Oculomotor and functional characteristics of bilateral central vision loss

Abstract

Central Vision Loss (CVL) is characterized by visual loss in the central posterior part of the retina, the light-sensitivity tissue in the back of the eye. CVL is often clinically presented as asymmetric bilateral vision loss, causing permanent asymmetric blindspots. To compensate for their vision loss, CVL observers adopt Preferred Retinal Loci (PRLs), healthy areas of the peripheral retina, for visually-guided tasks. Age-related macular degeneration (AMD) is one of the leading causes of CVL, affecting 11 million people in the United States. CVL negatively impacts quality of life and activities of daily living, economic and educational opportunities, and is associated with decreases in reading ability, visual acuity and contrast sensitivity. Despite the bilateral nature of this disease, a consistent methodological limitation to previous research is examining functional outcomes in only one eye, typically the better-seeing eye. The present research aims to expand on the current literature regarding bilateral CVL, and the binocular oculomotor and functional characteristics of people with CVL. Increases in life expectancy, magnifies the need for state and federally funded programs to provide low vision rehabilitation services for adults with AMD. Nearly half of the legally-blind cases in Massachusetts are diagnosed with macular degeneration, 92% of which were age-related, 70% identified as White and 62% as women. These findings support previous evidence that suggests women and White individuals with European ancestry are associated with increased risk of AMD development. Signs of early or intermediate AMD may go unnoticed in the United States, as many Americans do not have vision medical coverage and may not participate in yearly visual examinations. Moreover, eligibility for low vision services is state dependent, and in Massachusetts a person with AMD must have visual acuity 20/200 or worse to be eligible, and unfortunately by that point would already suffer severe visual impairment. In a sample of adults with inherited or age-related bilateral maculopathy, we examined fixation stability during a contrast sensitivity task under monocular and binocular viewing. The aim of this study was to investigate whether changes in fixation characteristics at different gaze directions affect visual function. The results suggest that increasing stability is associated with better contrast sensitivity and peak sensitivity at a higher spatial frequency. The results also showed that for some spatial frequencies, one eye may be better than two, but for other spatial frequencies there may be a benefit to binocular viewing. These findings could have direct implications for real-world behavior in low vision adults, as tasks involving orientation and mobility are more associated with lower and middle spatial frequencies, and other tasks like reading are more associated with higher spatial frequencies. However, idiosyncratic differences at the individual level suggest there is no model CVL observer, and that there is considerable heterogeneity in contrast function. There is no standardized way to measure monocular morphological changes in the diseased retina to binocular real-world function. To address this limitation, we developed a novel method to measure monocular and binocular PRLs in CVL observers. This method uses the optic nerve head (ONH) in each eye, perceptual blindspots that are devoid of light-sensitive cells, as landmarks to convert retinal space to visual field space. This approach uses 3D glasses, standard computer and an eye tracker to estimate binocular retinal locations of stimuli, and results support the feasibility of using this system to measure monocular and binocular PRLs. We measured disc-to-fovea distances (DFDs) in a sample of normally-sighted adults to provide approximate former foveae estimates in CVL individuals. Most binocular PRLs were demonstrated in the inferior and right visual field, locations that many be optimal for reading and visual resolution. Over half of the patients, adopted asymmetric PRLs in monocular and binocular viewing, and suggest asymmetric CVL influences PRL characteristics under different task and viewing conditions. Using this method, we developed a novel analysis to examine binocular PRLs during a fixation and contrast sensitivity task. The results provide evidence that PRL characteristics vary significantly with task and viewing conditions. Our results challenge the notion that there is one optimal PRL, even within the individual, implicating a more nuanced approach to low vision rehabilitation. This dissertation aimed to provide a more comprehensive understanding of oculomotor and functional characteristics in people with CVL and developed new methods to help clinicians and researchers understand more about the effects of asymmetric vision loss and what may provide the most effective rehabilitation interventions.--Author's abstract