Download Do women and myopes have larger pupils?

Reports
No. 7
1413
Investigative Ophthalmology & Visual Science, Vol. 31, No. 7, July 1990
Copyright © Association for Research in Vision and Ophthalmology
Do Women and Myopes Hove Larger Pupils?
Ronald Jones
It is commonly reported that the pupils of women are larger
than those of men and the pupils of myopes larger than
those of emmetropes. However, these reports are not supported by experimental procedures using objective measurements, controlled conditions, and adequate numbers of
subjects. In this report the open-loop pupil size was measured objectively using dynamic infrared pupillometry in a
sample of 48 subjects. Subjects were balanced for age and
equally divided between emmetropes and myopes, males
and females. The results did not support the contention that
females have larger pupils than males or that physiologic
myopes have larger pupils than emmetropes. Invest
Ophthalmol Vis Sci 31:1413-1415,1990
The size of the normal pupil is influenced by the
intensity of retinal illumination, proximity of the
stimulus, and emotional state of the individual.1
Pupil diameter varies between 8 mm when fully
dark-adapted to a minimum of 2 mm when the eye is
light-adapted to moderate photopic luminances.2 The
normal pupil has been shown to be relatively miotic
in the elderly,3 but other influences on the pupil are
not well established. It has been variously reported4'5
that the pupils of women are larger than those of men
and the pupils of myopes larger than those of emmetropes or hyperopes, but such reports do not appear
to have been documented by studies with objective
measurements, controlled conditions and adequate
numbers of subjects to consider them resolved.1 It is
the purpose of this report to determine the pupil size
in samples that are balanced for sex, age, and refractive error in order to evaluate these claims.
The pupillary light reflex is controlled by a negative
feedback system that continually adjusts pupil size in
response to changes in retinal illumination.6 The pupillary light feedback loop can be "opened" by preventing changes in pupil size from affecting retinal
illumination. This is achieved by placing an aperture
in front of the eye that is smaller than the natural
pupil. Artificial pupils of approximately 0.5 mm also
increase ocular depth of focus to the extent that accommodation is made ineffective.7 When viewing
through a monocular pinhole, accommodation and
convergence assume their resting states and the pupil
is open-loop. The pupils were measured under such
conditions in order to minimize any effects of external stimuli on pupillary size differences between
males and females, myopes and emmetropes.
Materials and Methods. The optical system is il-
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lustrated in Figure 1. The light source was an incandescent projection lamp (approximate color temperature 3200°K). Identical lenses L3, L2, and LI were
separated by twice their focal lengths, and the focal
point of LI was coincident with the entrance pupil of
the left eye. An aperture stop was imaged by LI and
L2 onto the pupil of the left eye to form a 0.5-mm
exit pupil for the viewing system. The fixation target
consisted of a Maltese cross that was positioned coincident with the subject's far point to assure that the
pinhole provided similar depth of focus for subjects
with all refractive errors. The target was 5° in diameter and provided a background retinal illuminance of
2.5 X 104 trolands. This photopic light level was chosen after pilot experiments indicated that it provided
an intermediate pupil response size, thus assuring
that the pupil response was not saturated.
The right eye was occluded with an infrared transmitting filter (Wratten no. 87; Kodak, Rochester,
NY). An infrared sensitive MOS video camera monitored the right eye through the filter and provided a
highly magnified video image to a dynamic pupillometer (model RK-416; ISCAN, Cambridge, MA).
The pupillometer was connected by digital interface
(Base-Board; TECMAR, Cleveland, OH) to a microcomputer (model 5150; IBM, Armonk, NY). The
pupillometer was calibrated with a series of artificial
apertures that were placed in the plane of the pupil. A
least-squares linear regression of the pupillometer
output against aperture diameter was used to obtain
coefficients for subsequent scaling of the pupil diameter by the computer. The pupillometer resolution was
0.05 mm.
Subjects: Subjects ranged in age from 18 to 26 yr
and had normal binocular vision as indicated by
20/20 or better corrected monocular visual acuities,
20" stereothreshold and ability to fuse at distance
through loose prisms of 4A base-in and 6A base-out.
Refractive error was determined by autorefraction or
lensometry of the subject's spectacles. Informed consent was obtained from each subject prior to testing.
A total of 48 subjects, equally divided between
males and females, simple myopes (-1.50 to —3.50
D) and emmetropes (+0.75 to -0.25 D) were tested
in these experiments. Preliminary experiments with
the pupillometer indicated a standard deviation of
pupil measurement of 0.25 mm. The precision was
limited by the presence of normal pupil oscillations.
1414
Vol. 01
INVESTIGATIVE OPHTHALMOLOGY b VISUAL SCIENCE / July 1990
CONDENSER
GROUND GLASS
FIXATION TARGET
APERTURE STOP
-INFRARED FILTER
Fig. 1. Schematic of the apparatus used to measure pupil size and
to open the accommodation, convergence, and pupillary loops.
Li-L 4 , lenses 1-4.
If alpha (one-tailed) and beta risks levels are 0.05, it
was estimated that significant differences between
groups exceeding 0.24 mm could be resolved with
this sample size.
Subject selection was intended to exclude individuals with pathologic myopia8 from this study. This
was attempted by selecting myopes having an age of
onset greater than 9 yr and limiting the maximum
refractive error to —4.00 D. The groups were closely
balanced for age, sex, and amount of refractive error.
Selection of subjects was continued until a minimum
of 12 subjects was obtained in each of the four categories. Subjects were then matched in priority of refractive error, sex, and age. Ties were resolved by random
selection. (A total sample of 72 subjects was tested in
order to obtain the balanced test sample).
Procedure: Data collection consisted of computercontrolled trials in which pupillary responses were
measured while accommodation, convergence, and
the pupil were open-loop. Subjects were positioned in
the apparatus with a dental impression bite-bar and
were light-adapted to the fixation target for 3 min
prior to stimulus presentation. The computer warned
the subject of the beginning of a presentation with an
audible tone so that blinks could be minimized. Then
a 5-sec sampling period was initiated in which pupil
size was measured at 60 sample/sec. The average and
standard deviation of the pupil diameter over the
5-sec period were computed and stored on disk. Data
samples taken during blinks or eye movements
greater than several degrees were eliminated from the
sample. Such artifacts were detected with circuitry
provided by the pupillometer and signaled to the
computer. Each measurement was repeated at least
once and the results of the separate trials were averaged.
Results. The mean values of pupil size and their
standard errors obtained for subjects in each of the
four measurement groups are presented in Table 1.
Two-way analysis of variance was used to evaluate
the effect of gender and refractive error on pupil size;
this statistical analysis is presented in Table 2. The
results indicate that there were no significant differences in the pupil sizes between myopes and emmetropes or between males and females. The mean size
of the pupil for the entire group of subjects was 5.71
mm (SE=0.12) for the open-loop conditions of the
experiment (retinal illuminance was 2.5 X 104 trolands).
Discussion. The results do not support the contention that females have larger pupils than males or that
myopes have larger pupils than emmetropes. The
Table 1. Characteristics of subjects
Group
Subject
Males
Characteristic
Emmetropes
Myopes
Totals
Diameter
6.00 ± 0.24
0.14 ± 0.10
22.58 ± 0.55
5.60 ± 0.22
0.21 ±0.09
22.83 ± 0.62
5.80 ±0.17
0.18 ±0.06
22.71 ±0.41
5.59 ± 0.20
-2.48 ± 0.24
22.50 ± 0.62
5.63 ± 0.30
-2.34 ±0.17
22.00 ± 0.66
5.61 ±0.18
-2.41 ±0.13
22.25 ± 0.44
5.80 ±0.16
-1.17 ±0.30
22.54 ± 0.40
5.61 ±0.18
-1.06 ±0.28
22.41 ±0.45
5.71 ±0.12
-1.12 ±0.20
22.48 ± 0.30
Rx
Females
Age
Diameter
Totals
Age
Diameter
Rx
Rx
Age
Data are means ± Standard errors.
Diameter, pupil diameter (mm); R», refractive error (diopters); age, subject age (yr).
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Reports
No. 7
1415
Table 2. Two-way analysis of variance of pupil diameter data from Table 1
Source
DF
Gender
Refraction
Interaction
Error
1
1
1
44
0.425
0.445
0.582
31.917
Total
47
33.369
MS
F ratio
P> F
0.425
0.445
0.582
0.725
0.586
0.613
0.802
0.551
0.562
0.624
DF, degrees of freedom; SS, sum of squares; MS, mean squares.
conditions of these experiments were designed to free
the pupil of influences from accommodation and
convergence by opening these oculomotor control
loops. Since the pupil also was open-loop, constant
retinal illuminance was maintained for the experiments, and the results permit comparison of pupil
sizes under controlled stimulus conditions. The relatively large number of subjects (48) and the balanced
design were employed to circumvent bias due to individual differences in the level of arousal. Further, the
use of a simple fixation target should have eliminated
any gender-dependent emotional factors associated
with the visual context of the stimulus.8
The dependence of the pupil size on gender or refractive error has yet to be determined by experiments in which the influence of accommodation,
convergence, and luminance is systematically varied.
The current results permit only the conclusion that
the pupil size is not dependent on gender or refractive
error in the absence of external oculomotor stimuli.
However, these results do cast doubt on the validity
of reports4'5 that are based on uncontrolled or unspecified stimulus conditions. It seems unlikely that the
results for gender would differ for closed-loop conditions. On the other hand, it is reasonable to expect
that uncorrected myopes will have a larger pupil at
near than will emmetropes, when accommodation is
closed-loop. This difference occurs because emmetropes will be required to accommodate more than
will myopes, and the synkinesis between accommodation and the pupil will result in relatively greater
pupillary constriction. Such pupillary size differences
lack physiologic significance and are simply artifacts
resulting from uncontrolled differences in accommodation between the two groups. Nevertheless, these
differences may account for the widely held clinical
impression that myopes have larger pupils than emmetropes or hyperopes.
These results do not allow conclusions about pupil
size in high or pathologic myopia, since physiologic
myopes were selected for this test sample. Physiologic
myopia is defined by Curtin9 as the refractive condition in which the apparently random combination of
normal optical components renders the eye myopic.
The etiology of this group of myopes is probably different than that of pathologic myopia, which is
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accompanied by significant structural changes in
the eye.
The results also suggest that the mean open-loop
pupil diameter (5.71 mm) is greater than that of the
closed-loop pupil, which is approximately 2.0 mm in
diameter at an equivalent retinal illuminance and
field size.10 Palmer" reported that opening the pupillary loop results in a significantly larger pupil than
when the same light flux passes through the natural
pupil. However, opening and closing the pupillary
feedback loop with pinholes concomitantly affects
accommodative feedback. Therefore, this pupil size
difference cannot be attributed only to the action of
pupillary feedback. Further investigation will be required to resolve this issue.
Key words: pupil, myopia, gender, near response, physiologic myopia
From Ohio State University, College of Optometry, Columbus,
Ohio. Supported by grant EY-06577 from the National Eye Institute, National Institutes of Health, Bethesda, Maryland. Submitted
for publication: May 30, 1989; accepted October 4, 1989. Reprint
Requests: Dr. Ronald Jones, The Ohio State University, College of
Optometry, 338 West Tenth Avenue, Columbus, OH 43210.
References
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3. Kumnich LS: Pupillary psychosensory restitution and aging. J
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4. Alexandridis E: The Pupil, Telger T, translator. New York,
Springer-Verlag, 1985, p. 11.
5. Zinn KM: The Pupil. Springfield, Charles C. Thomas Publisher, 1972, p. 44.
6. Stark L: Neurological control systems. Studies in Bioengineering. New York, Plenum Press, 1968, pp. 73-184.
7. Ripps H, Chin NB, Siegel IM, and Breinin GM: The effect of
pupil size on accommodation, convergence and the ACA ratio.
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acuity. Nature 187:1121, 1960.
11. Palmer DA: The size of the human pupil in viewing through
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