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Ann Elsner, Toco Chui, Lei Feng, Colleen McIntyre, Hongxin Song, Thomas Gast, Stephen Burns; Cone Distribution Variations for Young Healthy Subjects vs. Older Subjects. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1740.
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© ARVO (1962-2015); The Authors (2016-present)
To compare the individual differences in cone distribution with age, minimizing the effects of eye length and retinal disease. To distinguish between cone density variations related to foveal specialization, as opposed to more global factors or aging.
Subjects who had a normal fundus and axial length < 26 mm were divided into 2 groups, with age = 18 - 35 yr for the 47 subjects in the younger group and 50 - 70 yr for the 17 subjects in the older group. Cone densities were quantified using a second generation Adaptive Optics Scanning Laser Ophthalmoscope. Cones were imaged using 820 to 840 nm +/- 20 nm at approximately 100 microwatts, illuminating 530 x 550 microns of the retina. Samples collected in a + shape around the fovea were montaged. Cones were counted at a nominal 270, 630, 1480, and 2070 microns from the foveal center using custom software (Matlab, Mathworks). To reduce shadowing from large retinal vessels, temporal meridian data were used. Axial length measurements were used to correct the cone sampling positions and retinal area in mm2 (IOL Master, Zeiss Meditec). A subset of data were refit to a 2 parameter exponential model.
Cone density was greater on average for the younger group, but had substantial variability and overlap with the older group. The refit data from 36 younger subjects matched the 2 parameter exponential model sufficiently well that 97% had R2 > 0.9; but the parameters differed, and 92% of the data were slightly too steep. These cone densities averaged 43,200, 27,500, 15,000, and 12,200 microns per mm2, for the 270, 630, 1440, and 2070 micron locations, with a coefficient of variation of .14, .13, .10, and .10. Cone density at 630 microns was negatively correlated with the ratio of cones at 2070: cones at 630, p < .0001 and p = .0008 for the younger and older groups, respectively, and uncorrelated to the (unnormalized) density at 2070 microns, p = 0.71 and 0.37. The ratio of cones at 2070: cones at 630 averaged 0.45 vs. 0.39, for younger vs. older eyes.
Cone density is not characterized by a scalar factor for either eccentricity or age, nor a model with fixed exponents. Instead, cones from outside the fovea migrate centrally to provide foveal specialization, and aging further alters the distribution.
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