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WE Smollon, BR Wooten; The Role of Photopigment Self-screening in the Estimate of Macular Pigment Optical Density Using Heterochromatic Flicker Photometry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2952.
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Purpose: The purpose of the current study was to obtain a direct measure of the photopigment self-screening effect. This effect may introduce error into the estimate of the optical density (OD) of Macular Pigment (MP) when measured with Heterochromatic Flicker Photometry (HFP). HFP is a standard psychophysical method commonly used to measure the OD of MP (Wooten et al, 1999). Since HFP compares different retinal loci, a uniform relative spectral response at the receptor level is required for a perfect measure of MP. However, as pointed out by Sharpe et al (1998), since the outer segments of cones in the fovea are generally about twice as long as those in the parafovea, a substantial self-screening effect could result in a proportionate underestimate of MP. Method: We use an adapting background to systematically bleach the photopigments from 0% to near 100% thereby reducing self-screening to near zero. In the first experiment OD was measured at the peak absorbing wavelength of MP (460 nm) in 2 subjects at two different test loci, 30min. and 10 min. eccentric to the center of the fovea. In a second experiment spectral absorption estimates from 430 nm to 520 nm at two extremes of the bleaching range were measured for comparison with a well-accepted MP template (Bone et al., 1992). Results: Analysis of the data from Experiment 1 reveal a small effect of 0.01 OD at the 30 min. locus for both observers. At the locus 10 min from the center of the fovea the effect was larger yet still small; 0.04 OD for one observer, 0.07 OD for the second. The resulting spectral absorption curves from Experiment 2 are consistent with a self-screening model based on the Bone et al. (1992) template which is not subject to self-screening. Conclusion: The estimated size of the self-screening effect when measuring MP with HFP is significant but small. The size of the effect is on the order of the method’s variability.
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