Optometry and Vision Sciences - Theses
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ItemMeasuring central visual field loss using visual stimuli with natural scene statistics( 2021)Routine visual field assessments are performed using static automated perimetry that measures contrast detection performance for white luminance increment targets presented on a uniform luminance background. While offering a standardised measure of the visual field, such clinical visual field tests are not representative of natural viewing conditions. Previous studies show that some visual field defects are detectable via visual search behaviour analysis; for example, when watching a video or performing daily living tasks. Such measurements of performance using visual stimuli representing everyday tasks can provide information regarding how people with visual field loss perform day to day activities, but due to widely varying visual content, it is difficult to predict a generalised explanation of performance with these tasks to create a widely applicable screening tool. This thesis aimed to develop a method for detecting central visual field loss by measuring the number of fixations to find targets on a background with spatial frequency content similar to natural scenes (referred to as 1/f noise). An advantage of this proposed approach is the existence of an established theoretical framework [Najemnik, J., & Geisler, W.S. (2005), Nature, 434(7031), 387-391] that links the detectability of targets within an image to the number of fixations required by a Bayesian ideal observer to find the targets. The stimulus and behavioural methods used in this thesis were chosen to be consistent with those used to validate the Bayesian ideal observer model. Four experiments were explored in this thesis that stepped through creating the screening protocol for detecting central visual loss. In the first step, Experiment One computationally assessed the suitability of using similar stimuli and behavioural methods as Najemnik and Geisler (2005) to detect field loss. Experiment Two evaluated the need for an age-matched normative limit for setting the contrast of the target for the screening protocol using such a method. Experiment Three determined the requirement of age-matched normative data for the number of the fixations to find the targets on the 1/f noise background, the outcome measure for the screening protocol. The outcomes from Experiment One to Three were then applied to Experiment Four that developed a prototype test for detecting central visual field loss by measuring the number of fixations required to search for a target on a 1/f noise background. This experiment also tested if the developed prototype was able to detect central visual field loss in a group of glaucoma participants. The prototype test demonstrated 85% sensitivity for a fixed specificity of 95.2% in screening abnormal areas in central vision in the glaucoma group. The task requirements of the screening protocol were designed to be representative of natural visual environments and to be intuitive for participants to perform the test. While the prototype test was tested in participants with glaucoma, the developed methods in this experiment are designed to generalise to any form of central visual field loss.
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ItemVisual search and visual performance in infantile nystagmus syndrome( 2020)Abstract Infantile nystagmus syndrome (INS) is an involuntary ocular motor oscillation, which presents at or near birth and persists throughout life. The nystagmus intensity and visual acuity in INS may vary with gaze angle. The gaze with minimal nystagmus intensity and better visual performance is known as the null position. Nearly all the research on INS focused on visual acuity and the time needed to get the eyes onto the desired target (i.e., target acquisition time). In daily routines, we are constantly presented with search tasks that require us to find a target among distracters or tracking tasks where we need to identify moving objects and to estimate their speed of motion. These real-life visual activities entail complex visual functions, such as visual search and motion perception. However, research on INS and these visual functions is limited. Thus, this study aims to investigate how individuals with INS perform, compared with controls, when carrying out visual search tasks and motion perception tasks. Particularly, the study also aims to assess how the null position affects their visual performance. For visual search, two search conditions were presented: conjunction search and feature search. Search time and accuracy were used to assess visual search performance. For motion perception, three tasks were performed: coherent motion, velocity discrimination, and biological motion. Motion coherence thresholds, discrimination thresholds, and accuracy were measured for the three tasks, respectively. In visual search tasks, INS subjects showed poorer search performance, with longer search times compared to controls in both conjunction and feature search. No difference in accuracy between INS and control subjects was found. The null position did not affect the visual search performance in INS. In coherent motion and velocity discrimination tasks, INS subjects showed poorer performance, with elevated motion coherence and discrimination thresholds compared with controls. A positive null position effect was found only in velocity discrimination. In the biological motion task, no difference in accuracy between INS subjects and controls was found. In summary, visual search, coherent motion, and velocity discrimination were impaired in INS subjects, with the null position having a positive effect in velocity discrimination. However, biological motion perception was not affected by INS. Findings from this study could assist us in understanding of how INS actually affects the daily activities of patients, and aid us in developing new clinical visual function assessment for INS.