Several other studies measured MPOD profiles, but did not find rings, or did not look in detail to their particular distribution. Berendschot et al.
22 used a custom-built SLO to measure reflectance maps at 488 and 514 nm in studying the influence of lutein supplementation on MPOD. Data were analyzed with an exponential decaying function with eccentricity that seemed to provide a reasonable fit to the data. No obvious rings were seen at that time, perhaps because of the small sample (
n = 8). Chen et al.
43 used a modified fundus camera and a cooled CCD for in vivo imaging reflectometry. They found a mean ρ
1 = 0.19 ± 0.04 in a young aged group (
n = 24, mean, 24.8 ± 2.6 years), ρ
1 = 0.16 ± 0.03 in a middle-aged group (
n = 13, mean, 40.2 ± 8.3 years), and ρ
1 = 0.12 ± 0.02 in an old-aged group (
n = 17, mean, 67.5 ± 7.1 years). These ρ
1s are much lower (inferring a broader distribution) than those in the present study. In a fundus camera, stray light caused by vitreous backscatter and reflectance at the inner limiting membrane is hard to avoid. This factor may result in an underestimate of the MPOD. Indeed, absolute MPODs were low compared with those in other studies.
29 Also, it causes a broadening of the apparent spatial distribution, because the influence of the stray light is more pronounced for higher MPODs. As a result, the observed increase in ρ
1 with age may in part be an artifact caused by an increase in stray light with age. Chen et al.
43 noted shoulders on the MPOD profile; however, these were found at much larger eccentricities of approximately 4°. Bour et al.
44 used photographic images obtained with a fundus camera to study the MPOD distribution in 23 pediatric subjects. They found a mean ρ
1 = 0.25 ± 0.05. This rather low ρ
1, accompanied with a rather low absolute value,
29 may again be caused by stray-light problems as in Chen et al.
43 Ringlike structures were not mentioned. Trieschmann et al.
27 extracted MPOD from autofluorescence images at 488 nm only. They reported no ring structures. However, they did not perform a second reference measurement at a wavelength that is not or is hardly absorbed by the MP. Therefore, the observed MPOD distribution shows the real MPOD distribution, diluted by various lipofuscin and melanin concentrations as a function of eccentricity, the exact shape of which is unknown. This makes their results difficult to interpret. Robson et al.
45 used minimum motion photometry to determine MPOD profiles in 18 subjects. Broad shoulders at approximately 4° were observed in two subjects. Their spatial resolution of 0.5° was probably too low to find ring structures as were found in this study. They also measured autofluorescence images. However, as in Trieschmann et al., they used only one wavelength. Wüstemeyer et al.
30 used reflectance and autofluorescence maps to determine MPOD distributions by comparing maps obtained at 488 and 514 nm. However, they looked only at the peak values and did not study the spatial profiles in detail.