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MiYoung Kwon, Rong Liu; Increased equivalent noise may explain a loss of foveal contrast sensitivity in glaucoma. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2565.
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© ARVO (1962-2015); The Authors (2016-present)
Glaucoma is a leading cause of world blindness, characterized by progressive loss of retinal ganglion cells. Evidence has shown that glaucomatous damage involves the macula even in the early stages (Hood et al., 2013). In parallel with the anatomical evidence, functional deficits such as a loss of contrast sensitivity have been also reported for the macular region of the glaucomatous eyes (Chien et al., 2017). However, the exact mechanism by which glaucomatous macular damage impairs visual sensitivity still remains unclear. Here we examine whether a loss of contrast sensitivity in glaucoma is a result of a decrease in the sampling efficiency, an increase in the equivalent noise (internal noise), or both.
The study included 7 glaucoma patients (mean age = 64.43 yrs) and 8 age-similar normally-sighted adults (mean age = 62.63 yrs). The classical equivalent input noise paradigm (Pelli, 1990) was adopted to factor visual sensitivity into the efficiency and the equivalent input noise. For each subject, threshold contrast for orientation discrimination was measured using an adaptive procedure. The measurements were made at the fovea. A subject’s task was to judge the orientation (tilted to the left or right) of a target grating embedded with varying levels of external noise. To compare changes in parameter values of the internal equivalent nose and the efficiency, the data of log threshold contrast energy versus log noise level (power spectral density) was fitted with the linear amplifier model.
Compared to age-similar normal controls, glaucoma patients exhibited a significantly larger equivalent noise level (p<0.01). However, no significant decrease in the efficiency was found in glaucoma patients (p>0.05). In other words, glaucomatous damage impaired target discrimination performance at low noise levels while it had very little effect at high noise levels. Furthermore, the internal equivalent noise data were significantly correlated with Pelli-Roberson contrast sensitivity data (r =-0.69, p<0.01).
Our results show that contrast sensitivity deficits in glaucoma is largely accounted for by an increase in internal noise. Our findings further suggest that glaucomatous macular damage appears to undermine macular function by increasing internal noise.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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