Macular pigment optical densities measured with the Maxwellian
system and the free-viewing LED tabletop device are presented in
Table 1 . As shown in
Figure 6 , the two methods provide highly similar individual and mean values and
are highly correlated (
y = −0.03 +
1.06
x,
r = +0.95). Note that the intercept
of the line is near 0 and the slope is near 1, showing that there is no
systematic difference between the two techniques. In fact, the average
absolute change in these values is lower (mean = 0.04; SD =
0.03) than the differences reported for studies that obtained repeated
measures using a single method on different days.
18 To
obtain a reliability estimate for MP measured with the new device, the
MP density of four subjects was measured 10 times over a period of 2 to
4 weeks. An analysis indicated that the data were reliable (Cronbach’sα
= 0.89). This level of reliability is comparable to that
obtained measuring MP on different days using naive subjects but
Maxwellian view optics (e.g., Cronbach’s α =
0.85
13 and 0.68
19 ).
As shown in
Table 1 , of the subjects whose lens density was measured,
differences between the MP values obtained with the two techniques were
not related to individual differences in lens density. The lack of
influence of lens optical density on the MP measures is further
indicated by the observation that the difference in MP measured with
the two techniques is unrelated to age (
r = −0.02). If
one method was more influenced by lens optical density, then one would
expect the differences in measured MP to increase with age because lens
optical density increases with age.
To confirm that differences in retinal illuminance associated with
differences in lens optical density should have little effect, we
conducted a small control experiment varying retinal illuminance. To
this end, we used the LED tabletop device to measure MP density while
changing the radiance of the background field in 0.25 intervals over a
1–log unit range. For the three subjects that were tested, MP density
averaged 0.60 ± 0.14, 0.61 ± 0.14, 0.55 ± 0.17,
0.61 ± 0.21, and 0.61 ± 0.25, respectively. Thus, in
effect, changing the background from dim (simulating a dense lens) to
bright (simulating a clearer lens) does not significantly change the
measured MP values.
For two subjects, we also tested the effects of pupil size by measuring
MP with the LED tabletop device before and after pupil dilation with a
mydriatic. The MP values when measured with nondilated pupils (0.16 and
0.42) were very similar to the values obtained during dilation (0.11
and 0.43, respectively).
We also tested the effects of head movement on the MP values of two
subjects, using the tabletop device. The limiting factor in the lateral
direction is the ability to see the stimulus. A subject can only move
approximately 1.5 cm to the right or left before the stimulus is
occluded by baffling. However, when subjects were misaligned to the
allowable limit, no differences were found in their MP values (range of
differences = 0.02). In the Z direction, subjects can move at
least 10 cm forward or backward without any change in their MP values
(range of differences = 0.04).
Individual differences in the average MP density of the individuals in
this small sample tended to be consistent with our past observations on
determinants of individual differences in MP density in different
populations. For example, the average MP of the women (mean =
0.21, SD = 0.123, n =16) was lower than the average MP
of the men (mean = 0.30, SD = 0.20, n =14). The
average MP of the smokers (mean = 0.215, SD = 0.24, n =4) was lower than the MP density of the nonsmokers
(mean = 0.26, SD =0.13, n = 26). Finally, the MP
of the blue-eyed subjects (mean = 0.199, SD = 0.139, n =7) was lower than the MP of the green/hazel-eyed subjects
(mean = 0.29, SD = 0.21, n =5) or the
brown/black-eyed subjects (mean = 0.35, SD = 0.14, n = 16). The sample sizes of these groups were too
small to assess the statistical significance of these differences, but
the trends indicate that this population is representative of other
groups whose MP has been studied.