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Allison McKendrick, Geoff Sampson, Mark Walland, David R. Badcock; Contrast discrimination and adaptation in glaucoma. Journal of Vision 2009;9(14):66. doi: 10.1167/9.14.66.
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© ARVO (1962-2015); The Authors (2016-present)
Assessment of the loss of function in glaucoma typically involves measuring contrast detection thresholds. Contrast processing in natural vision is more complicated than contrast detection, often involving the discrimination of different supra-threshold contrasts. Optimal visual performance also requires the visual system to adapt to the ambient contrast conditions. This study compared performance of individuals with glaucoma to those of approximately age-matched controls on several measures of suprathreshold contrast processing. Psychophysical contrast detection and discrimination thresholds were measured in central vision. The stimulus was a vertically oriented D6 of 3 c/deg. Thresholds were measured with and without adaptation to a low (15%) and high (70%) contrast vertically oriented 3 c/deg sinusoidal grating. Thirty adults (15 with glaucoma, and 15 approximately age-matched controls) participated. Full contrast discrimination (dipper) functions were also measured for a subset of three individuals with glaucoma and three controls. The glaucoma group demonstrated elevated thresholds relative to the controls for both detection and discrimination tasks in central vision (detection: t(28) = 2.42; p=0.02; discrimination: F(1, 28) = 13.10, p=0.001). For the discrimination task there was a significant interaction between contrast and group (p 〈0.001) with a greater difference between groups when tested with low contrast stimuli. For the subset of additionally tested participants, normalised contrast discrimination functions had similar shapes for all observers, revealing that contrast discrimination performance was largely predictable by individual contrast detection performance. Glaucoma group thresholds were less influenced than controls by contrast adaptation for the discrimination task (significant interaction between adaptation level and group: p=0.003). There was also a trend for a reduced effect of adaptation for the detection task, but it did not reach statistical significance (p=0.10). The relationship between contrast detection thresholds and the magnitude of adaptation to the 15% contrast grating (r = −0.03, p = 0.90) was not significant suggesting that a simple reduction in adapting grating strength due to contrast detection impairment is insufficient to explain the reduction in adaptation efficacy in the patient group. In summary, glaucoma causes complex alterations to contrast processing, including alterations to detection and discrimination thresholds in addition to an altered magnitude of effect of adaptation. This study adds to a growing body of work that demonstrates suprathreshold contrast abnormalities in glaucoma (Sun et al, Vision Research, 2008: 2633–41; McKendrick et al, IOVS, 45: 1846–53).
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