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Erica N Woertz, Melissa A Wilk, Joseph Carroll, Edgar A. DeYoe; Examining the relationship between cone density and Vernier acuity in human albinism. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5858.
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© ARVO (1962-2015); The Authors (2016-present)
Albinism is a family of genetic diseases that cause disrupted melanin synthesis and lead to abnormal retinal development, altered decussation at the optic chiasm, and aberrant cortical retinotopy. Vernier acuity (VA) is a psychophysical measure that, in healthy individuals, is believed to be limited by neural sampling in the primary visual cortex,1 and thus may provide insight into post-receptoral visual circuitry. We examined the relationship between cone density (CD) and VA in human albinism.
Five subjects with genetically-confirmed albinism (2 M, 3 F, age 15-31) and four normal control subjects (3 M, 1 F, age 24-41) were recruited. Images of the cone mosaic were obtained using adaptive optics scanning light ophthalmoscopy and CD was estimated using a semi-automated cone counting algorithm.2 VA thresholds along the horizontal meridian of the left visual field were measured at 10-11 loci ranging from 0-20° eccentricity relative to fixation using a three-dot vertical Vernier stimulus.
Peak CD (mean ± SD) was 128,378 ± 25,667 cones/mm2 in controls and 69,796 ± 19,995 cones/mm2 in albinism subjects, consistent with previous reports.2 Foveal VA (mean ± SD) was 0.498 ± 0.127 arcmin in controls and 5.926 ± 6.227 arcmin in albinism subjects. Albinism subjects had greater variability for foveal VA than for peak CD. At all eccentricities tested, VA was worse and more variable among albinism subjects than controls. When VA was plotted as a function of eccentricity (E) for each subject and fitted to a power function (VA = a(E)b), the exponential relationships were similar in controls (mean b ± SD: 0.867 ± 0.092) and in albinism subjects (mean b ± SD: 0.874 ± 0.104). Interestingly, the scaling constant (a) was negatively correlated with peak CD in controls (r2=0.894) but not in albinism subjects (r2=0.074), indicating greater variability in the relationship between VA and CD in albinism.
This demonstrates worse VA in albinism compared to normal controls at all eccentricities measured. Additionally, the relationship between CD and VA in albinism varies between individuals. This supports previous reports of albinism as a spectrum of visual system phenotypes2 and suggests that the visual deficits in albinism may be a result of altered numbers and/or connectivity of post-receptoral neurons in retinogeniculostriate pathways.1. Duncan, PMID: 12765616; 2. Wilk, PMID: 24845642
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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