Abstract
Myopia is a common refractive condition which normally results from an increase in the size of the eye. The incidence of myopia is increasing worldwide and has now reached epidemic proportions in some Asian countries, making myopia a significant public health concern. While myopia has a strong genetic input, environmental factors are also thought to contribute to myopia development and progression. These environmental factors are not fully understood but visual input is known to regulate eye growth. Furthermore, myopic eye growth itself has the capacity to modify the visual input received due to changes that occur within the visual system. This thesis employs various psychophysical and clinical measures to assess the impact of these changes on visual performance.The initial experimental chapters measured foveal detection and resolution acuity to investigate sampling limited visual performance in central vision. An equivalent noise approach for a contrast detection task, using both circular and annular stimuli, was then employed to determine individual factors contributing to foveolar and extra-foveolar visual performance. Higher level processing was subsequently evaluated using global motion discrimination tasks and classification images. Finally, the impact of retinal anatomical changes on visual performance was assessed in an interesting case study of a highly myopic subject without a foveal pit.
Sampling limited visual performance (i.e. aliasing) was identified as a feature of central vision in some myopic individuals, however there was no correlation between the degree of myopia and the likelihood of aliasing being present. Additionally, high myopes had higher internal noise and lower sampling efficiency, which resulted in higher contrast detection in noise thresholds, but not higher thresholds for global motion processing. Furthermore, it is shown that, even in the absence of a foveal pit, visual acuity can be within normal limits in high axial myopia.
Myopic growth can produce deficits of lower level visual processing in the form of aliasing, increased thresholds for contrast detection, and increased noise. However, there may be compensatory changes in the visual cortex of myopic individuals, which optimise the degraded input from lower visual areas to achieve normal visual performance. This potential to manipulate low-level visual input means that higher level processing may have a role in controlling myopic growth.
| Date of Award | 2014 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Graeme Kennedy (Supervisor) & Niall Strang (Supervisor) |