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P. D. Shah, M. Sharifzadeh, P. S. Bernstein, W. Gellermann, S. Richer; Validation Test for Heterochromatic Flicker Photometry Based Measurements of Human Macular Pigment. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2134.
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© ARVO (1962-2015); The Authors (2016-present)
Heterochromatic flicker photometry, HFP, is a psychophysical method used to measure the optical density of macular pigment, MP, in the human retina. The method exists in various instrument configurations measuring MP densities at a single or at several discrete eccentricities with respect to the fovea. The purpose of this work is to validate HFP methods with an objective test.
In HFP, a light stimulus is projected onto the human retina. It alters rapidly between blue and green colors, stimulating the subject’s photoreceptors with wavelengths within and outside the absorption band of MP, respectively. A subject performing HFP measurement is asked to view the flickering stimuli in two projections, one in which the stimuli are centered on the fovea, and one in which they are projected at discrete locations in the peripheral retina. MP density values are determined from the difference of attenuation densities determined to achieve minimum flicker perception. To test the validity of the HFP method we simulated additional MP densities in a subject’s retina by placing cuvettes with well defined lutein solutions in the beam path of the stimuli. Subjects were asked to carry out the HFP test as usual except to view the light stimuli through the cuvette for the case of foveal projection.
HFP measurements were carried out by up to 19 volunteers, using three different instruments, and lutein solutions with zero, low (~0.3), medium (~0.4), and high (~1.0) optical density levels at the blue stimulus wavelength. Perceived optical densities increased in most cases. However, large deviations from the expected optical density increases were seen for many subjects even when the added optical densities were low. For an instrument measuring pigment density at 7 degree eccentricity, only 30% of the subjects were able to perceive an optical density increase of 0.26 to within 10% of the expected value. For another instrument, measuring MP densities at several discrete eccentricities, deviations from expected levels were even higher and varied strongly between different eccentricities.
Placement of lutein solutions in the measuring beam path of HFP instruments is a simple test to validate instrument performances in the measurements of MP levels in human subjects. HFP perceived MP levels can differ significantly from their true values in a large percentage of subjects. A potential explanation for this effect may be a difference in photoreceptor quantum efficiencies between subjects for blue and green light detection.
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