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M.S. Eckmiller; Optical Filtering by Macular Pigment Facilitates the Alignment of Foveal Cone Photoreceptors: Implications for Age–Related Macular Degeneration . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3509.
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Purpose: The pathomechanism of macular degeneration has been proposed (Eckmiller, Prog. Ret. Eye Res. 23: 495, 2004) to involve disturbances in the alignment of foveal cones. This study sought to clarify the possible involvement of macular pigment (MP) in cone alignment: the optimal direction of alignment for a cone is expected to depend on the combined effects of its spectral sensitivity (± optical filtering by MP) and ocular chromatic aberrations. Methods: Spectral sensitivity measurements reported for human medium– (M) and long– (L) wavelength–sensitive cones (most of those in the fovea) as a function of wavelength () were modified by subtracting the variable filtering expected to result from variable amounts of MP (e.g., at different retinal eccentricities or with pathologically reduced MP levels). Measurements of the chromatic difference of refraction (defocus) in human eyes as a function of were obtained from the literature. These measurements were combined in a novel way so as to become independent of , by plotting the values of M or L cone sensitivity (± variable MP filtering) as a function of defocus (diopter, D). The area under these curves was integrated and the median defocus value was calculated for each type of cone, ± variable MP filtering. Results: The curves for M and L cone sensitivity as a function of defocus were skewed, shifted, and narrowed in comparison to these curves as a function of . The sensitivity/defocus curve for both cone types was significantly altered by different amounts of MP filtering, which reduced the total area by up to 31% (L cones) or 36% (M cones). In the presence of MP filtering, the (M or L) cone sensitivity/defocus curve was particularly narrow and the median defocus value was close to that of maximal sensitivity. Without MP filtering, the (M or L) cone sensitivity/defocus curve was wider and the median defocus value was displaced from that of maximal sensitivity by nearly –0.2 D. Thus, the directional characteristic for cone alignment was more slim with MP filtering than without it. Conclusions: These findings show that the alignment of foveal M and L cones is subject to less chromatic aberration and hence more accurate, if the light they receive has been filtered by adequate levels of MP. This analysis demonstrates that MP facilitates the accurate alignment of foveal cones, supporting the relevance of foveal cone alignment in macular degeneration. This important function of MP as an optical filter in the normal human retina may be sufficient to provide significant protection against the pathological disturbances of age–related macular degeneration.
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