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R.A. Bone, J. Gibert, J.T. Landrum; Macular pigment and the edge hypothesis of heterochromatic flicker photometry . Invest. Ophthalmol. Vis. Sci. 2004;45(13):1294.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To investigate the hypothesis that macular pigment optical density (MPOD) is determined at a retinal eccentricity corresponding to the edge of the visual stimulus in heterochromatic flicker photometry (HFP). Methods: A flicker photometer (460 vs 540 nm, 900 cd/m2, 1.5° stimulus, 8° eccentric fixation mark) was modified to provide discoid stimuli of 1.5 and 1.17° diameter, and annular stimuli of diameters 0.42 to 0.56° (mean 0.49°), 0.67 to 0.82° (mean 0.74°), 0.92 to 1.17° (mean 1.05°) and 1.38 to 1.50° (mean 1.44°). Ten experienced subjects participated. The subject adjusted the 460 nm intensity to obtain a flicker null while centrally fixating each stimulus, and eccentrically fixating the 1.5° discoid stimulus. Five settings were made with each stimulus and both eyes were tested. Results: The annular stimuli produced MPOD distributions within the central 1.5° that generally declined with eccentricity. For both eyes of all subjects, the 1.5° discoid stimulus produced a higher MPOD than the annular stimulus of the same outer diameter. The average eccentricity at which the former MPOD matched that of the distribution was 0.38° ± 0.13°, i.e. at an eccentricity that was 51% of the 1.5°discoid stimulus radius. The MPOD values obtained with the 1.17° discoid stimulus were also higher than with the corresponding annular stimulus of the same outer diameter in 80% of the eyes tested. Conclusions: HFP does not provide MPOD at an eccentricity corresponding to the edge of the flickering stimulus, at least under the operating conditions of our flicker photometer. This result contradicts a common assertion to the contrary. We suggest that HFP with a discoid stimulus determines MPOD at an eccentricity (here 51% of the stimulus radius) such that the modulation–amplitude at all points within the stimulus is at or below threshold.
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