title: Stimulus edge effects in the measurement of macular pigment using heterochromatic flicker photometry.
abstract: Heterochromatic flicker photometry (HFP) is the most common technique of measuring macular pigment optical density (MPOD). Some data strongly suggest that HFP samples MPOD specifically at the edge of center-fixated circular stimuli. Other data have led to the conclusion that HFP samples over the entire area of the stimulus. To resolve this disparity, MPOD was measured using HFP and a series of solid discs of varying radii (0.25 to 2.0 deg) and with thin annuli corresponding to the edge of those discs. MPOD assessed with the two methods yielded excellent correspondence and linearity: Y=0.01+0.98X , r=0.96. A second set of experiments showed that if a disc stimulus is adjusted for no-flicker (the standard procedure) and simply reduced in size, no flicker is observed despite the higher level of MPOD in the smaller area. Taken together, these results confirm that MPOD is determined at the edge of the measuring stimulus when using stimulus sizes in the range that is in dispute (up to a radius of 0.75 deg). The basis for this edge effect can be explained by quantitative differences in the spatial-temporal properties of the visual field as a function of angular distance from the fixation point.
authors.affiliation: Brown University, Department of Psychology, 190 Thayer Street, Providence, Rhode Island 02912, United States.
authors.forename: William E
authors.initials: WE
authors.lastname: Smollon
authors.affiliation: Brown University, Department of Psychology, 190 Thayer Street, Providence, Rhode Island 02912, United States.
authors.forename: Billy R
authors.initials: BR
authors.lastname: Wooten
authors.affiliation: University of Georgia, Brain and Behavioral Sciences, 125 Baldwin Street, Athens, Georgia 30602, United States.
authors.forename: Billy R
authors.initials: BR
authors.lastname: Hammond
mesh_terms: D000328:Adult; D003720:Densitometry / Q000295:instrumentation / Q000379:methods; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D005425:Flicker Fusion / Q000502:physiology*; D006801:Humans; D008266:Macula Lutea / Q000502:physiology*; D065348:Macular Pigment / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D057054:Molecular Imaging / Q000379:methods*; D010775:Photic Stimulation / Q000295:instrumentation* / Q000379:methods; D010783:Photometry / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D055815:Young Adult
chemical_list: D065348:Macular Pigment