Optometry and Vision Sciences - Theses
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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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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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ItemThe non-neural contributions to the pigmented rat ERG( 2009)Introduction: The aim of this study is to consider the contribution of the RPE and Müller cells to the pigmented rat ERG. Materials and Methods: Groups of Long-Evans pigmented rats (n = 4 to 9) were anesthetized (Ketamine and Xylazine) and treated in one eye with different combinations of pharmacological agents. Intravitreal injection of BaCl2, APB and PDA, as well as intravenous injection of sodium iodate were administered in vivo, in order to isolate the specific contribution of each cell populations. Long flashes (2.5 s) were generated by LEDs in a full field Ganzfeld bowl, and ERG recordings were sampled at a rate of 2.5 Hz at 15 levels of luminance. The effect of each of the treatments was analyzed in comparison to a vehicle control group (n = 7), and between each drug treatment, using a two-way ANOVA with a Bonferroni post-hoc test to consider the intensities levels at which significant effects were noted. Results: Inhibition of Müller cells with BaCl2 treatment caused a b-wave enhancement attributable in part to the loss of the slow PIII. However, the slow PIII loss could not completely account for the enhancement. In addition, BaCl2 also alters what seems to be a bipolar cell driven PII enhancement, and induced an additional positive component on the ascending limb of the b-wave. A reduction of the c-wave could also be attributed to the loss of the slow PIII. The slow PIII exhibits an overshoot and a relative positivity after 1 s. Inhibition of RPE cells with sodium iodate caused a larger trough following the b-wave, attributed to the loss of the positive PI, but not a c-wave reduction. Conclusions: Pigmented rats exhibit a unique component structure where Müller cell slow PIII is the generator of the c-wave, while the RPE contributes a small PI component which peaks at the c-trough just following the b-wave.
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ItemExploring the mechanisms of Rarebit perimetry( 2009)Visual field testing, or perimetry, measures peripheral visual loss in eye diseases such as glaucoma. Rarebit Perimetry (RBP) is a new and novel perimetric method, introduced in 2002 by Lars Frisén (2002), with the aim of detecting low degrees of neural damage within the retina. RBP is unlike conventional perimetric methods that measure levels of retinal sensitivity, but instead uses very bright (i.e. suprathreshold) and very small targets to detect tiny areas of absolute blindness within otherwise normal areas of vision. RBP thus claims to locate miniscule gaps in the receptive field matrix of neurons in the retina, with the assumption that dead neurons leave gaps in this matrix. The most useful application of this idea is to detect progressive eye disease in the earliest stages (Frisén, 2002). Current research shows that RBP correlates with other standard visual field tests (Brusini, Salvetat, et al., 2005; Frisén, 2003; Gedik, Akman, et al., 2007; Martin & Wanger, 2004), but may afford greater sensitivity by detecting very mild visual losses missed by other tests (Martin, Ley, et al., 2004; Martin & Nilsson, 2007; Nilsson, Wendt, et al., 2007). To date, there are no studies that definitively test the theoretical basis of RBP, so in this thesis I aim to explore the proposed underlying mechanisms and assumptions of this test. In particular, the proposed mechanism of RBP leads to specific predictions as to how responses will alter when the luminances of the RBP targets are systematically decreased. I therefore compared RBP responses of mean hit rate as a function of target luminance and found results to be inconsistent with the proposed RBP mechanism. Mathematical simulations were performed to explore reasons for the differences between the two groups (Chapter Seven).
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ItemSegregation within afferent pathways in primate vision( 2009)The current knowledge of the visual pathways in primates includes the patterns of projection from the retina through the dorsal lateral geniculate nucleus (dLGN) to the striate cortex (V1) and the extra-striate projections towards the dorsal and ventral streams. Cells with short wavelength sensitive cone (S-cone) inputs in the dLGN have been studied extensively in New World marmosets but not in Old World macaques. This thesis presents results from studies in the macaque monkey which are more relevant to humans since humans are closer in evolution to Old World than New World monkeys. The spatial, temporal, chromatic and orientation preferences of neurons in the dLGN of the macaque were investigated by electrophysiological methods. The physiological findings of cells with S-cone inputs were compared to cells with opponent inputs from the long and medium wavelength sensitive cones (L-cones & M-cones, respectively). The cells receiving S-cone inputs (blue-yellow or B-Y cells) preferred lower spatial frequencies than the cells with opponent L-cone and M-cone inputs (red-green or R-G cells). Orthodromic latencies from optic chiasm stimulation were measured where possible to distinguish differences in conduction velocity between the cell groups. Although the B-Y cells usually had longer latencies than R-G cells, there wasconsiderable overlap between the cell groups. The recorded cells were localised through histological reconstruction of dLGN sections stained for Nissl substance. The distribution of B-Y cells within the dLGN was compared to the distribution of R-G cells. The majority of B-Y cells were located within the intercalated koniocellular layers as well as the koniocellular bridges (extensions of the koniocellular layers into the adjacent parvocellular layers). The B-Y cells were also largely segregated within the middle dLGN layers (K3, P3, K4 & P4). The R-G cells were mainly concentrated within the parvocellular layers (P3, P4, P5 & P6) and were evenly distributed throughout the middle and outer layers of the dLGN. The study also included recordings from the extra-striate middle temporal area (MT) to determine whether a fast S-cone input exists from the dLGN to area MT which bypasses V1. The pattern of cone inputs to area MT neurons was investigated before and during inactivation of V1. The inactivation was done through reversible cooling with a Peltier thermocouple device or focal inactivation with y-amino butyric acid (GABA) iontophoresis. Precise inactivation of V1 to the topographically matching visual fields of the recording sites in area MT revealed a preservation of all three coneinputs in many cells. The subcortical sources of these preserved inputs are discussed with their relevance to blindsight, which is the limited retention of visual perception after V1 damage. Analysis of the latencies of area MT cells revealed a rough segregation into latencies faster or slower than 70 ms. Cells both with and without a significant change in response during V1 inactivation were present in each group. The findings reported in this thesis indicate that some of the preserved inputs in area MT during V1 inactivation may be carried by a direct input from the dLGN which bypasses V1.