Optometry and Vision Sciences - Theses

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    Using structural information to improve perimetry test procedures
    BALLAE GANESHRAO, SHONRAJ ( 2015)
    Existing literature on perimetry suggests that current perimetric test procedures suffer from two major limitations; 1) test-retest variability and 2) sampling density. In this thesis, I aimed to improve perimetric testing by leveraging information about an individual’s retinal structure. Current perimetric test procedures are designed to be applicable for a widespread population and are not based on individual information about a given patient. Optical Coherence Tomography (OCT) provides us with some ready information about an individual’s RNFL thickness which might be useful for perimetric testing. This thesis explores ways to incorporate information gained from OCT into perimetric test procedures. In Experiment 1, an individual’s RNFL thickness information is used to bias the prior of a Bayesian perimetric test procedure. Experiment 2 studies the limitations of the relationship between Retinal Nerve Fiber Layer (RNFL) thickness, as measured by OCT data, and perimetric thresholds. In Experiment 3, an individual’s RNFL thickness information is used to customise visual field test locations. The results of this thesis suggest that an individual’s RNFL thickness information can be used to improve the accuracy, precision and test duration of perimetric testing (Experiment 1). The strength of the structure-function relationship in glaucoma can be better revealed improve by relating both measures using customised optic nerve head sectors rather than choosing fixed optic nerve head sector boundaries (Experiment 2). This thesis also shows that customising visual field locations based on the individual’s RNFL thickness improves the chances of detecting abnormal visual field locations (Experiment 3).
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    Customized, automated stimulus location choice to improve visual field procedures
    CHONG, LUKE ( 2014)
    It is well accepted in the literature that test-retest variability is a key limitation in current automated perimetric procedures, especially with moderate to advanced visual field loss. This obfuscates the ability to discriminate between true progression of the visual field and measurement noise. Studies have shown that sampling the visual field at a greater spatial resolution not only reduces global measures of test-retest variability, but can also describe the spatial extent of scotomata with more accuracy and detect scotomata otherwise missed in conventional perimetry. However, to date there has been no automated application of these principles, and previous approaches which have employed high resolution sampling have sacrificed test duration in order to measure more locations than standard stimulus distribution patterns. The primary objective of this thesis was to address each of these problems by developing a novel, automated, perimetric approach which customises test location choice based on spatial information and observer response during the examination. It was not only important that new procedures did not increase test times compared to current procedures, but also are computationally feasible to implement. The experimentation reported in this thesis concentrated on computer simulation (Experiments 1 to 3) in order to develop and tune these new procedures before testing their performance on human observers (Experiment 4). The outcome measures were precision of threshold estimates (test-retest variability), accuracy (absolute error) and efficiency (number of presentations). It was discovered that one of the developed approaches, Gradient-Oriented Automated Natural Neighbour Approach (GOANNA), improved accuracy and precision in areas surrounding scotoma borders without increasing test duration compared to current procedures (Chapter 3, Experiment 2). This led to further exploration of GOANNA, whereby it was demonstrated (through computer simulation) that the improvement in scotoma characterisation seen in Experiment 2 gave rise to earlier and more accurate detection of glaucomatous progression. It was assumed that the assumptions made in the simulation studies hold true for real data. This thesis lends support to previous findings that implementing high resolution grids is beneficial in glaucomatous progression detection, and that the conventional 6° rectangular grid of fixed locations may not be the most suitable stimulus arrangement for characterising and monitoring all visual field defects. It also reports on initial approaches that didn’t work, which may be useful for future investigators in this field of research. Most importantly, it provides a novel, principled, automated approach of locally increasing sampling spatial resolution without having to sacrifice efficiency.
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    Exploring the mechanisms of Rarebit perimetry
    Hackett, Deborah Anne ( 2009)
    Visual field testing, or perimetry, measures peripheral visual loss in eye diseases such as glaucoma. Rarebit Perimetry (RBP) is a new and novel perimetric method, introduced in 2002 by Lars Frisén (2002), with the aim of detecting low degrees of neural damage within the retina. RBP is unlike conventional perimetric methods that measure levels of retinal sensitivity, but instead uses very bright (i.e. suprathreshold) and very small targets to detect tiny areas of absolute blindness within otherwise normal areas of vision. RBP thus claims to locate miniscule gaps in the receptive field matrix of neurons in the retina, with the assumption that dead neurons leave gaps in this matrix. The most useful application of this idea is to detect progressive eye disease in the earliest stages (Frisén, 2002). Current research shows that RBP correlates with other standard visual field tests (Brusini, Salvetat, et al., 2005; Frisén, 2003; Gedik, Akman, et al., 2007; Martin & Wanger, 2004), but may afford greater sensitivity by detecting very mild visual losses missed by other tests (Martin, Ley, et al., 2004; Martin & Nilsson, 2007; Nilsson, Wendt, et al., 2007). To date, there are no studies that definitively test the theoretical basis of RBP, so in this thesis I aim to explore the proposed underlying mechanisms and assumptions of this test. In particular, the proposed mechanism of RBP leads to specific predictions as to how responses will alter when the luminances of the RBP targets are systematically decreased. I therefore compared RBP responses of mean hit rate as a function of target luminance and found results to be inconsistent with the proposed RBP mechanism. Mathematical simulations were performed to explore reasons for the differences between the two groups (Chapter Seven).