to eye dominance. The results of the binocular dominance tests indicate plausible effects of graded eye dominance affecting how the visual scene is perceived under true binocular viewing conditions. Spectacle treatment can reduce symptoms for MTBI patients but with marginal effects on visual performance.