Optometry and Vision Sciences - Theses
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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.