The effect of aging on spatial visual process outside the fovea
AffiliationOptometry and Vision Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-04-24. This item is currently available to University of Melbourne staff and students only, login required.
© 2018 Dr. Malavita Mohottalalage Menaka Samanmali Malavita
Studies have repeatedly shown that physiological aging impacts contextual spatial vision in central vision. Some visual functions are different between foveal and non-foveal vision and we have limited understanding of parafoveal contextual vision in aging. My thesis addressed how healthy aging influences spatial vision in non-foveal areas. Tasks involved in Experiment 1 were visual crowding and surround suppression of contrast detection. In the literature, these functions are often poorly differentiated The experiments described in this thesis investigated the consequences of healthy aging on visual crowding and surround suppression of contrast detection and determined whether such tasks were related to visuospatial attention as measured by visual search. Surround suppression of contrast detection increased in healthy older adults whereas visual crowding was relatively unchanged, supporting the existing evidence that crowding and suppression are distinct phenomena. Older adults showed longer reaction times for visual search, but visual search was not predicted by performance in either the crowding or surround suppression task in either age group. Although attention is considered important mechanistically for both visual search and visual crowding, the results did not reveal a role for attention in the magnitude of crowding. Since visual crowding relatively remained unchanged in older adults, Experiment 2 and 3 continued to explore surround suppression of contrast detection. Contrast detection, however, does not change across the visual field in a globally equivalent fashion, instead there are orientation biases that depend on eccentricity. The effect of surround on these anisotropies are unknown. Experiment 2 investigated if there are aspects related to stimulus orientation of surround suppression of contrast detection at different eccentricities and the role of centre-surround orientation using psychophysical methods. The results indicated that suppression is increased for horizontal centre-surround stimuli at 6ᵒ eccentricity while radial stimulus showed increased suppression at 15ᵒ eccentricity. Suppression was greater for a horizontal centre than for a vertical centre regardless of its surround orientation (parallel or orthogonal) at 6ᵒ and a radial centre was more susceptible for suppression than a tangential centre only for parallel surround condition at 15ᵒ. Experiment 3 determined if biases of suppression strength according to stimulus orientation and retinal eccentricity are observed in older adults. Orientation anisotropy of surround suppression changed from a horizontal bias to a radial bias moving from 6ᵒ to 15ᵒ in older groups indicating similar eccentricity dependency of orientation biases of surround suppression of contrast detection in both young and older adults. The experiments described in this thesis broadened our understanding of how aging effects non-foveal spatial vision. Advanced understanding of the effects of healthy aging in non-foveal vision is not only important for better understand of fundamental aspects of visual perception, but also for developing remediation strategies in diseases such as Age-Related Macular Disease (AMD).
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