Abstract
purpose. Increasing evidence indicates that the macular pigments (MP) protect
the central retina and may retard macular disease. For that
reason, a practical method for measuring MP that does not require
elaborate optics and can be applied to diverse populations by operators
with a modest amount of experience was developed and validated.
methods. A small tabletop device based on light-emitting diodes (LEDs) as the
light source with electronic controls was constructed. Macular pigment
was measured with the tabletop device with a 1° test stimulus at 460
nm using heterochromatic flicker photometry, and the results were
compared with measurements using a traditional three-channel Maxwellian
view system with a xenon-arc source.
results. Macular pigment density of 30 subjects (age range, 16–60 years) was
measured with both stimulus systems. Macular pigment measured with the
LED tabletop device in free view was highly correlated with MP measured
in Maxwellian view (y = −0.03 + 1.06x, r = +0.95). The average absolute difference between
the two techniques was 0.04 (SD, 0.03). The new technique was not
significantly affected by variations in lens optical density, pupil
size, or small head movements.
conclusions. Psychophysical measurement of MP provides a unique opportunity to
make repeated noninvasive assessment of the concentration of a
protective nutrient in the retina. The availability of this new device
should make this measurement technology accessible to a wide variety of
investigators for application to diverse
populations.
Several epidemiologic studies have found associations between
dietary intake or blood concentrations of the carotenoids
lutein and zeaxanthin and protection from age-related macular
degeneration.
1 2 3 4 However, other populations did not show
these relationships.
5 6 Although differences between the
populations may have contributed to the conflicting results, an
important caveat is that all these studies used measures of lutein and
zeaxanthin status that may not adequately reflect the concentrations of
these pigments within the macular retina itself. Because the
relationship between measures of dietary intake and blood
concentrations of lutein and zeaxanthin and retinal concentrations of
these pigments is weak,
7 using diet and blood values to
predict retinal concentrations of these nutrients is not optimal.
It seems likely that lutein and zeaxanthin protect the retina
locally.
1 Lutein and zeaxanthin are most dense in the
inner retinal layers of the foveal retina and are referred to as the
macular pigments (MP) that create the yellow color of the macula lutea.
A growing body of evidence indicates that lutein and zeaxanthin (as
measured in the macula) protect the retina from oxidative damage that
accrues with age.
8 9 This protection may be mediated
through passive filtering of short-wave light
10 or
actively by quenching photosensitizers or reactive oxygen
species.
1 Evidence supporting a protective role for MP
comes largely from experimental data on nondiseased subjects (reviewed
in
Ref. 9 ; a noted exception is data by Landrum et al.
11 ).
To date, no epidemiologic data are available on the relationship
between retinal concentrations of lutein and zeaxanthin and risk of
age-related macular degeneration.
The lack of clinical data on this relationship is partially due to
difficulties in measuring MP in vivo. Macular pigment has traditionally
been measured using complex optical systems.
12 These
optical systems require extensive training to operate and cannot be
easily moved from the laboratory. Thus, most studies on MP have been
conducted in university laboratories using populations composed of
students and faculty rather than samples drawn from the general public.
To obtain data from a wider more representative sample, a simple, more
accessible method of measurement was needed.
In the present article, we describe a simplified device for measuring
MP optical density that can be applied to diverse populations. This
device implements noninvasive procedures that are similar to past
studies (see review in
Ref. 12) but uses a design that is less
expensive, physically robust, and easy to use. Furthermore, it is more
comfortable for the subject because the stimulus is presented in free
view and the subject does not need a bite-bar for head stabilization.
As a first step in validating the new device, we compared MP density
measured using a traditional Maxwellian view optical system with MP
density measured with the new device. Measurement of MP with the
Maxwellian view system has been validated previously by varying the
test wavelength to derive the MP density spectrum and demonstrating
that it is nearly identical to the MP measured ex vivo.
13
Methods
Measurement of MP Optical Density
Subjects.
Macular pigment density of 14 men and 16 women (age range, 16–60
years) was measured using the two different techniques. The ordering of
the measurements was counterbalanced to avoid possible order effects.
Twenty-six of the subjects in our sample had never participated in a
psychophysical task before this study and were experimentally naive.
The remaining four subjects were experienced in psychophysical tasks
and were aware of the purpose of the study. Because this study involved
direct comparisons between two methods, no exclusion criteria were used
in sample selection. Informed consent was obtained from all subjects,
and the tenets within the Declaration of Helsinki were followed.
Procedure.
All measurements were made with the right eye only. Macular pigment
density is highly similar in the left and right eyes.
13 We
selected the right eye to maintain consistency with past studies, but
the device can be used to measure the MP density of either eye with
only minimal adjustments. The procedure for measuring MP was the same
whether measured with the conventional Maxwellian view system or with
the new device. For an expanded discussion of the procedure see
Snodderly and Hammond.
12 In brief, visual sensitivity was
measured using a test wavelength that is maximally absorbed by MP, 460
nm, and a reference wavelength that is not absorbed by MP, 550 or 570
nm. These measurements were made at a retinal locus where MP is most
dense, the center of the fovea, and in an area where MP density is
minimal, 4° or 6° in the temporal retina. Sensitivity was measured
using flicker photometry, which presented the two test stimuli in
temporal square wave alternation at 12 to 15 Hz for the foveal
condition, and 6 to 7 Hz for the parafoveal condition. Temporal
resolution for small stimuli presented as described above is higher in
the fovea than in the parafovea
14 and therefore requires
the flicker rate to be lowered when making parafoveal measurements. If
a range of test wavelengths is used, this procedure for measuring MP in
vivo yields an optical density spectrum for the pigments that matches
the extinction spectrum of MP measured ex vivo.
13
Maxwellian View Measurement
A conventional three-channel Maxwellian view system with a 1000-W
xenon arc light source (power source: Raytheon, Lexington, MA; housing:
Kratos Analytical, Ramsey, NJ) was used for the measurements. A
schematic of this system is shown in
Figure 1 . The exit pupil of the system was 2 mm. One channel provided a 460-nm
background field, and two other channels were combined to produce the
flickering measuring stimulus. The second channel provided the test
wavelength, the intensity of which was adjusted by the subject via a
2.0–log unit circular neutral density wedge. The third channel
provided the reference wavelength, the intensity and wavelength
composition was constant. Light from the second and third channels was
presented in square wave alternation for the purpose of flicker
photometry. The alternation was accomplished by using a sectored
first-surface mirror rotated by a highly regulated Bodine
motor (Electro Sales, Somerville, MA). The wavelength of the test field
was produced by a grating monochromator with a nominal half bandwidth
of 7 nm (model H-20; Instruments SA, Metuchen, NJ); blocking
filters eliminated stray light and higher-order spectra. The wavelength
of the background and the reference fields was produced by Ditric
Optics interference filters (half-bandpass = 7 nm). Subjects were
positioned for Maxwellian view by using an auxiliary pupil viewer (made
with a comparator/reticle used in conjunction with a beam splitter
placed immediately before the final focusing lens). Stabilization of
the head was maintained by use of an adjustable dental impression
bite-bar and headrest assembly.
The circular 1° measuring stimulus alternated between a 2.7-log
Troland (Td), 550-nm reference field, and a 460-nm test field.
The intensity of the test field was adjusted by the subject. The
circular measuring field was presented near the center of a 10°,
2.5-log Td, 460-nm background. A small black dot placed on a slide
transparency in the background channel provided a fixation point on the
edge of the background in the nasal visual field. This stimulus
configuration is shown in
Figure 2 . To make the foveal setting, the subject looked directly at the small
measuring field. For the parafoveal setting, the subject looked at the
fixation point.
Measurement with the New Tabletop Device
Figure 3 is a diagram of the optical system that we used to measure MP density
in free view. The light for the background is provided by a source (S1)
that consists of three LEDs with peak energy at approximately 470 nm
and half-widths of approximately 20 nm. For the sake of simplicity, S1
is diagrammed as a linear array in
Figure 3 . In fact, S1 was
constructed by packing the three LEDs (3 mm in diameter) as tightly as
possible in a triangular array. This was done by first grinding off the
lens of each LED and then embedding them within a brass tube with an
inner diameter of 0.258 inches filled with an epoxy resin. After
curing, the front surface of the resin was ground flat and polished.
Light from S1 was collimated with a planoconvex lens (L1, 10-cm focal
length). A 1.75-inch circular aperture (A1), which defined the 6°
background field, was located approximately 2 inches beyond L1 at a
position where collimated light from the three LEDs overlapped. A1 was
constructed by exposing high-density photographic mylar film with a
computer-generated image of the aperture. The mylar film was then
affixed with optical grade glue to the smooth side of a diffuser, D1.
The polycarbonate diffuser is a high-efficiency holographic type
(Physical Optics Corporation, Torrance, CA) with a circular
diffuser angle of 20°. A1 was viewed by the subject reflected through
a beam splitter (BS) and appeared evenly filled with the light
transmitted through the diffuser. The subject’s eye was located
approximately 16 inches from the front surface of the beam splitter.
Light from S2 was collimated with a planoconvex lens (L2, 10-cm focal
length). S2 was composed of two LEDs with peak wavelengths of 458 nm
and one LED with a peak at 570 nm (half-bandwidths of 20 nm).
Construction was as for S1. A 0.3-inch aperture (A2), defining the 1°
measuring field was placed as for L1. The construction and composition
of the aperture-diffuser sandwich were identical to that of the S1
channel. The subject viewed A2 directly through the beam splitter,
which combined the two beams.
The entire optical system was enclosed within an opaque,
Plexiglas box. The subject peered into the system through a
1-inch circular hole (H) that was centered on the optical axis. When
properly positioned, the subject saw the 1° target superimposed on
the 6° background with the slightly larger and out-of-focus edge of
the hole (H) concentric with the background. A tiny (5-minute) opaque
spot was located on the extreme left side of the background to serve as
a fixation point for the parafoveal measuring condition. Another spot
(also 5 minute) was located in the center of the target to serve as a
fixation point for the foveal condition.
The configuration of the stimulus is illustrated in
Figure 2 . The 1°
target was located within the background such that its center was
displaced 4° from the fixation point located on the extreme left. A
photocell (model PIN-10; UDT Sensors, Hawthorne, CA) was used
to measure the relative radiance of the target and background.
Because of the 20° diffusing angle, head position was not critical.
The subject was merely instructed to make sure that the viewing hole
was concentric with the background. A chin and forehead rest were
sufficient to help the subject maintain position, and a bite bar was
unnecessary.
Calibration and Stimulus Control
In preliminary tests we found that the peak spectral energy of
individual LEDs varies considerably within a category defined as the
manufacturer’s catalog number. We chose each LED with the desired
spectral energy distribution in mind. For the shortwave component of
the measuring field we wanted peak energy to be within 2 nm of 460 nm,
which is close to the peak absorption of MP. The long-wave component of
the measuring field is less critical because it only needs to be
outside the main absorption band of MP (greater than 520 nm) and of
reasonable luminance. We chose 570 nm for that value. For the peak
energy of the background, we chose a value of 475 nm as the best
compromise, considering such factors as luminous efficiency and the
spectral absorption of rods and shortwave cones. We decided to use a
maximum of three LEDs for each source, because a triangular packing
gave a good compromise between maximum radiance and compactness. Thus,
we used three LEDs (model NSPB300A; Nichia, Mountville, PA) for
S1, each with peak wavelength near 470 nm. For the measuring source,
S2, we used two LEDs (Nichia Corp., Model NSPB300A) with peak
wavelengths near 460 nm, leaving the third position for the LED peaking
at 570 nm. We should emphasize that by combining both measuring lights
into one compact source, the necessity of combining them with a
light-losing beam splitter is avoided.
The stimuli were calibrated by placing a spectroradiometer-photometer
(model 650; Photo Research Inc., Chatsworth, CA) at the
position of the subject’s eye.
Figure 4 shows the relative spectral energy of the background (squares) and two
measuring components (diamonds for the shortwave components; triangles
for the long-wave component). The radiance of the background was set at
1.5-log Tds, the highest value that allowed a good adjustment range for
the small superimposed measuring field. The 570-nm reference wavelength
was set at 1.7-log Tds, the highest value that allowed a wide range of
settings for the 460-nm test wavelength. The radiance of the 460-nm
test wavelength is adjusted by the subject to minimize flicker (i.e.,
it is the dependent variable).
The LEDs are driven by constant current supplies. Radiance is
controlled by delivering brief (1.5 μsec), rectangular pulses at a
rate that can be varied from approximately 300 to 300,000 Hz. The pulse
rate for each LED is individually adjustable, with the radiance levels
being monitored by a photocell and the relative values displayed on a
digital display. Thus, the predetermined radiance values of each field
can be precisely set at the beginning of each experimental session. In
addition, the radiance of the 460-nm measuring beam can be varied by a
current adjustment that allows pulse frequency values to correspond to
absolute radiance values across experimental sessions.
There is a major advantage to using pulse frequency to control radiance
in the particular way that we measure MP density. Our calculation of
the pigments’ optical density (OD) is simple, as shown below:
\[\mathrm{O.D.{=}Log10\ R}_{\mathrm{f}}\mathrm{/R}_{\mathrm{p}}\]
where R
f and R
p are
simply the radiances of the 460-nm test beam that results in minimum
flicker with respect to the 570-nm reference at the foveal and
parafoveal loci, respectively. Because the pulse frequency should be
proportional to the radiance, with a slope of 1.0 and origin of 0.0, it
follows that:
\[\mathrm{MP\ O.D.{=}Log10\ F}_{\mathrm{f}}\mathrm{/F}_{\mathrm{p}}\]
where F
f and F
p refer
to the frequency of the 460-nm test beam at the foveal and parafoveal
positions, respectively. We tested this conclusion by placing a
spectroradiometer at the eye’s position and measuring the actual
integrated radiance as pulse frequency was varied over a large range.
The range of energies we explored was a conservative estimate of a good
working region for the measurement of MP.
Figure 5 shows that log energy ratio of the radiance values plotted against the
log frequency ratio of the corresponding pulse frequency values in the
range of adjustment needed to make MP measurements. Notice that the
data points fall very close to the straight line, which has a slope of
1.0 and an intercept of 0.0. The conclusion is that frequency ratios
can substitute for radiance ratios. This greatly simplifies the use of
the instrument because pulse frequency is easily determined with
inexpensive and accurate electronic devices, whereas an accurate energy
calibration requires a fairly expensive and elaborate analogue device.
The use of pulse frequency is a major simplifying aspect of the device.
Measurement of Lens Optical Density
For a subset of subjects (
n = 10), we also
measured lens optical density (see
Table 1 ). For a discussion of the technique, see the recent chapter by
Snodderly and Hammond (1999).
12 Scotopic
thresholds were obtained using two channels of the Maxwellian view
system. One channel provided a dim blue 20-minute fixation point. A
second channel provided a 2.8° test stimulus. The test stimulus was
either a 410-nm (high lens absorbance) or 550 nm (minimal lens
absorbance
14 ) light. These wavelengths were selected
because of equal absorption values on the rhodopsin
curve.
15 Lens optical density was calculated by directly
subtracting the log relative sensitivity value at 410 nm from the log
relative sensitivity value at 550 nm without referring to the rhodopsin
curve itself. For a discussion of the rationale behind using the“
balanced rhodopsin” technique see the original description by van
Norren and Vos (1974).
16 Test field exposures were
determined by a Uni-blitz electromagnetic shutter (Rochester, NY).
The test stimulus was always presented at 6° in the nasal visual
field. Scotopic thresholds were obtained after subjects were dark
adapted for 40 minutes. For a discussion of the procedure used for
measuring these absolute thresholds see Hammond et al.
(1997)
17 and Snodderly and Hammond.
12
Results
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.
Discussion
Past techniques for measuring MP in vivo include fundus
reflectometry,
20 21 fluorophotometry,
22 23 and psychophysical methods using
Maxwellian View Optics.
17 18 19 24 25 26 27 28 29 30 Of these techniques,
the psychophysical technique has been used the most often and provides
reliable data.
18 Nonetheless, a limited number of
laboratories have Maxwellian View systems, and skilled operators are
necessary to obtain reliable data. Moreover, these systems are not
typically mobile and therefore not amenable to field study. In this
report we have described the development of a technique for measuring
MP in vivo that is easy to use, is relatively inexpensive, and can
therefore be more widely disseminated.
The major difference between stimuli viewed in Maxwellian view and free
view, is that in the Maxwellian view the stimuli enter the eye as a
narrow pencil-shaped beam centered in the pupil. In contrast, free view
utilizes the whole area of the pupil. Thus, an advantage of Maxwellian
view optics is that variations in pupil size have no effect on the
final retinal illuminance because the exit pupil of the optical system
is smaller than the smallest diameter that can be obtained by the
pupil. Moreover, free view cannot obtain the higher retinal illuminance
levels obtainable with Maxwellian optics.
In this study we tested whether measurements of MP were affected by
variations in pupil size or retinal illuminance by comparing MP
measured using Maxwellian view optics with MP measured using a free
viewing situation. Subjects of different ages and lens optical
densities were tested. As shown in
Table 1 , a high correlation was
found between MP measured using the different techniques, and no
systematic differences were found between the two techniques. In fact,
variation across the two techniques is equal to what would be expected
given the across-session variance using the Maxwellian view system
alone.
18 The similarity in the MP values derived from the
two systems underscores the basic hardiness of this psychophysical
procedure for measuring MP. For example, the test stimulus in
Maxwellian view was composed of wavelengths with a narrower band-pass
(7 nm as opposed to 20 nm) and higher retinal illuminance, which were
referenced to a peripheral point located at 6° as opposed to 4°.
The similarity in the derived values indicates that a wide number of
conditions can be altered without affecting the ultimate reliability of
the measurements. This suggests that we may be able to use the method
on a variety of subjects under varying conditions.
A simple method of measuring MP is useful for a variety of
applications. As outlined in the introduction, measuring MP would
provide a direct assessment of the relationship between local
concentrations of lutein and zeaxanthin and macular disease. Before
this assessment is made, however, future work must focus on validating
the psychophysical method of measuring MP on patients with eye disease.
Our preliminary investigations (unpublished) indicate that even
individuals with cataractous lenses can perform the task, but no
information is currently available showing what types of patients with
retinal pathology can perform the task, and whether data from patients
will be valid measures of MP. Currently, the most convincing validation
is to show that it is possible, using the task, to derive an optical
density spectrum for MP that matches the extinction spectrum of MP
measured ex vivo.
13 For rigorous application of the
psychophysical methodology to patients, it will be necessary to repeat
the validation experiments that have previously been performed with
normal subjects using carefully characterized clinical populations.
Measuring an individual’s MP density over time with our methodology is
practical due to the nondestructive nature of the measurement. Unlike
other techniques for measuring tissue concentrations of a nutrient
(e.g., adipose biopsy), our technique is noninvasive and therefore does
not alter the tissue itself. Thus, repeated measurement of the
concentrations of lutein and zeaxanthin in the retina can be made.
Tissue measures of this type would be a significant addition to the
repertoire of techniques available to nutritional scientists studying
the metabolism of nutrients within the body. The ability to obtain
repeated measures of the macular pigments also allows for the
assessment of interventions without the added variability of measuring
different biopsies of tissue. Finally, to the extent that this
technology can be made widely available, clinicians may be interested
in adding this measurement to their usual assessment procedures.
Accurate information regarding the nutritional status of the retina is
of interest to patients with early signs of disease but also to
nondiseased individuals concerned with maintaining their health.
Submitted for publication January 7, 1999; revised May 18, 1999; accepted June 3, 1999.
Commercial relationships policy: P.
Corresponding author: Billy R. Hammond, Jr., Department of Psychology, University of Georgia, Athens, GA 30602. E-mail:
bhammond@egon.psy.uga.edu
| Subject | Iris Color | Age | Sex | MPOD-free | MPOD-Maxwell | OD Change | Smoking Status | Lens OD |
| LJ | Brown | 46 | M | 0.34 | 0.26 | 0.08 | NS | |
| PJ | Black | 23 | M | 0.39 | 0.46 | −0.07 | NS | |
| AT | Blue | 24 | F | 0.34 | 0.46 | −0.12 | NS | |
| RC | Brown | 30 | F | 0.42 | 0.505 | −0.09 | NS | 1.69 |
| HJ | Blue | 22 | M | 0.32 | 0.38 | −.06 | NS | 1.5 |
| ML | Black | 48 | F | 0.39 | 0.32 | 0.07 | NS | |
| HC | Brown | 19 | M | 0.11 | 0.02 | 0.09 | NS | |
| WA | Brown | 25 | M | 0.46 | 0.47 | −0.01 | NS | 1.63 |
| EA | Brown | 31 | F | 0.07 | 0 | 0.07 | NS | 1.37 |
| ZL | Black | 30 | M | 0.47 | 0.41 | 0.06 | PS | |
| VM | Brown | 23 | F | 0.314 | 0.25 | 0.06 | NS | 1.51 |
| VN | Brown | 16 | F | 0.27 | 0.22 | 0.05 | NS | |
| RC | Brown | 41 | F | 0.219 | 0.178 | 0.04 | PS | |
| HR | Green | 34 | M | 0.43 | 0.42 | 0.01 | NS | 1.65 |
| LM | Brown | 30 | M | 0.223 | 0.2 | 0.02 | NS | 1.46 |
| GA | Blue | 31 | M | 0.103 | 0.03 | 0.07 | NS | |
| SJ | Blue | 28 | M | 0.19 | 0.15 | 0.04 | NS | |
| TM | Brown | 23 | F | 0.15 | 0.13 | 0.02 | CS | |
| BB | Blue | 58 | F | 0.222 | 0.214 | 0.01 | NS | |
| KM | Brown | 28 | M | 0.164 | 0.111 | 0.05 | NS | |
| RB | Blue | 33 | M | 0 | 0.03 | −0.03 | CS | |
| TC | Brown | 51 | F | 0.188 | 0.142 | 0.04 | PS | |
| MM | Hazel | 49 | F | 0.292 | 0.267 | 0.03 | PS | |
| WB | Brown | 56 | M | 0.595 | 0.594 | 0 | NS | |
| RR | Brown | 29 | M | 0.59 | 0.55 | 0.04 | CS | 1.71 |
| WA | Green | 51 | F | 0.24 | 0.3 | −0.06 | NS | 1.64 |
| AT | Brown | 29 | F | 0.02 | 0 | 0.02 | NS | |
| SS | Blue | 20 | F | 0.17 | 0.19 | −0.02 | NS | 1.1 |
| BJ | Hazel | 21 | F | 0.12 | 0.12 | 0 | NS | |
| VJ | Green | 60 | F | 0.141 | 0.138 | 0.00 | NS | |
| Mean ± SD | | 33.6± 12.7 | | 0.265± 0.15 | 0.251± 0.17 | 0.01 ± 0.05 | | |
The authors thank Robert K. Moore for designing the electronics and
Ken DeLucia for technical assistance.
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