Optometry and Vision Sciences - Theses

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    A structural and functional profile of high myopia as a function of eye size
    Jaworski, Alexandra Anne ( 2018)
    The prevalence of myopia and its associated sight threatening pathology is anticipated to increase, making high myopia a global health concern, especially in our ageing population. Although pathological sequelae and visual dysfunction have been attributed to excessive elongation in the highly myopic eye, a link between axial elongation and visual dysfunction in the absence of significant pathology is less well described. Furthermore, emmetropia and high myopia are variably defined, and in most instances, defined according to refractive error and not eye size. This thesis was designed to define and distinguish emmetropic and highly myopic eyes with regard to axial length and vitreous chamber depth and to apply these definitions to studies exploring the impact of ocular expansion on visual function and ocular structure. The first experiment (Chapter 2) used meta-analysis to predict the axial length (23.47 ± 0.07 mm) and vitreous chamber depth (16.12 ± 0.14 mm) of an emmetropic eye. Based on gender matched studies, male eyes not selected for refractive error were found to be larger with respect to both axial length (+0.52 ± 0.04 mm) and vitreous chamber depth (+0.40 ± 0.04 mm) relative to female eyes. Male emmetropic eyes displayed longer vitreous chamber depths (+0.47 ± 0.15 mm) relative to female emmetropic eyes, but axial lengths of emmetropic eyes did not vary with gender. Linear and non-linear meta-regressions predicted minimum dimensions of -5, -6 and -8 Dioptre (D) eyes, with a – 5 D high myope characterised by an axial length and vitreous chamber depth of at least 24.66 and 16.94 mm, respectively. Refractive errors of -5, -6 and -8 D were selected as they have previously been used to define high myopia in other studies, and were the refractive error groupings investigated in Chapters 3 and 4 of this thesis. Systematic review showed that vitreous chamber depth is presented less frequently than axial length in published literature. This may reflect instrumentation used to perform biometry and suggests that vitreous chamber elongation is mostly assumed but not demonstrated by researchers. In such cases, the role of posterior segment elongation in the development of structural and/or functional sequelae is hypothetical. The second experiment (Chapter 3) contrasted luminance and S-cone pathway spatial processing of axial high myopes ( -8 D) and emmetropes using psychophysical tools. Luminance and S-cone pathways were both probed to discriminate between pathway selective and non-selective visual dysfunction, given the known redundancy of neuronal elements comprising the S-cone pathway and reports of altered colour vision in high myopia. High spatial frequency loss and increased critical area indicated increased separation of neural elements. Critical area enlargement was consistent with a non-uniform posterior pole model of ocular expansion. Models previously reported in the literature have included non-uniform, posterior pole and global expansion. However, unlike the wrok presented here, these studies did not assess visual function with regards to vitreous chamber depth. Reduced contrast sensitivity for spatial summation tasks in the presence of retained sensitivity at lower spatial frequencies suggested non-selective post-receptoral dysfunction due to ocular enlargement and either normal or enhanced photoreceptor sensitivity. The third experiment (Chapter 4) utilised readily available clinical tools, customised automated perimetry and optical coherence tomography, to investigate structure-function relationships in enlarged highly myopic eyes ( -5 D). Generalised choroidal thinning for the central 4 mm and localised retinal thinning confined to the central ±1 mm were evident for high myopes. Although overall visual sensitivity decreased with increasing eye size, regional relationships between sensitivity and structural thickness or eye size were not evident. However, the nasal region appeared predominantly thinner with ocular enlargement suggesting that it may be more susceptible to visual dysfunction in longer eyes. The finding of marked nasal thinning of the ocular structural layers has been reported previously and is consistent with choroidal watershed zones predominantly affecting the nasal posterior pole. This reduced blood supply could underpin structural and functional changes in eyes demonstrating posterior pole expansion. The findings support that vitreous chamber elongation causes high myopia, visual dysfunction, and generalised choroidal and localised retinal thinning. Psychophysical and available clinical tools supported non-uniform ocular expansion in high myopia, with the nasal region predominantly thinned and potentially at risk of visual dysfunction. Although meta-analysis is not a novel tool, its application to defining emmetropia and high myopes with regards to axial length and vitreous chamber depth is novel, and has application in subsequent research and clinical settings.
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    Characterising the ocular phenotype in a murine model of Alzheimer’s disease
    Lim, Jeremiah ( 2018)
    This thesis shows that amyloid beta found in the brain is also present in the retina of a murine model of Alzheimer’s disease. By applying non-invasive retinal techniques, we show that neuronal structural and functional changes occur early in this model and that these are associated with vascular dysfunction. Such retinal hallmarks differentiate Alzheimer's changes from healthy ageing mice and provide evidence that the retina is a viable biomarker for dementia.
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    Age-related changes in structure, function and response to stress in the rat retina
    Paul, Joseph ( 2016)
    Ageing is a key risk factor for ocular diseases, though age-related changes in the eye have not been fully characterised. This study investigated age-related changes in retinal function, structure and their response to acute and chronic stress in Long Evans rats. With age, both retinal structure and function decline and the retina loses its ability to cope with acute stress. When exposed to mild chronic stress, older eyes suffered greater functional damage than younger eyes.
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    Subcortical pathways for colour vision
    Szmajda, Brett A. ( 2006-09)
    Visual sub-modalities, such as colour, form and motion perception, are analysed in parallel by three visual “pathways” – the parvocellular (PC), magnocellular (MC) and koniocellular (KC) pathways. This thesis aims to further elucidate some properties of the subcortical pathways for colour vision. The experimental animal used throughout is a New World monkey, the common marmoset Callithrix jacchus. (For complete abstract open document)
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    Functional correlates between the rat electroretinogram and visual evoked potential
    Tsai, Tina I-ting ( 2012)
    The ERG and VEP are sequentially-activated responses, widely used for diagnosis of eye and brain diseases. Measuring both simultaneously provides additional information to help localise where in the visual pathway injury has occurred. This thesis shows how retinal information streams are encoded in the VEP. In addition, it shows that changes to ERG components can predict the amount of loss downstream in the retina. However, retinal loss may not predict VEP changes.
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    Susceptibility of the diabetic rat retina to intraocular pressure challenge
    Wong, Vickie Hoi Ying ( 2012)
    This thesis shows that hyperglycaemia makes retinal function and ocular blood flow more sensitive to acute intraocular pressure (IOP) elevation. Increased functional susceptibility was associated with a reduced capacity to upregulate endothelial nitric oxide synthase. Chronic IOP elevation (4 weeks) produced retinal dysfunction and exacerbated the susceptibility of ocular blood flow to IOP challenge in diabetic but not healthy rats. These data suggest that hyperglycaemia-induced blood flow anomalies may contribute to the functional susceptibility of the diabetic eye to IOP.
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    Wide-field ganglion cells in primate retina
    Percival, Kumiko A. ( 2011)
    The anatomical circuitry involving distinct neuronal types within the retina, form the basis for the functional properties of ganglion cells. Such properties ultimately represent the nature of visual information carried by these cells for further processing in subsequent areas of the visual system, and are therefore crucial to our understanding of the visual system as a whole. The projects outlined in chapters 2, 3 and 4 investigate the anatomical connections of a group of ganglion cell types (wide-field ganglion cells) retrogradely labelled and photo-filled in the retina of the common marmoset (Callithrix jacchus). This animal model is ideal for our purposes, as ganglion cell types identified in marmoset retina to date, have closely resembled those of Old world primate species. Chapter 1 outlines the anatomical connectivity, morphological features and physiological properties of retinal neurons. Chapter 2 investigates the density and distribution of synaptic inputs onto the two dendritic tiers of small bistratified ganglion cells with the aim of elucidating the retinal connectivity behind their blue-ON/yellow-OFF chromatic responses. The density of presumed bipolar and amacrine cell inputs onto small bistratified cells were measured. All cells analysed showed a consistent pattern, where bipolar input to the inner tier is approximately fourfold greater than bipolar input to the outer tier. This structural asymmetry of bipolar input may help to balance the weight of cone signals from the sparse S cone array against inputs from the much denser M/L cone array. Chapter 3 investigates the density and distribution of synaptic inputs onto two wide-field ganglion cell types with largely contrasting morphological features. Large sparse cells have sparsely branching dendritic trees that are narrowly stratified close to the ganglion cell layer. Broad thorny cells have densely branching dendritic trees broadly stratified within the center of the inner plexiform layer. The density of presumed synaptic inputs was similar for the two cell types, indicating that the larger number of synapses on broad thorny cells may be attributable to the larger membrane surface area of this cell type. Synaptic input density was comparable to previous values for midget and parasol ganglion cells, suggesting that functional differences between these cell types do not arise from variation in synaptic input densities. Chapter 4 investigates the contribution of wide-field ganglion cells to the retinal circuitry subserving foveal vision. The fovea is a region of the retina specialised for high acuity photopic vision, in which the role of the midget pathway has been implicated. The question of whether wide-field ganglion cell types and their neural partners are present within the first 1mm of the foveola was examined. Bipolar cells were immuno-labelled in vertical sections of marmoset and macaque retina. At least three types of wide-field ganglion cells previously described in peripheral retina were present at the fovea. A subset of bipolar cells likely to be involved in providing inputs onto wide-field ganglion cells at the fovea was also found, supporting the view that retinal pathways involving wide-field ganglion cells contribute to foveal vision.