Optometry and Vision Sciences - Theses
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ItemThe effects of task-induced stress and mental workload on visual performance and eye movement behaviour in nystagmus and controls( 2016)Background and aims: During daily activities, individuals may experience stress when a visual task is combined with an irrelevant mental task (e.g. conversing). Clinical examinations of patients with infantile nystagmus syndrome (INS) may elicit subjective reports of worsened nystagmus under internal states such as stress. Although the negative effects of stress on visual function have been widely studied in healthy subjects, few studies are available for patients with INS. Previous findings have demonstrated that INS is influenced by task conditions (i.e. visual demand and internal states such as stress); however, these studies limited their methodologies to only the null or central gaze position. Gaze position, solely, can affect visual function in INS and can affect INS parameters such as foveation time. By acknowledging that gaze position can affect INS, one gaze position (e.g. central) may not be enough to measure the changes in INS arising from task variations. Here, we hypothesised that a task-induced change from baseline in INS parameters at the null position would differ from task-induced changes at the gaze position away from the null. Patients with INS may also complain of being slow to see. Target acquisition time and the following visual processing time can affect the total period taken to react to a visual object. Surgical improvement in foveation has resulted in a reduction in both target acquisition time and visual recognition time. A few studies have reported that stress exacerbates nystagmus by increasing its intensity and reducing foveation time. Since tenotomy has been reported to decrease target acquisition time in INS, there was a possibility that changes in INS parameters under internal states such as stress affected target acquisition time. Thus, our second hypothesis stated that the effects of stress on visual timing would differ between control and INS subjects. The objective was to measure the extent to which saccades, target acquisition and visual processing times are affected by cognitively induced stress in INS subjects in comparison to the control group. Methods: To test our first hypothesis, we conducted a visual acuity experiment. We varied task conditions by manipulating levels of mental load and visual demand, thus inducing varying stress levels. Participants with idiopathic INS were required to determine the direction of Tumbling-E targets, which varied in size and contrast, using a staircase procedure across ±25° gaze positions, with 5° steps from the centre. To test our second hypothesis, we conducted a saccade experiment, which required INS and control subjects to respond to the direction of horizontal E targets presented randomly at the post-saccade position, set at ±25° with 5° step, away from the centre. Visual task performance was measured by subjects’ reaction time and their response accuracy to the direction of the post-saccade target. Each of the above tasks was performed under two conditions: alone (low mental load) and with mental arithmetic and time restriction (induced high mental load). Perceived workload across these task conditions was assessed via changes in heart rate as well as other physiological measurements such as skin conductance level, and subjective ratings. The subjects’ eye movements and visual task performance were recorded across varying task conditions. Results: The induction of the mental arithmetic task and time restriction increased mental workload. For both visual acuity and saccade experiments, the metrics of task performance worsened under high mental load. For the visual acuity experiment, there was a significant interaction between mental load and gaze position for foveation time and task performance. Interaction effects were interpreted as follows; during high visual demand, the change in foveation time due to mental load was greater at the null than away from it. Similarly, the increase in last optotype size from the low to high mental load condition was more pronounced at the null than away from it. During both low and high mental load, saccade latency, target acquisition time, and visual recognition time were longer in the INS group in comparison to controls. For both control and INS subjects, saccade latency and target acquisition time increased with high mental load. The LATER model (Linear Approach to Threshold with Ergodic Rate), implemented to analyse saccade latency in the control group, highlighted that the rate of rise in saccade decision signal decreased during high mental load. Visual recognition time increased with high mental load only in the INS group. The effects of mental load on the gain of prosaccades and target acquisition time varied between INS and control subjects. Discussion: We suggested that the extent to which foveation duration contributes to visual function varies across both task conditions and gaze positions. We agreed with the notion that an extended foveation time does not guarantee an improvement in visual task performance in INS. Previous studies have highlighted that INS subjects are late in detecting a new target, which can result in a delay in target acquisition. Consistent with the literature, our study concluded that difficulty in the accurate programming of eye movements used to acquire a new target can also delay target acquisition time. Our results suggest that a further increase in target acquisition time with stress worsens the phenomenon of being slow to see in INS. We concluded that an increase in saccade latency with high mental load is due to a reduction in the capacity of information processing resources in both individuals with and without INS. However, visual recognition time in INS is multifactorial and can show additional increases under stress (e.g. driving) when compared to normal subjects. We have suggested that the efficacy of visual processing further decreases with effort to see in INS. Conclusion: Possible main and interaction effects of task condition and gaze position on foveation time can lead to an overestimation of visual function in INS. Accuracy and speed of the subjects’ response to visual targets should be measured with a mental load resembling real world conditions. The results can be useful in relation to work safety and clinical assessment of INS, especially in terms of treatment outcomes.
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ItemThe influence of playing video games as an attention rehabilitation technique in patients with traumatic brain injury( 2016)Introduction: Video game playing has been suggested to modify various aspects of visual attention in healthy adults. Moreover, it has been tried as a training regimen to improve less than normal cognitive and perceptual abilities in groups such as the elderly and individuals with amblyopia and dyslexia. However, so far there is little evidence regarding possible game related modifications on overt allocation of visual attention (i.e. saccadic eye movements). Therefore, our first aim was to investigate whether or not observed benefits of gaming on covert allocation of attention can be related to any modifications in eye movement behaviour. Then, the influence of video game training as a possible attention rehabilitation technique in patients with traumatic brain injury (TBI) was examined. Methods: Three experiments were conducted to address these aims. In experiment one, saccadic eye movement characteristics of 33 gamers and 18 non-gamers were compared using four simple saccade paradigms along with two attentional measures. In experiment two, 40 healthy non-gamers were allocated to play either an action video game (Call of Duty: Modern Warfare 2) or a control card game for 10 hours to investigate the causal influence of gaming on saccadic eye movements in five visual search paradigms. In experiment three, four patients with history of traumatic brain injury were trained on the game Call of Duty and their eye movement characteristics and attentional abilities were assessed using a single case experimental design. Results: Except for a shorter antisaccade latency in video gamers, there were not any significant differences between gamers and non-gamers in their eye movement characteristics as tested with simple paradigms. However, gamers tended to be more impulsive, as measured with a continuous performance test. In the second experiment, non-gamers who were trained on an action game did not show any significant modification of their eye movement behaviour while performing visual search tasks. However the vertical distribution of their fixations became narrower in the game-related search task, suggesting that they only learned the likely distribution of targets important in the game. A few significant improvements were observed in each patient with TBI as a result of the action game training, including shorter memory-guided saccade latency and faster search time in patient 1, shorter antisaccade latency and higher rate of self-paced saccade generation in patient 2, more accurate prosaccade gain, higher rate of self-paced saccade generation and faster search time in patient 3 and faster search time and shorter fixation duration in patient 4. Discussion: The findings generally suggest no eye movement modifications as a result of video gaming in neurologically intact people obtained both in cross-sectional and game training experiments. This finding might be related to already optimal eye movement functioning, as we normally move our eyes 2-3 times a second, therefore experiences such as video games cannot further enhance this function. A few improvements were observed after action game training in each patient with TBI in this exploratory study. The differences seen in the patients may reflect differences in underlying pathology. However to propose this technique for attention rehabilitation, more evidence, perhaps using custom made video games adaptable to patients’ needs, is required.
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ItemImproving adaptive optics image quality in high powered eyes( 2016)Adaptive optics (AO) retinal imaging has been widely used for high resolution in vivo imaging studies in the human eye over the last two decades. More recently, this technology has been applied to image the retina of small animals such as rodents. Small animal eyes offer many advantages for the scientific study of retinal diseases, and theoretically have potential for superior image quality compared to human eyes. Unfortunately, to date AO image quality in practice is inferior to state-of-the-art image quality obtained from the human eye, and the cause of these limitations has not been fully understood. This work aimed to investigate and address the limitations to AO image quality in rodent eyes, and explore novel techniques in adaptive optics correction and image processing to overcome these limitations. The first experiment used optical modelling to test several commonly made assumptions in human AO imaging. These assumptions were found to be inappropriate for extrapolation to rodent eyes, due to their very high power and dioptric thickness of the retina. Specifically, results showed that AO image quality for the human eye is robust against positioning errors of the AO corrector and to differences in imaging depth and wavelength compared to the wavefront beacon. On the other hand, image quality for the rat eye declined sharply with each of these manipulations. The second experiment used a purpose-built flood-illumination AO ophthalmoscope designed for rat eyes to validate the modelling results from the first experiment, using physical model eyes of varying optical power. It was found that in general, AO image quality from the lower powered (60 D) model eye was much less susceptible to manipulations similar to those described in the first experiment, compared to the higher powered (220 D) model eye, thus confirming the optical modelling results. The same optical system was then used to account for these limitations as much as possible, while imaging adult pigmented Long-Evans rats. However the images obtained showed only modest improvement compared to the pilot experiment. It is argued that this occurred primarily due to intra-ocular scatter and the challenging wavefront sensing step in the rat eye which are exacerbated in our flood illumination setup. The third and fourth experiments explored two novel techniques, HiLo imaging and non-sensing AO, to address problems with intra-ocular scatter and wavefront sensing respectively. Both experiments resulted in noticeable improvements in AO image quality from the rat eye. In conclusion, with almost twice the numerical aperture, retinal image resolution for rodent eyes is potentially far better than the human eye, provided aberrations can be corrected with AO. This thesis explored the limitations to AO image quality in rodent eyes, as well as practical methods to improve image quality by addressing these limitations. Although the final flood-illumination AO image quality from the rat eye was still inferior compared to scanning AO modalities, the findings from this thesis could be used to improve the AO image quality for all imaging modalities for both human and rat eyes.
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ItemEye movements in neurocognitive disorders and frontotemporal dementia( 2016)Introduction/Objective: Frontotemporal dementia (FTD) is a young onset heterogeneous neurodegenerative condition which presents with changes to behaviour and language and causes degeneration of a number of cortical regions, particularly the frontal and temporal lobes. Onset of the disease is often insidious, and a wide variety of presentations can be seen clinically. Signs and symptoms can be difficult to quantify early in the disease and there is a large overlap with a range of other diagnoses. Neuropsychological testing can often under-represent the deficit seen behaviourally by collateral observers and return a non-specific profile. Vision is a dominant sense and the brain pathways and networks used for eye movements are widespread and well-documented. Therefore the aim of this thesis was to characterise eye movements in frontotemporal dementia across a hierarchical range of movements and to investigate if eye movements could be linked to neuropsychological deficits occurring in the disease. Method: FTD and control participants were recruited to examine eye movement changes in the disease. A hierarchical battery of eye movements were examined beginning with the simplest movements, saccades, which require only a short pathway through the brainstem, and progressed to examining scan paths in more complex tasks including facial emotion recognition, visual search and driving, thus recruiting additional cortical regions and networks. These tasks were selected as FTD patients are reported to have difficulty correctly undertaking them. Saccadic eye movements included horizontal and vertical reflexive saccades, which were then built upon by examining higher-order saccades including antisaccades, self-paced, predictive and memory-guided saccades. Results were examined with regard to eye movement characteristics reported in the literature for other neurocognitive conditions including parkinson’s-plus disorders, schizophrenia and Alzheimer’s disease. Results: A total of 28 FTD variant and 25 controls were recruited to participate. Not all participants undertook all tasks. Some participants were unable to undertake specific tasks either due to complexity or in the case of the driving task because they had never held a licence. In addition some participants had to stop partway through a task due to simulator sickness. Within the FTD group insufficient semantic dementia and progressive non-fluent aphasia patient participants were able to be recruited and so their data is presented in each task as pilot data. There was also a reduction in data due to some trials being excluded for poor tracking. Saccadic eye movements revealed an increased latency across all basic movements. Scan paths to emotional faces from the Ekman Pictures of Facial Affect series revealed a reduced gaze time to the right eye for both implicit and explicit face viewing but otherwise resembled those of controls. Eye movements whilst driving revealed gaze times not significantly different to any area of interest other than to pedestrians, which elicited less gaze time from bvFTD participants. Eye movements during visual search revealed an increased reaction time intercept but a similar processing time per item to control participants. Scan paths to all tasks were qualitatively normal. Discussion: Eye movements were analysed across a hierarchical range of movements, starting with simple eye movement circuitry and progressively incorporating more brain regions and networks. Stimuli were selected to specifically target tasks known to produce errors in FTD. Our study revealed bvFTD patients display eye movement changes sharing two prominent features: a psychomotor slowing and a disruption of salience, possibly due to changes in their assignment of mind. Psychomotor slowing has been evidenced in both saccade and visual search tasks as patients were able to correctly undertake the task but at a significantly slower pace, seen in increased reaction times and increased latencies. The alterations in salience noted including reduced gaze time to the right eye and a lack of gaze time to pedestrians whilst maintaining qualitatively normal scan paths and gaze durations to other areas of interest, leads to the conclusion that social cues are detected but not processed and interpreted correctly. Qualitatively normal scan paths are in stark contrast to those seen in a number of overlapping neurocognitive diseases including schizophrenia and Alzheimer’s disease, which have been reported in the literature to lead to hypo-scanning and disorganised scan paths respectively. Our study did not differentiate different pathological groups; however there were a wide range of responses in each task which may reflect different underlying pathologies, this finding warrants future studies examining saccadic paradigms other than reflexive and antisaccades. Scan paths, such as to faces, may be able to be utilised diagnostically in conjunction with other examinations to differentiate between bvFTD and other degenerative groups. Performance on the driving simulator task suggests potential for a driving task to be developed to examine safety to drive.
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ItemInto the void: exploring the experience of vision loss through visual art( 2016)This thesis explores a personal experience of vision loss caused by the onset of juvenile macular dystrophy, through the framework of a studio art practice. It asks, “What does vision loss look like?” or, more specifically, “what does macular dystrophy look like to me?” seeking to find those answers in the very arena where its impact is felt most – in the field of vision as it is experienced in everyday life, and through the very visually expressive language of art.
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ItemAge-related changes in structure, function and response to stress in the rat retina( 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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ItemUsing the eye as a surrogate for the brain: Imaging blood-neural barriers with fluorescein angiography( 2016)The eye allows for direct and non-invasive imaging of the vasculature. Using fluorescein angiography, this thesis explored whether blood vessels in the eye may be a surrogate for those in the brain. The vasculature in both tissues showed similar fluorescence dynamics. Systemic disruption of blood vessel integrity affected both the eye and brain, with effects in the brain lasting longer. Vascular leakage in the eye may provide information about the integrity of the vasculature in the brain.