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W.E. Smollon, B.R. Wooten, B.R. Hammond; Stimulus Edge Effects When Measuring Macular Pigment Using Heterochromatic Flicker Photometry . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4569.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: : Empirical data regarding the sampling method used by most techniques for measuring macular pigment (MP) is lacking. One exception is the method called heterochromatic flicker photometry (HFP). Some data suggests that HFP samples MP directly at the edge of the stimulus area. More recent data, however, has raised questions regarding the generalizability of this edge effect: Does the edge effect hold true across subjects of varying MP density? At what eccentricity does this effect break down? This study was designed to address these questions. Methods: Eight subjects with widely varying MP were assessed. MP density was measured at peak absorbance in Newtonian view using a series of disc stimuli of varying radius (0.25, 0.50, 0.75, 1.0, 1.5 and 2.0°) and corresponding annuli. The width of the bands of the annuli increased with eccentricity (from 5–25’). Subjects fixated a small central point so that the centers of bands of the annuli were coincident with the edge of the solid stimuli. Results: Overall, MP measured across sites with the two stimulus configurations was highly correlated (r = +0.96, y = 0.01 + 0.98X). Nonetheless, deviations did occur for the three subjects with the highest MP density at the most eccentric points. When MP density measures for all subjects were averaged, deviations between the two stimulus configurations were <0.03 at all loci with the exception of 1.5 degrees eccentricity where the deviation was 0.06. Conclusions: Despite large differences in geometric area, the two stimulus configurations yielded values that were generally quite similar. Thus, edge effects do appear to dominate these types of flicker perceptions under these specific conditions. The edge effect predicts MP density very well for subjects with low MP density at all locations that we tested. For subjects with higher MP density, however, the edge effect predicts MP density well only within the central one–degree. Past this eccentricity, substantial deviations were noted, albeit not systematically in the same direction. Differences in results between the present and past studies will be compared and discussed.
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