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R. A. Bone, J. T. Landrum, M. Adams, J. C. Gibert; Validity of Macular Pigment Optical Density Measurements by Heterochromatic Flicker Photometry. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2131.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the validity of one of the basic assumptions of heterochromatic flicker photometry when used to determine macular pigment optical density (MPOD), viz. that differences in the photopic luminosity function (V) between fovea and parafovea are attributable to macular pigment only. An apparent, non-zero MPOD above 540 nm would call such an assumption into question.
A flicker photometer was modified so that the test wavelength, normally set at 460nm (the peak of the MPOD spectrum), could be varied from 410 to 680 nm. The reference wavelength was 540 nm where MPOD is negligibly small. Subjects made 5 to 10 luminance matches between the test and reference wavelengths for both central and 8° peripheral viewing of a 1.5° stimulus.
Measurements made at 10 nm intervals in the range 410 to 540 nm revealed an MPOD spectrum that was consistent with expectations based on spectrophotometry of lutein and zeaxanthin mixtures. However, measurements made at 20 nm intervals in the range 540 to 680 nm revealed an apparent MPOD that increased continuously with wavelength, reaching a value of ~ 0.25 AU at 680 nm in one subject. Measurements made in 6 subjects with a test wavelength of 620 nm only indicated an apparent MPOD at that wavelength that ranged from ~ zero to 0.28 AU. Using published cone fundamentals, we have calculated the long-to-medium wavelength-sensitive cone ratios (LWS:MWS) in the fovea and parafovea that would account for the apparent MPOD that we observed above 540 nm. (Short wavelength-sensitive cones were assumed to have negligible input to V.) Normalizing the LWS:MWS ratio to unity in the fovea, the required ratios in the parafovea ranged from ~ 0 to 4.15 for different subjects. Correspondingly, MPOD at 460 nm was underestimated by up to ~ 0.07 AU.
Our results clearly indicate that V should not be assumed to be the same (after subtracting the influence of MP) in the fovea and parafovea in all subjects. Generally the sensitivity to longer wavelengths relative to mid-wavelengths is less in the fovea compared with the parafovea. However, by determining the apparent MPOD at, say, 620 nm, a correction to the measured MPOD at 460 nm can be determined.
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