Optometry and Vision Sciences - Theses
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ItemTowards improving the detection of diabetic retinal disease( 2023-08)Diabetic retinopathy is a visual complication of diabetes clinically diagnosed by the presence of visible microvascular lesions in the retina. Since the hallmarks of diabetic retinopathy are vascular in nature, the assumption is that the retinal microvasculature is the main disease casualty. However, the impact of diabetes on the retina is widespread, affecting multiple components within the retinal neurovascular unit. More recent evidence reveals that retinal neuron damage, which can be manifested as visual function deficits and degeneration of the retinal layers, occurs prior to classical diabetic microvascular features. The American Diabetes Association has acknowledged diabetic retinopathy as a neurovascular complication of diabetes (Solomon et al., 2017). Additionally, some groups have also proposed the term “diabetic retinal disease” instead of diabetic retinopathy, as the terminology of the former more comprehensively characterises any retinal change owing to diabetes (Abramoff et al., 2018; Sun et al., 2021). Although new research reinforces vascular retinopathy as a late consequence of diabetic retinal disease, visual acuity (which remains preserved until the end stage of the disease) continues to serve as the main functional outcome for diabetic retinopathy in clinical research. Clinical detection of diabetic vascular lesions associated with an increased risk for visual impairment is also the only management strategy available for individuals at the early stages of the disease. Screening for diabetic retinopathy via ophthalmoscopic inspection remains the mainstay of clinical optometric practice and is still recommended by authoritative guidelines. The cost for diabetic retinopathy screening programmes is significant as persons with diabetes are (1) often working age, (2) will have to undergo several screening eye examinations in their lifetime, and (3) left without any therapeutic options until their vision is close to being compromised. New clinical markers, especially those that are likely to herald vascular damage, have the potential to improve detection of diabetic retinal disease. Numerous studies across the years have revealed a range of visual functional deficits in individuals with diabetes. Advances in retinal imaging technology, notably optical coherence tomography (OCT), have revolutionised clinical diagnosis of several retinal diseases. In the context of diabetic retinal disease, progressive thinning of the retinal neural layer can be measured by OCT in some individuals without diabetic retinopathy, supporting the notion of retinal neurodegeneration. OCT-angiography (OCTA), an update of OCT, can also detect microvasculature changes prior to overt vascular retinopathy. The focus of this thesis will be on clinical assessments of diabetic retinal disease. This thesis explored functional and structural outcomes measures from novel psychophysical techniques as well as structural OCT and OCT-A that could predict future diabetic retinopathy. Altogether, this thesis highlights the need for comprehensive assessment of diabetic retinal disease, which has future implications for improving how clinicians screen for and manage those with diabetic retinopathy.
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ItemCentral and peripheral motion perception in healthy older adults and its potential relevance to driving( 2021)Aging of the population is a common phenomenon in many countries. This increase in the number of older individuals consequently implies an increase in the number of older drivers. Older adults have changes to vision that could potentially influence driving behavior, as visual information is the predominant sensory input for operating a motor vehicle. One visual function that is potentially relevant for driving performance is motion perception, as both the motor vehicle and the surroundings are in motion. This project explored differences to motion perception between older and younger adults and considered in the applied context of driving. Experiment One explored the differences in performance of healthy older and younger adults on different aspects of motion perception under daylight viewing conditions. A battery of seven psychophysical motion perception tasks was applied to a cohort of participants belonging to two age groups (older and younger adults). Motion perception was also studied comparing central and peripheral vision. This experiment demonstrated that older adults had different results to their younger counterparts for some aspects of motion perception. In addition, for most of the tasks, the effects of aging were similar in central and peripheral vision. In Experiments Two and Three, four motion perception tasks were selected from the battery of tasks used in Experiment One to further test under viewing conditions that are commonly described as problematic by older drivers. These conditions included vision at mesopic light levels, such as those found during nighttime driving (Experiment Two) and driving under headlight glare similar to that of oncoming cars (Experiment Three). These two viewing conditions were simulated in a laboratory-based testing procedure. The results showed that thresholds were in general poorer under low light levels in both age groups. Experiment Three demonstrated that the presence of a continuous glare source simulating car headlights did not impact performance on the selected motion perception tasks. Experiment Four explored the relationship between motion perception and the measurement of the ability to predict potential traffic hazards in a computerized video test (the hazard perception test). The results of this experiment showed that two motion perception tasks (Dmin and motion contrast) were statistically related to the scores in the hazard perception test, and better predicted performance than measurements of visual acuity. This thesis, therefore, contributed to the knowledge of how aging impacts different components of motion perception, not only under photopic viewing conditions, but also under mesopic light levels and under simulated glare. This thesis demonstrated that some motion perception tasks clearly distinguished between age groups (Dmin, motion contrast and biological motion), but these group differences were absent for other tasks (global motion coherence). In addition, some motion perception tasks presented a wide range of interindividual differences in performance, suggesting that aging is a very individual process that cannot be assumed from chronological age.
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ItemPsychophysical explorations of the illusion underpinning frequency doubling perimetry in glaucoma( 2006-01)The spatial frequency doubling illusion (FDI) occurs when the contrast of a low spatial frequency sinusoidal grating is modulated at high temporal frequencies – its apparent spatial frequency increases. Earlier suggestions were that the FDI is generated by a specific class of retinal ganglion cells, which are preferentially lost in the early stages of glaucoma. Based on this linking theory, frequency doubling perimetry (FDP) was developed and several clinical reports confirmed its high efficiency in diagnosing early glaucomatous vision loss. However, this linking theory is not universally accepted and newer suggestions posit that the illusion arises because of temporal frequency related difficulties in temporal phase encoding ability. This thesis psychophysically examines the spatiotemporal characteristics of both the FDI and temporal phase encoding ability with achromatic and equi-luminant (both red-green (RG) and blue-yellow (BY)) gratings at a range of spatiotemporal parameters including those eliciting the FDI. (For complete abstract open document)
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ItemMechanisms of top-down processing in visual perception( 2013)Visual attention allows the brain to selectively process only what is relevant from the rich visual world that surrounds us. This selection process can be biased by both bottom-up processes that are stimulus-driven and top-down influences that are goal or expectancy driven. Top-down processes of attention, in turn, can be sub-divided into two systems: a location-based system, where stimuli are selected on the basis of their location in the visual field, and a feature-based system, where selection of stimuli is based on their featural properties (e.g. colour, direction of motion), regardless of location. Since both location- and feature-based attentional systems rely on processing within two inter-connected but distinct pathways in the brain, the mechanisms underlying each are separable, leading to the widely disputed question of whether and which system dominates attentional processing. This thesis had two primary goals – the first was to determine whether the effects of location-based and feature-based attention were different. Experiments 1 and 2 explored this possibility using psychophysical techniques that incorporated a unique attention-demanding global motion-perception task. In Experiment 1, location- and feature-based attention were deployed using three types of cues - location of motion, direction of motion and colour cues. Differential effects were elicited depending on the type of cue employed. In general, location-based effects were larger than feature-based effects of colour and direction of motion. In Experiment 2, the effect of adding a highly salient distracter to the tasks was examined. It was found that the presence of the distracter affected performances significantly only when features were cued and not when locations were cued. Furthermore, the effect of the distracter when features were cued depended on the similarity between the target and the distracter. The second goal of this thesis was to highlight the importance of the primary visual cortex (V1) in the attention neuro-circuitry. This was accomplished in Experiment 3, using a combination of functional imaging and psychophysical techniques. It was hypothesized that the size of V1 could determine the individual attention capacity in a visual search task. Consistent with this expectation, it was found that people with larger V1s tended to perform faster searches and hence had larger attention magnitudes. It was further hypothesized that the size of V1 could predict individual reading speed. Although this relationship was not elicited, a strong positive correlation was found between attention and reading speed, consistent with what was previously reported in the literature. The results from this study provide support for a location-based model of attention. They also provide insights into the effect of attentional capture by a distracter during focused attention conditions. This helps us appreciate the various constraints of attentional processing within the brain. Finally, the results from Experiment 3 are perhaps the first demonstration of a morphological link between the brain and a cognitive ability like visual attention. Together, the findings from this study set the stage for further research into the mechanisms and structural morphology underlying attention.
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ItemPerceptual centre-surround processing in older adults( 2012)The aim of this thesis was to investigate the effects of perceptual centre-surround suppression in older adults. It has previously been shown that older adults show increased contrast-contrast surround suppression for textured centre-surround stimuli. As the amount of centre-surround suppression is known to depend on stimulus parameters such as contrast, orientation and size, the purpose of the current experiments was to use a variety of stimulus parameters in order to assess under which conditions older observers demonstrate increased surround suppression compared to younger adults. Two groups of adult observers one young and one old participated in centre-surround contrast matching tasks. Experiment 1 aimed to investigate border cues between centre and surround stimuli. Surround suppression was measured when centre and surround were presented in-phase and when they were presented out-of-phase. Older observers produced greater amounts of suppression for both conditions when compared to younger observers indicating that the phase information at the border is not responsible for the increases in surround suppression. Additionally, Experiment 1 revealed that increases in surround suppression cannot be attributed to decreased contrast sensitivity of the older groups. Experiment 2 aimed to investigate surround suppression in older observers for drifting stimuli. Observers performed the contrast-contrast task as well as a motion discrimination task which has also been used to measure perceptual centre-surround suppression. Consistent with Experiment 1, older observers showed increased surround suppression for the contrast-contrast task, however performed similarly to younger observers for the motion discrimination task implying that the two tasks involve different mechanisms. Finally, Experiment 3 investigated the contrast ratios between the centre and surround. Results revealed that older adults showed increased surround suppression when contrasts were low, more specifically when centre contrast was low. When centre-surround contrasts were high (80/80%), younger and older observers performed similarly. The findings of this thesis demonstrate that perceptual contrast surround suppression is strengthened in older adults when compared to younger observers for a variety of stimulus conditions. The findings of Experiment 3 provide a possible explanation for the differences found between analogous perceptual tasks (contrast vs. motion tasks), with results enabling some inferences regarding neurophysiological mechanisms responsible for the age-related differences. Additionally, the results presented herein suggest that a series of perceptual tasks are needed in order to measure the balance of excitation and inhibition within the human visual system.
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ItemThe effects of ageing and visual field loss on visuomotor control( 2012)Aspects of visual localisation, such as foveal hyperacuity, decline with age. However, the effect of ageing on the visual localisation judgements required for goal-directed reaching movements has not been studied. It is also unknown whether pointing precision to such visual stimuli are altered with ageing. Further, the effects of visual impairment on these tasks are not understood. This thesis aimed to investigate the effects of ageing and compromised peripheral visual function on visual localisation and pointing precision. The first part of this thesis aimed to investigate the effects of ageing on visual localisation and pointing precision (Chapter 4). Experiment 1 consisted of a cohort of younger and older observers performing visual localisation and pointing tasks. The results suggest that both the visual localisation and pointing systems remain largely intact with ageing. However, the visual localisation precision of older adults was more affected by the removal of visual references than younger observers. These findings are encouraging for older adults; especially with the increasingly active part they play in the workforce, and society at large. In order to further probe the state of the older visuomotor system, the second part of this thesis investigated the pointing and visual localisation precision of older observers with compromised visual status (Chapter 5). Older adults with glaucoma, a chronic eye disease that results in reduced visual field sensitivity, were used as the model for compromised visual status in Experiment 2. Results suggest that patients with glaucoma show a reduced ability to locate objects both visually and manually. However, perimetry – a clinical measure of visual field sensitivity – provides only a small indication of the degree of this difficulty. Observers with glaucoma also showed a reduced benefit of binocularity compared with their older controls for visual localisation tasks, suggesting that reduced visual field sensitivity may inhibit aspects of binocular processing.