Optometry and Vision Sciences - Theses
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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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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.