For the data nearer the fovea, the results are similar (
Figs. 4–
8). There were significant differences between the younger and older groups for cone packing density (
Fig. 4) and ONL + HFL thickness (
Figs. 5,
6), with older subjects having on average lower cone packing densities but thicker ONL + HFL (
P < 0.0001 for each analysis). Again, the individual differences led to overlap of the data for the two age groups. The mean cone packing densities were 43.9, 25.7, and 13.7 × 10
3 cones/mm
2 at 1.2, 2.1, and 4.5 degrees retinal eccentricity, respectively. The mean ONL + HFL thicknesses were 98.4, 84.6, and 74.3 μm at 1.2, 2.1, and 4.5 degrees retinal eccentricity, respectively, and include a significant contribution from the HFL thickness.
As expected, cone packing density decreased with increasing eccentricity (
P < 0.0001;
Figs. 1,
3,
4). ONL + HFL thickness, which includes the Henle fibers and supporting structures, such as portions of the Muller cells, decreased significantly with increasing retinal eccentricity (
P < 0.026;
Figs. 4,
5). At this more central retinal location, the number of cone photoreceptors decreases rapidly with eccentricity. This decrease is greater in the younger subjects, who have higher foveal cone densities than the older subjects, so there is a significant interaction of age and eccentricity for cone packing density (
P < 0.0001). The topographic mapping demonstrates the differences in cone packing density and ONL + HFL thickness between the two age groups, including the steeper fall-off and meridional differences for cone packing density of the younger subjects in the central region but higher values and smaller meridional differences for ONL + HFL thickness for the older subjects. The finding of fewer cones in the center for older subjects along with a different distribution of cones and their axons for the two age groups is in agreement with previous studies exploring cone density, cone photopigment, macular pigment, and foveal birefringence.
3,4
Cone packing density also depended upon meridian (
P < 0.001) as expected from the data set of Song et al.
7 The ANOVA with nasal and temporal combined to contrast with inferior and superior indicated that cone packing density was higher along the horizontal meridian (
P < 0.001). Younger subjects had an average over all samples of 27.1 × 10
3 cones/mm
2 in the horizontal meridian, but 22.9 × 10
3 cones/mm
2 in the vertical meridian, while older subjects had 23.5 × 10
3 cones/mm
2 in the horizontal meridian but only 20.8 × 10
3 cones/mm
2 in the vertical meridian (
Fig. 4). This asymmetry is so great that older subjects had greater numbers of cones on average in the horizontal meridian than younger subjects had in the vertical meridian. Note that the actual average cone density across these regions of rapid decrease is lower than the average of the samples, since the more eccentric samples with lower values represent relatively larger regions.
ONL + HFL thickness depended on meridian as well (P < 0.005), with the ONL + HFL thickest on average in the nasal retina (88.1 μm) and thinnest in the inferior retina (78.1 μm). None of the interaction analyses for ONL + HFL reached significance.
The ratio of the cone packing density-to-ONL
2 was larger on average for the younger than for the older group 3.75 vs. 3.01 (
P < 0.0001;
Figs. 7,
8). As with more eccentric locations, the range of the ratios overlapped for the younger and older subjects (
Fig. 7), however to a lesser degree. The ratio did not decrease significantly with increasing eccentricity (
P < 0.08). The ratio, however, differed with meridian (
P < 0.028). For older subjects the ratio of cone packing density-to-ONL
2 was 2.96 for the vertical and 3.06 for the horizontal meridians, compared to 3.48 and 4.01, respectively, for younger subjects. The HFL accounts for appreciable amounts of ONL + HFL thickness in this central region, yet nevertheless the central and more peripheral regions had similar ONL + HFL thicknesses for younger compared to older subjects.
A comparison of
Figures 4 and
9 illustrates that younger subjects have cone density peaked near the fovea and density dropped off rapidly, but more rapidly for the vertical meridian than the horizontal. Surprisingly, the horizontal versus vertical contrast of the ONL + HFL thickness (
Fig. 9) indicates a fairly radially symmetric ONL despite the asymmetry of the cone packing density. The data for older subjects show a different pattern, in that there is a lower peak of cone packing density near the fovea and there still is some asymmetry for horizontal versus vertical meridian. Again, the horizontal versus vertical contrast of the ONL + HFL thickness indicates fairly symmetric data.
The previously reported cone packing data and reproducibility measurements indicate that age, meridian, and eccentricity vary across individuals, but that there is considerable variation within an age group and overlap across younger versus older age groups.
7 The reproducibility data for the ONL + HFL thickness indicate that the pixel-to-pixel variation is not likely an important factor, although the criterion used for segmentation has some impact on the size of trends in the data. For the inner macular region, there was no significant difference in the ONL + HFL thickness for Sessions 1–2 measurements (
P = 0.485), while there were significant effects of age and eccentricity (
P = 0.001, 0.003, respectively). Since we limited the reproducibility analysis to horizontal measurements, there was less statistical power, and also no analysis for horizontal versus vertical meridian. For the outer macular region, there was no significant difference in the ONL + HFL thickness for Sessions 1–2 measurements (
P = 0.660). The effect of age was not large enough to overcome the loss of the contribution from the thinnest meridian (inferior) and the decrease in statistical power (
P = 0.055). ONL + HFL thickness was varied significantly with eccentricity, and there was an interaction of age and eccentricity (
P = 0.002 and 0.012, respectively). The individual variations within each age group were large. Failing to control for eccentricity or meridian could result in the lack of an effect of age. The reproducibility data supported our finding that the thickened ONL + HFL with age is not due to segmentation errors, since these were small relative to the effect.