Optometry and Vision Sciences - Theses

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    Optical modelling of visual performance
    Liou, Hwey-Lan ( 1996-06)
    The aim of this thesis is to develop a method of optical modelling that can be used to predict visual performance of the eye. It is intended to give visual acuity estimates under normal circumstances and under a wide range of optical treatments such as photorefractive keratectomy (PRK) to correct refractive error. Visual performance refers to the performance of the eye under various conditions such as decreased object contrast, defocus and change in pupil size. (For complete abstract open document)
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    Problems and solutions in the optometric management of presbyopic airline pilots
    Ungerer, Jan Louise ( 1986)
    It is well known that the near visual tasks involved in flying pose some special problems for presbyopic pilots because charts, flight and engine instruments and the overhead panel are at widely varying distances from the eye. (For complete summary open document)
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    An analysis of colour vision standards in the transport industry
    Vingrys, Algis Jonas ( 1984)
    Colour vision standards were introduced by transport industries at the end of the nineteenth century. They have remained relatively unchanged since introduction, despite advances in transport technology and improved knowledge on the characteristics of human colour vision. No agreement exists on the precise level of rigour that standards should set, or if they are needed at all, which has led to an inconsistent diversity of standards used by many transport authorities. This thesis reports a critical analysis of the use of colour coding within the transport industry and concludes that colour still serves an important role in information transfer, despite an increasing reliance on electronic aids for navigation and communications. Disabilities suffered by colour defective observers identifying colour codes are also reviewed. Experimental studies reported in the literature show persons with abnormal colour vision are more likely to make colour confusions, have slower reaction times and reduced visual ranges to coloured signal lights. Accident statistics suggest certain colour defective groups may have a higher risk of accident than persons with normal colour vision. Thus it is concluded that standards of colour vision remain justified and should be retained, especially for operators of public transport vehicles. Experiments conducted and reported in this thesis consider problems specifically related to certain types of colour defective observer or the testing of colour vision. Protanomals and protanopes were found to have a similar reduction of visual range to deep red colours, even though protanomals possess a red-absorbing cone photopigment. Therefore protans should not be considered safe for employment where distant signals, especially red signals, must be recognized. The history of tests used for enforcement of colour vision standards is reviewed. Lantern tests, introduced in 1875, remain the principal test methods for the administration of colour vision standards. It is argued that lantern testing should continue because clinical tests poorly predict an observer's ability to recognize coloured signal lights. However, protanomals pass some lanterns and a clinical diagnosis should supplement these lantern tests to identify and reject protans. Ergonomic solutions, in the selection of colours, do not help colour defective observers reliably recognize a red-green-white colour code, even after allowing an extended practice trial at the recognition of these colours. Protanopes show least benefits from these engineering changes and give the worst performance. However, colour identification by other observer groups improves using a bluish-green or deeper red than the present orange limit recommended by the CIE. Finally, a rational approach is suggested for setting colour vision standards by transport authorities. This approach considers the variables of task demand, exposure, consequences of accident and community attitudes. It suggests international implementation of a set of equitable standards based upon these factors, rather than loosely or poorly defined standards that may be subject to various interpretations. Three classes of standard are recommended, namely, colour safe, colour normal and colour defective safe. These classes define the intent of the standard with each recommended fail criterion establishing the degree of difficulty set.
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    Further studies on the acquired tritanopia of dominantly inherited juvenile optic atrophy: and, the mechanisms of acquired dyschromatopsia
    Smith, Damien Patrick ( 1974)
    A clinical investigation of two families purporting to show congenital tritanopia confirmed that a condition of familial tritanopia unassociated with neuro-retinal disease does exist and is a unique nosological entity. The investigation refuted a recently promulgated view that congenital tritanopia does not exist and cases mistakenly reported as such in the past have actually been secondary to an undiagnosed condition of dominantly inherited juvenile optic atrophy (DIJOA). The clinical investigation also exposed differences in the clinical manifestation of the congenital and acquired forms of tritanopia and these differences were further examined in an experiment which measured the colour-naming functions of congenital tritans. That experiment did not specifically confirm the two-hue theory of dichromasy because the subjective spectrum of congenital tritans was found to be multi-hued, and because the yellow-green rather than yellow portion of the spectrum was seen as desaturated. When compared with observers exhibiting the acquired tritanopia of DIJOA, the congenital tritans failed to show gross desaturation of the short and middle wavelength parts of the spectrum, but showed greater confusion between blue and green. It was concluded that the differences in colour-naming behaviour reflect different physiological mechanisms underlying the congenital and acquired dyschromatopsias, in particular, that the acquired tritanopia is accompanied by attenuation of the green cone system. The two-colour increment threshold technique developed by W. S. Stiles was used to examine the characteristics of it mechanisms in the acquired tritanopia of DIJOA and in several other acquired dyschromatopsias. A method of graphical analysis was innovated to enable t-v-r curves to be objectively fitted to increment threshold data. The acquired tritans did not exhibit the blue-sensitive mechanisms, iii, '2 and i3, and the remaining mechanisms showed an anomalous relationship between the increment threshold response and the effects of the adapting field. When stimulus wavelengths of 445 nm and 480 nm were employed, after threshold had risen about 1 log unit from its absolute level, the determination of threshold response shifted to a second mechanism which was less sensitive to the stimulus wavelength. As well, the remaining mechanisms showed an enhanced sensitivity to the wavelength of the adapting field, and that sensitivity could not be confirmed in measurement by the absolute threshold method. The increment threshold response in other acquired dyschromatopsias was also anomalous. For instance, in the acquired protan defect of Stargardt's disease, the i5 mechanism is depressed and the i2 mechanism appears to determine threshold to all stimulus wavelengths. In the acquired deutan defects of alcohol amblyopia and diabetic retinopathy, the '4 mechanism shows loss of short wavelength sensitivity and the i5 mechanism loss of long wavelength sensitivity. A number of hypotheses were proposed to examine possible mechanisms at inner and outer retinal levels whereby the abnormal responses arise. It was concluded that the two-colour increment threshold technique is an appropriate and fruitful procedure for the investigation of acquired dyschromatopsia.
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    Dominantly inherited juvenile optic atrophy: an investigation of the clinical characteristics and acquired dyschromatopsia in three new pedigrees
    Smith, Damien P. ( 1969)
    The finding by the author of a 13 year old female with bilateral optic nerve atrophy, reduced vision and acquired tritan dyschromatopsia prompted an investigation into the identity of the affection. Similar findings in the father and siblings of the propositus, indicated the condition was familial. There were no apparent neurological complications. The inheritance of a primary optic atrophy suggested Leber's disease, but the dominant inheritance pattern with transmission by a male, and the clinical manifestations, were clearly distinct from that disease. The condition was identified as Dominantly Inherited Juvenile Optic Atrophy, but only after recourse to the basic literature. Standard text-books of ophthalmology and ophthalmological genetics proved unenlightening. For instance, Duke-Elder (1959) and Kestenbaum (1961) describe only Leber's disease under the heading of hereditary optic atrophies. Cogan (1966), Duke-Elder (194?) and Ballantyne and Michaelson (1962) give only 9, 8 and 5 lines, respectively, to what is essentially just an acknowledgement that optic atrophy with dominant inheritance has been reported. Walsh (195?) does not differentiate clinically between Leber's disease and other inherited optic atrophies with post-natal onset. The genetic texts of Waardenburg, Franceschetti and Klein (1963) and Francois (1961) do not agree on the classification of the hereditary optic atrophies. Waardenburg et., al. (1963) do not attempt to draw up a consistent clinical picture from the literature they discuss. Francois (1961) allows only 6 lines to the clinical manifestations of the dominant form of hereditary optic atrophy. That is, the text-books either omit the condition, or give it passing reference only. Nowhere is there a detailed clinical picture presented. Subsequent to the finding of the young girl by the author, two other propositi were found with the assistance of practising optometrists, and the investigation embraced all three unrelated families. The investigation followed two directions. The first aimed at defining and verifying the clinical picture of the disease, and the second at fully evaluating the disorder of colour vision which accompanies it. The report is accordingly in two major parts, with the findings integrated to form conclusions in a third.
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    An investigation of the blue-sensitive mechanisms of human colour vision
    Cole, Barry Leighton ( 1971)
    The characteristics of the blue-sensitive process of human colour vision have been explored by means of the two-colour increment threshold technique of W. S. Stiles and the measurement of retinal directional sensitivity. The objectives were a) to find points of difference between the three independent blue-sensitive mechanisms π1, π and π3 which might lead to reasonable hypotheses concerning their physiological basis and functional role, and b) to establish whether the unique threshold behaviour of the blue process at the fovea is due to an absence of blue cones as Wald (1967) has concluded. It has been found that the π3 mechanism has similar characteristics to π1; it is absent in tritanopia, exhibits extensive complete spatial summation and has high directional sensitivity. The π2 mechanism appears to be less directionally sensitive. Both π1 and π3 have been demonstrated at the central fixation area of the fovea. On the basis of these results and evidence in the literature a theory of the blue-sensitive mechanisms is proposed.
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    The mobility performance of people with Retinitis pigmentosa as a function of residual vision and psychological variables
    Haymes, Sharon Ann ( 1996)
    The purpose of this investigation was to explain the difference in the mobility performance of people who have retinitis pigmentosa (RP). I proposed that while the difference in performance is chiefly explained by differences in the amount of residual vision, psychological variables are also relevant. I measured the mobility performance of a group of eighteen subjects with RP on three real-world courses using Percentage Preferred Walking Speed (PPWS). I investigated the importance of residual vision using clinical measures of visual acuity, contrast sensitivity and residual visual field. In addition, I investigated the importance of psychological variables, such as anxiety, depression, self-esteem, locus of control and intelligence. I found a highly significant correlation between each of the vision variables and mobility performance. However, contrary to expectations derived from reading the literature, I did not find a significant correlation between psychological variables and mobility performance. I obtained the strongest correlation for a new measure of the residual visual field; the RP Concentric Field Rating. I was able to find a clinically useful model that explained 64% of the variance in functional mobility performance. The model incorporates the RP Concentric Field Rating and the Pelli-Robson measure of peak contrast sensitivity.
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    Oxygen levels at the human cornea during contact lens wear
    Efron, Nathan ( 1981)
    Contact lenses can restrict the amount of oxygen available to the cornea, often resulting in corneal swelling and visual dysfunction. However, despite the obvious importance of corneal oxygen availability, few studies have attempted to measure the level of oxygen at the anterior corneal surface of the in vivo human eye during contact lens wear. The aim of this thesis is to acquire such information. The rate at which the unanaesthetised cornea consumes oxygen from the membrane of a polarographic oxygen sensor forms an arbitrary index of corneal oxygen demand. An equivalent oxygen technique was modified for use in the in vivo human eye; this technique involved equating the corneal oxygen demand following a test condition (such as contact lens wear), to that following exposure to known oxygen levels. Human eye models relating static equivalent oxygen percentages (EOP) to lens oxygen transmissibility (Dk/L) were derived using seven hard and soft gas-permeable contact lenses to allow predictions to be made of the likely physiological response to contact lens wear. The discrepancies between these and other models were to be expected in view of the differing criteria upon which they were based. The effects of blinking and lens fit on the EOP beneath hydrogel lenses were found to be minimal, confirming earlier predictions. The significant increases in rigid lens EOP results upon blinking could be explained in terms of the tear pumping mechanisms known to exist with such lenses. An EOP of 7.7% O2 is available to the cornea of the closed eye. In some instances, a significant amount of oxygen is derived not only from the paipebral conjunctiva, but also from the atmosphere as a result of an imperfect paipebral aperture seal. Attempts to derive and EOP v Dk/L model for contact lens wearing under closed-lid conditions were unsuccessful; individual variations in the supply of oxygen to the cornea under these conditions were thought to contribute to this failure. The validity of similar models was questioned in view of the simplified assumption which were made.
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    Changes in selected physiological parameters of the human cornea with age
    Horton, Peregrine ( 1989)
    The effects of age on corneal oxygen flux and the corneal oedema response to contact lens induced hypoxia were studied by comparing the measurements of these variables made on two subject groups of differing ages. No significant difference in corneal oxygen flux between the groups was detected, although a trend towards a decrease in oxygen flux with age was found. A difference was detected, however, in the amount of corneal oedema which occurred in response to eye closure, both in the presence of a thick hydrogel contact lens, which caused corneal hypoxia, and without a lens, with older corneas swelling significantly more than those of the younger group. The implications of this finding for the clinical management of contact lens wearers are discussed. These findings are of particular relevance to the fitting of extended wear lenses to elderly patients, as they cast doubt on the criteria commonly used to determine "acceptable" levels of corneal oedema during lens wear. An attempt was also made to correlate several aspects of corneal structure and physiology with age and each other. A significant difference in the thickness of the corneal epithelium was found to exist between central and peripheral locations - a previously unreported phenomenon. A mathematical model was constructed, relating the various corneal parameters measured.