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Ji C. He; Macular Pigment: A Possible Contributing Factor for Myopia Development. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6318.
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The human eye has longitudinal chromatic aberration (LCA) due to the dispersion of its ocular media. Macular pigment (MP) filters short-wavelength lights and thus has effect to reduce image degradation caused by the LCA. Meanwhile, the blue filtering could also shift axial location of the best image plane, which is a guiding signal for emmetropization. The purpose of this study was to model the effect of OD of the MP on axial shift of the ocular images.
LCA between 400 and 700 nm (Atchison & Smith, JOSA 2005) was adopted for the model in a theoretical eye with a 5.0 mm pupil. Geometrical optics was used to derive power distribution of the image at each wavelength for a distant point source, and the point spread function (PSF) at each axial location was calculated by summing the power distributions across the whole spectra. Axial MTF volume was then determined. Three lighting conditions: daylight (Illuminant D65), incandescent light (Illuminant A) and fluorescent light (Illuminant F2) were modeled, and OD of the MP was varied from 0.1 to 1.0. Two models of neural responses were tested: the luminous channel only and the luminous channel with short-wavelength-cone (S-cone) response.
For the model of luminous channel only, peak MTF volume point doesn’t axially shift with OD of MP under any lighting condition. But the width of half-maximum MTF volumes (depth-of-focus, DOF) changes in amount of 0.04, 0.02, 0.01D for the D65, A and F2 conditions respectively, mainly at shorter wavelength side, when OD of MP was changed from 0.1 to 1.0. For the model of luminous channel with S-cone, the peak MTF shifted toward longer wavelength direction when OD of MP was changed (0.21D for D65; 0.12D for A and 0.32D for F2). As compared to the luminous channel model, more changes in half-MTF point at the shorter wavelength side with the OD of MP were also found for all three lighting conditions (0.45D for D65, 0.33D for A, and 0.70D for F2).
Both axial location of the best image plane and the depth of focus of the ocular images depend on optical density of macular pigments when S-cone contribution is taken into account. The effect of macular pigment on ocular images changes with spectral distribution of lighting source, with a maximum shift under the fluorescent lighting as compared to daylight and incandescent lightings. In considering of its specific spatial distribution of the macular pigment at the retina, gradually decreasing from the fovea toward the peripheral in its optical density, the macular pigment could produce relative hyperopic defocus at the fovea, thus might contribute to myopia development in the eyes with relatively high optical density of the macular pigment.
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