Optometry and Vision Sciences - Theses

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    Contrast processing in ageing and early glaucoma
    LEK, JIA JIA ( 2014)
    A natural visual experience commonly requires an ability to differentiate objects of supra-threshold contrast (contrast discrimination) and to adjust to changes in contrast (contrast adaptation). While the loss of contrast sensitivity with glaucoma has been well documented in the literature, the effect of glaucoma on supra-threshold contrast processing is less studied. This thesis investigated the effects of early glaucoma on contrast discrimination and rapid contrast adaptation. Additionally, alterations in retinal and cortical responses to supra-threshold contrast stimuli with glaucoma were investigated using electrophysiology techniques. As age is a risk factor for glaucoma, this thesis also considered the effects of normal ageing on contrast processing. The results of this thesis suggest that glaucoma can result in supra-threshold contrast deficits, with patients having early glaucoma demonstrating a reduction in rapid contrast adaptation and poorer contrast discrimination. As ageing did not alter rapid contrast adaptation, the assessment of contrast adaptation might be a useful functional tool in early glaucoma, although further studies are required to develop this. Further studies are also required to consider the implications of supra-threshold contrast deficits with glaucoma on natural vision. The smaller cortical deficits relative to retinal deficits revealed with electrophysiological recordings suggest that post-retinal abnormalities are minimal in patients with early glaucoma. Hence, there is a possibility that the supra-threshold contrast deficits seen in early glaucoma in this thesis may mainly arise from retinal abnormalities. There is also the possibility that post-retinal compensation resulted in minimal cortical deficits in patients with early glaucoma. Further experiments involving patients with more advanced glaucoma may help to elucidate post-retinal contrast processing mechanisms with glaucoma. In older adults, elevated cortical responses in the presence of reduced retinal responses suggest the possibility of age-related reduction in cortical inhibition. Altogether, the results of this thesis provide further understanding of the mechanisms underlying supra-threshold contrast deficits in early glaucoma and ageing.