Optometry and Vision Sciences - Theses
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ItemOptical modelling of visual performance( 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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ItemDeveloping eye care and an analysis of eye conditions in Papua New Guinea( 2007)Accessible and affordable eye care is only a dream for much of the population of developing countries. Strategies for improving the visual welfare of these people need to be appropriate to the local situation. In 1992 a proposal was devised to address the lack of eye care in Papua New Guinea. This thesis examines the outcome of this proposal and reports on the ophthalmic data collected by these trained eye nurses.Method: In 1994, 11 National nurses were trained in a 3 month intensive course to become ‘eye nurses’. A basic set of equipment was provided to each eye nurse. Appropriate follow-up and annual conferences supported this initial training. A second group of 14 eye nurses were trained in 1997. Monthly eye clinic reports from the eye nurses provide significant data on eye conditions and visual welfare in PNGResults: After 6 years 80% of the eye nurses were still actively working in eye care. An analysis was made of the eye conditions of the 30,000 patients examined by the eye nurses over this 6 year period. The data is generally consistent with previous ophthalmic data from Papua New Guinea. The eye nurses were able to provide appropriate eye care for 80% of the presenting patients without Optometric or Ophthalmic assistance.Conclusions: Training nurses to become ‘eye nurses’ functioning as basic optometrists is an effective strategy in improving eye care in developing countries. The eye nurses were able to deliver sustainable, accessible, affordable and appropriate eye care, independently treating and managing the most common eye conditions in Papua New Guinea.
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ItemSubcortical pathways for colour vision( 2006-09)Visual sub-modalities, such as colour, form and motion perception, are analysed in parallel by three visual “pathways” – the parvocellular (PC), magnocellular (MC) and koniocellular (KC) pathways. This thesis aims to further elucidate some properties of the subcortical pathways for colour vision. The experimental animal used throughout is a New World monkey, the common marmoset Callithrix jacchus. (For complete abstract open document)
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ItemProblems and solutions in the optometric management of presbyopic airline pilots( 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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ItemSynaptic connectivity of visual pathways in the primate retina( 2005)The retina contributes to visual submodalities by anatomically and functionally distinct parallel pathways. In this thesis, the synaptic connectivity of cell types in parallel visual pathways in marmoset (New World primate) and macaque (Old World primate) retinas are studied. In Chapter 1 the main anatomical and physiological properties of parallel pathways in the primate retina are described. Diffuse bipolar (DB) cells receive synaptic input from multiple cones and provide output to ganglion cells, but their synaptic connectivity in the inner plexiform layer is not well understood. In Chapter 2, the stratification and synaptic connectivity of DB6 axon terminals are described. It is shown that the axons of DB6 cells stratify in the same region as rod bipolar cells and blue-OFF/yellow-ON ganglion cells. The majority (86%) of their synaptic output is onto amacrine cells; only 14% goes to ganglion cells. The DB6 cells may synapse with amacrine cells in the rod pathway and the blue-OFF/yellow-ON ganglion cells. The inhibitory neurotransmitter glycine is used by about half of all amacrine cells. Using immunohistochemical methods, the experiments in Chapter 3 investigated whether different bipolar and amacrine cell types differ with respect to the expression of glycine receptor (GlyR) subtypes. Postembedding electron microscopy showed the postsynaptic location of the a1, a2 and a3 subunits of the GlyR. Double immunofluorescence demonstrated that firstly, the three a subunits are clustered at different postsynaptic sites, and secondly that OFF bipolar types are predominantly associated with the a1 subunit, whereas ON bipolar types are predominantly associated with the a2 subunit. This shows that different amacrine cell types at synapses express different types of glycine receptors. The midget pathway is involved in processing red-green colour vision and high spatial acuity. In Chapter 4, the synaptic connectivity of OFF midget bipolar cells was investigated in the central retina of marmosets and macaque. The OFF midget bipolar cells and their synapses were identified immunohistochemically. Midget ganglion cells in marmosets were retrogradely labelled from the parvocellular layers of the dorsal lateral geniculate nucleus. Consistent with previous studies of Old World primates, it is shown that in marmoset the midget bipolar cells contact midget ganglion cells at a ratio of 1:1. The number of output synapses was quantified for 330 OFF midget bipolar cells (n = 104, dichromatic marmoset; n = 108, trichromatic marmoset; n = 118, macaque). The average number of output synapses per axon terminal was comparable for all animal phenotypes. In all animals the number of output synapses per axon terminal showed a unimodal distribution. Our results suggest that the midget circuitry in central retina is comparable in dichromatic and trichromatic animals. The midget pathway in mid-peripheral retina has been suggested to involve colour selective wiring of midget bipolar to midget ganglion cells. The question whether there is anatomical evidence for colour selective wiring was addressed in Chapter 5, in double labelled preparations of marmoset retina where OFF midget bipolar and OFF midget ganglion cells were identified. The relationship of the bipolar axon terminal mosaic and the dendritic fields of midget ganglion cells was analysed. No anatomical evidence for colour-selective connectivity in the inner retina of marmosets was found.
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ItemPsychophysical explorations of the illusion underpinning frequency doubling perimetry in glaucoma( 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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ItemA development of Chinese near logMAR visual acuity charts( 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.
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ItemAn analysis of colour vision standards in the transport industry( 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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ItemFurther studies on the acquired tritanopia of dominantly inherited juvenile optic atrophy: and, the mechanisms of acquired dyschromatopsia( 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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ItemDominantly inherited juvenile optic atrophy: an investigation of the clinical characteristics and acquired dyschromatopsia in three new pedigrees( 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.