Optometry and Vision Sciences - Theses
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ItemIntegration of auditory and visual temporal rate in aging( 2017)This thesis investigated how aging affects the integration of visual flicker (the temporal modulation of luminance) with auditory flutter (the temporal modulation of sound amplitude) to produce a unified audiovisual percept of temporal modulation rate. A group of younger and older adults judged the temporal rate of an auditory and/or visual stimulus oscillating at 10 Hz. Whichever sensory modality discriminates temporal rate more precisely, contributes more to the audiovisual percept. Consequently, the first experiment explored how aging affected the precision of auditory temporal rate discrimination relative to vision. Auditory temporal rate discrimination in older adults was degraded by an age-related impairment in sensitivity to auditory amplitude modulation. In subsequent audiovisual experiments, auditory modulation depth was individually tailored to equate flutter and flicker temporal rate discrimination thresholds to normalise for this age-related sensory loss. When auditory and visual rates were conflicting, partial integration distorted perceived rate such that the auditory or visual rate subjectively equivalent to a reference was nonveridical. Distortions in perceived rate were unaffected by older age, indicating that the ability to integrate conflicting auditory and visual rates is preserved in aging. However, younger adults’ heightened sensitivity to auditory amplitude modulation was sufficient to increase the influence of audition on perceived rate when the modulation depth of auditory flutter was the same as the average older adult. Therefore, the age-related impairment in auditory rate discriminability is expected to increase visual influence on audiovisual rate perception in older adults. When auditory and visual rates are identical, temporal rate discrimination thresholds improved in line with statistically optimal integration in younger but not older adults. This indicates an age-related impairment in integration, which will be further compounded by the age-related decline in auditory temporal rate discriminability under natural conditions. These findings indicate that older adults will perceive audiovisual temporal rate differently to younger adults. These age-related changes in audiovisual rate perception will be the complex product of the age-related interaction between rate congruence and integration ability, and the age-related decline in auditory temporal rate discrimination.
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ItemAudiovisual synchrony perception in older adults( 2015)The purpose of this project was to examine differences in the behavioural and neural measures of audiovisual synchrony perception between younger and older adults. To accurately perceive events in the natural scene, the human perceptual system needs to combine related and segregate unrelated auditory and visual stimuli. The amount of temporal asynchrony is one of the key properties that define whether an auditory and a visual stimulus are related, and whether they should be perceived as originating from the same event. As older adults experience reduced visual contrast and hearing sensitivities, this project investigated audiovisual synchrony perception in older adults after scaling stimulus levels to individual detection thresholds. It was also investigated if older adults are able to realign their perception of audiovisual synchrony after adapting to asynchrony - an ability that is potentially important for correctly perceiving audiovisual pairs across distance. Additionally, this project investigated the impact of older age on the underlying neural time course for audiovisual synchrony perception. The results from this project demonstrated that older adults were less sensitive to audiovisual asynchrony even after scaling the stimuli to their own detection thresholds for visual contrast and sound intensity. Older adults also adapted less to sound-lag asynchrony adaptation as compared to the younger adults. In addition, older adults showed additional neural activity in the frontal and parietal regions in order to form perceptual decisions for audiovisual synchrony. The findings from this project could imply that older adults find it more difficult to correctly perceive related audiovisual events in a natural scene, particularly for distant events. The additional involvement of the frontal and parietal areas in older adults may indicate that they may compensate by recruiting extra neural resources to perform the same perceptual task as younger observers. This project also considered some future work that are required to understand the real-life implications of older age on audiovisual synchrony perception like the effect of spatial clutter (i.e. in crowds) and the effect of rapid adaptation. As a whole, the findings from this project added knowledge to the body of work of ageing on audiovisual synchrony perception.
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ItemThalamocortical transformation of visual signals: role of geniculate orientation biases and intracortical circuits in orientation selectivity of striate cortical neurons( 2013)One of the fundamental questions in our understanding of the visual system is posed by various neurons along the afferent visual pathway, from eye to the brain, which differ in their response properties. While the neurons in the retina and the lateral geniculate nucleus (LGN) of cats and most mammals respond reasonably to diffuse patches of light, most primary visual cortical neurons (striate cortex) respond only to elongated stimuli of particular orientation. Hubel and Wiesel’s Nobel prize winning discovery of this orientation selective property of striate cortical neurons has opened up an enormous field for investigation. They proposed that striate cortical orientation selectivity arises from excitatory convergence of several non-oriented LGN afferents whose receptive fields are arranged along the long axis of the striate cell’s receptive field. The emergence of orientation-tuned responses among these striate cortical neurons from relatively poorly tuned thalamic neurons has been a topic that courted an intense debate. This comes from the findings of alternative models of orientation selectivity that have shown that the excitatory input from the LGN is almost circular, that the major excitatory drive comes from within the cortex and intracortical inhibition may have a role in the generation of orientation selectivity. These models claimed that inhibitory circuits within the visual cortex are required to sharpen the broadly tuned input from the LGN. One such model, called anisotropic LGN-driven recurrent model (ALD-R model), suggests an alternative scheme that exploits the orientation biases present in the responses of subcortical neurons, with the intracortical inhibitory and excitatory networks sharpening this broadly tuned subcortical inputs at the cortical level. Several experiments were designed in this study to test the predictions of the ALD-R model, including the role played by geniculate orientation biases and non-specific inhibition in the orientation tuning of cat striate cortical neurons. Firstly, the contribution of broadly tuned inhibition on geniculate orientation biases was tested using a novel electrical stimulation protocol within the LGN. The effect of this LGN electrical stimulation was also studied in the topographically corresponding striate cortical region. Our results showed that the non-specific inhibition brought about by electrical stimulation within the LGN resulted in significant sharpening of the LGN orientation biases as well as a mild broadening of the orientation selectivity of neurons in the corresponding striate cortical region. The effect of non-specific inhibition generated by this electrical stimulation paradigm was also compared with pharmacologically induced inhibition on geniculate relay cells and the results were comparable. These findings add support to the ALD-R model, where even non-specific inhibition acting on geniculate orientation biases could lead to sharper orientation tuning similar to those seen among striate cortical neurons. The dependence of striate cortical orientation preferences on LGN orientation biases was tested in the second experiment, where simultaneous paired extracellular recordings were performed from a geniculate cell and a cortical cell, which had overlapping receptive fields. The rationale behind this study is that a geniculocortical pair with matched orientation preferences would show higher coherence than a pair that has near orthogonal orientation preferences. The degree of coherence between the spikes from each neuronal pair was compared with the differences in optimal orientations exhibited by each geniculocortical pair. The results support the prediction of ALD-R model that orientation preferences of LGN neurons can predict the orientation preferences of striate neurons that they purportedly project to. The next experiment tested whether contrast invariance of orientation tuning, a property that is often pointed out as a drawback of Hubel and Wiesel’s model, is a subcortical property that gets transferred to striate cortical neurons. Extracellular responses of LGN neurons for varying stimulus contrasts were studied and their orientation sensitivities for the high and low contrast stimuli were compared. The results show that cat LGN neurons exhibit contrast invariance of orientation tuning and striate neurons could acquire this property from the geniculate excitatory inputs. Together, the results of the above studies substantiate the claims of ALD-R model where broad orientation selectivities present in geniculate inputs predict striate cortical orientation tuning and also other properties such as contrast invariance. The contribution of intracortical inhibition, especially those that are broadly tuned or non-specific is pivotal for the generation of orientation selectivity of striate cortical neurons.
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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.