Optometry and Vision Sciences - Theses

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Subject: vision × Start date: 2000 × End date: 2009 ×

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    Psychophysical explorations of the illusion underpinning frequency doubling perimetry in glaucoma
    Vallam, Kunjam ( 2006-01)
    The spatial frequency doubling illusion (FDI) occurs when the contrast of a low spatial frequency sinusoidal grating is modulated at high temporal frequencies – its apparent spatial frequency increases. Earlier suggestions were that the FDI is generated by a specific class of retinal ganglion cells, which are preferentially lost in the early stages of glaucoma. Based on this linking theory, frequency doubling perimetry (FDP) was developed and several clinical reports confirmed its high efficiency in diagnosing early glaucomatous vision loss. However, this linking theory is not universally accepted and newer suggestions posit that the illusion arises because of temporal frequency related difficulties in temporal phase encoding ability. This thesis psychophysically examines the spatiotemporal characteristics of both the FDI and temporal phase encoding ability with achromatic and equi-luminant (both red-green (RG) and blue-yellow (BY)) gratings at a range of spatiotemporal parameters including those eliciting the FDI. (For complete abstract open document)
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    A development of Chinese near logMAR visual acuity charts
    Hao, Yan Mei ( 2000)
    Near visual acuity (VA) test charts using simplified Chinese characters were designed based on the logMAR principle. Two experiments were conducted on native Mandarin-speaking subjects with normal vision. In experiment one, the relative legibility of characters with different strokes (‘complexity’ of from 4 to 10 strokes per character) and the effect of different viewing distances on VA were investigated. No significant difference were found between test distance but significant differences were present between subjects and character complexity. In experiment two, a clinical method of rating VA (VAR) was compared with the determination of a psychometric threshold in 28 subjects, using charts with a 0.05 logMAR step. Results showed that VAR was not significantly different from the 75% psychometric threshold. The assumption made in this thesis that increased stroke density will reduce legibility was confirmed by the experimental results. Two near Chinese logMAR charts were designed with mixed-stroke of characters based on the findings of the first two experiments. In experiment three, a direct comparison of the Chinese near logMAR charts with the established Bailey-Lovie near logMAR chart were performed, using 6 bilingual subjects, a consistent pupil size of artificial pupil, and simulated low vision conditions by defocus. Regression analysis showed a highly correlation between the Bailey-Lovie near logMAR chart and the Chinese near logMAR chart. A correction factor was determined for all Chinese near VA charts that allows for direct comparison to be made with the established Bailey-Lovie near VA charts. A finding of a similar visual acuity from both charts confirmed the Chinese chart as comparable to the current clinical standard.