Since adaptive optics was first used to correct the monochromatic aberrations of the eye over a decade ago there has been considerable interest in correcting the ocular aberrations beyond defocus and astigmatism. In order to understand the prospective benefits of correcting these higher-order aberrations it is important to study their effect on visual performance. From a clinical perspective it is important to know how different types of aberration can affect visual performance so that wavefront measurements can be better understood. Visual performance is determined by a combination of optical and neural factors. It is important to consider how degradations in the optical quality of the eye can impact the neural processes involved in visual tasks such as object recognition. \ud In this thesis we present a study of the effects of three types of aberration, defocus, coma and secondary astigmatism, on letter recognition and reading performance. In the course of this work we also characterise the repeatability of the Zywave aberrometer, which we used to measure our subjects' ocular wavefronts. We use stimuli that have these aberrations applied in their rendering to examine the differences between these aberrations and how they differ with respect to the visual task. We find that secondary astigmatism causes the largest impairment to both letter recognition and reading performance, followed by defocus. Coma causes comparatively smaller degradations to performance but its effect is different depending on the visual task. We can predict the reduction in performance based on a simple cross-correlation model of letter confusability. The relationship between these predictions and the experimental results are the same for all three aberrations, in the case of single letter recognition. In reading however, the relationship is different for coma. We suggest that coma causes lateral masking effects and may additionally disrupt the planning of eye movements. Coma slows reading, but does not specifically impair word identification whereas defocus and secondary astigmatism do. We attribute disruptions in word identification to the dramatic effects defocus and secondary astigmatism have on the form of a letter
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