Optometry and Vision Sciences - Theses
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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).