Optometry and Vision Sciences - Theses
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ItemReceptive field properties and dynamics in mammalian primary visual cortex( 2015)The functional properties and structure of receptive fields in primary visual cortical (V1) neurons represent how visual information is processed in the mammalian neocortex. Cortical receptive fields are diverse and highly dynamic to accommodate the constantly changing visual environment. The mechanisms behind the organisation of different types of receptive fields are still highly debated after David Hubel and Torsten Wiesel first described the fundamental properties of cortical receptive fields half a century ago. These pivotal discoveries were conducted in the classic animal models of vision research: cats and monkeys. In recent years, fuelled by the opportunities for genetic and molecular manipulation, mice have rapidly become a major model for studying cortical visual processing. It is essential to recognise the similarities and differences between mouse V1 and that of the well-established animal models. A major goal in this thesis is to compare the receptive field properties of mouse V1 (area 17) and cat V1 (area 17 and 18). Cortical neurons are largely composed of excitatory pyramidal cells and GABAergic inhibitory cells. Compared to excitatory neurons, the receptive field properties of inhibitory neurons are poorly understood due to the difficulty in identifying the diverse inhibitory subpopulations. In Chapter 4, by separating inhibitory and excitatory neurons based on their spike waveform shapes, I was able to examine the inhibitory receptive fields in both mouse and cat V1 and demonstrate differences in orientation selectivity and response linearity between these cell types in two species. In addition, I was also able to show that inhibitory cells were significantly over-represented in layer 1 of cat V1 and were less sensitive to low contrasts, as a population, compared to excitatory cells. Based on receptive field structures and response properties, V1 neurons are classified into simple cells and complex cells. Simple cells are thought to have spatially segregated ON and OFF subfields and are thus highly selective for the spatial phases of oriented edges. Complex cells have intermingled ON and OFF subfields and are largely phase-insensitive. Recent evidence reveals that some complex cells in cat and monkey V1 show increased phase sensitivity in their spiking activity as stimulus contrast is reduced, which suggests a shift towards a simple-like receptive field at low contrasts. By employing drifting sine-wave gratings (Chapter 5) and contrast-reversing gratings as visual stimuli (Chapter 6), I demonstrated the same effect in mouse V1 neurons. Furthermore, through intracellular recording I also observed contrast-dependent phase-sensitivity in the subthreshold membrane potentials of the cells as well as their spiking responses. This confirmed that the contrast-driven effect was a result of altered synaptic inputs and not the non-linear transformation from membrane potential to spike output.
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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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ItemFunctional correlates between the rat electroretinogram and visual evoked potential( 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.