![Refractive astigmatism induced by optically imposed astigmatism of two different magnitudes.
(A) Distributions of inter-ocular differences in refractive astigmatism for the four treatment groups with cylindrical lenses of two magnitudes ([H]:+4.00DS/-8.00DC and [L]:+2.00DS/-4.00DC) and two axis orientations (H90, H180, and L90; n = 20 in each group; L180, n = 18). See caption for Fig. 2 and text for details. (B) The box plots of refractive astigmatism include the values of median, maximum, minimum, upper and lower quartiles for each group (see Fig. 2 for details). The levels of significant difference in the magnitudes of refractive astigmatism across the treatment groups (lines above the boxes, two-sample t-tests), or between treatment and control group (lines below the boxes, Tukey’s post hoc tests) are indicated by asterisk: * p≤0.05, *** p≤0.001.](https://www.researchgate.net/profile/Geoffrey-Chin-Hung-Chu/publication/273757271/figure/fig12/AS:342771102699523@1458734407694/Refractive-astigmatism-induced-by-optically-imposed-astigmatism-of-two-different_Q320.jpg)
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RESEARCH ARTICLE
Effects of Optically Imposed Astigmatism on
Early Eye Growth in Chicks
Chin Hung Geoffrey Chu*, Chea Su Kee
School of Optometry, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China
*geopicasso@yahoo.com
Abstract
Purpose
To determine the effects of optically imposed astigmatism on early eye growth in chicks.
Methods
5-day-old (P5) White Leghorn chicks were randomly assigned to either wear, monocularly,
a“high magnitude”(H: +4.00DS/-8.00DC) crossed-cylindrical lens oriented at one of four
axes (45, 90, 135, and 180; n = 20 in each group), or were left untreated (controls; n = 8).
Two additional groups wore a “low magnitude”(L: +2.00DS/−4.00DC) cylindrical lens orien-
tated at either axis 90 or 180 (n = 20 and n = 18, respectively). Refractions were measured
at P5 and after 7 days of treatment for all chicks (P12), whereas videokeratography and
ex-vivo eyeshape analysis were performed at P12 for a subset of chicks in each group
(n = 8).
Results
Compared to controls, chicks in the treatment groups developed significant amounts of re-
fractive astigmatism (controls: 0.03±0.22DC; treatment groups: 1.34±0.22DC to 5.51±0.26DC,
one-way ANOVAs, p0.05) with axes compensatory to those imposed by the cylindrical
lenses. H cylindrical lenses induced more refractive astigmatism than L lenses (H90 vs.
L90: 5.51±0.26D vs. 4.10±0.16D; H180 vs. L180: 2.84±0.44D vs. 1.34±0.22D, unpaired
two-sample t-tests, both p0.01); and imposing with-the-rule (H90 and L90) and against-
the-rule astigmatisms (H180 and L180) resulted in, respectively, steeper and flatter corneal
shape. Both corneal and internal astigmatisms were moderately to strongly correlated with
refractive astigmatisms (Pearson’s r: +0.61 to +0.94, all p0.001). In addition, the charac-
teristics of astigmatism were significantly correlated with multiple eyeshape parameters at
the posterior segments (Pearson’s r: -0.27 to +0.45, all p0.05).
Conclusions
Chicks showed compensatory ocular changes in response to the astigmatic magnitudes im-
posed in this study. The correlations of changes in refractive, corneal, and posterior
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 1/22
OPEN ACCESS
Citation: Chu CHG, Kee CS (2015) Effects of
Optically Imposed Astigmatism on Early Eye Growth
in Chicks. PLoS ONE 10(2): e0117729. doi:10.1371/
journal.pone.0117729
Academic Editor: J. Fielding Hejtmancik, National
Eye Institute, UNITED STATES
Received: August 28, 2014
Accepted: December 31, 2014
Published: February 12, 2015
Copyright: © 2015 Chu, Kee. This is an open access
article distributed under the terms of the Creative
Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any
medium, provided the original author and source are
credited.
Data Availability Statement: All data underlying the
findings in our study are freely available in the
Supporting Information file, S1_Dataset.xls.
Funding: Grant support was provided by RGC
General Research Fund #561209; Internal
Competitive Research Grant G-YK90, The Centre of
Myopia Research and The Niches Areas-Myopia
Research Fund (J-BB7P) from The Hong Kong
Polytechnic University. The funders had no role in
study design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing Interests: The authors have declared
that no competing interests exist.
eyeshape indicate the involvement of anterior and posterior ocular segments during the de-
velopment of astigmatism.
Introduction
Astigmatism is a very common refractive error but its etiology remains elusive [1–4]. Uncor-
rected astigmatism not only degrades the contrast of retinal image at both distance and near,
the presence of significant astigmatism with specific orientation has also been associated with
amblyopia [5–8] and myopia development [9–11]. The prevalence of astigmatism usually de-
clines during childhood [8,12]. However, in American Indian, a population known to exhibit
high prevalence of significant astigmatism [13–15], the prescription of spectacles correction
even during early school years did not appear to improve visual functions to normal level [16].
These findings, together with asthenopia [17], tilted optic disc [18–20], and abnormal retinal
electrophysiology frequently found in astigmats [21], spur the needs for understanding the eti-
ology of astigmatism with new approach. Although several factors including genes [22,23], eth-
nicity [8,24–29], nutrition [30], age [31,32], and spherical refractive errors (i.e., myopia and
hyperopia) [33,34] have been associated with astigmatism in humans, the effect of environ-
mental factor is still unclear.
Visual experience plays an important role in refractive development. In response to form
deprivation and spherical defocuses, a wide variety of animal models developed refractive er-
rors [35–40,40–44]. By the way of illustration, both chicks and macaque monkeys developed
ametropia primarily axial in nature, with the former animal model responsive to a broader
range of spherical defocus than the latter (-30.00D to +15.00D [45,46] vs. -3.00D to +6.00D
[47]). However, could the growing eye alter its ocular components to compensate for astigmat-
ic errors? Different laboratories have investigated this question, but the results were contradic-
tory. An initial study in chicks showed partial compensation for optically imposed astigmatism
with significant effects of axis orientation, the highest magnitudes of induced astigmatism was
found when imposing oblique astigmatism, and about 50% of these induced astigmatism at-
tributed to the cornea [45,48]. However, similar results were not replicated subsequently, in
chicks [49–54] or in monkeys [55,56]. On the other hand, although the presence of astigmatism
produced a slight myopic or hyperopic shift in some studies [45,49,51–53,57], it did not appear
to affect the compensatory response to spherical defocus [54].
The inconclusiveness of previous studies has questioned about the capability of the eye to
compensate for astigmatic errors. The primary purpose of this study was to examine how the
chick eye responds to imposed astigmatism with crossed-cylindrical lenses of different axis ori-
entations and magnitudes. The secondary purpose was to determine the correlations between
refractive, corneal, and eyeshape parameters in astigmatic eyeball.
Materials and Methods
Animal Subjects
Eggs of White Leghorn chickens (Gallus gallus domesticus) were hatched in the university’s
central animal facilities. The chicks were reared in a temperature controlled (22°C) animal fa-
cility on a 12-hour light/12-hour dark lighting cycle (from 7:00am to 7:00pm) with food and
water provided ad libitum. The average light illuminance was approximately 100 lux at the
chick’s eye level. Care and use of the animals were in compliance with the ARVO Statement for
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 2/22
the Use of Animals in Ophthalmic and Vision Res and the protocol was approved by the Ani-
mal Subjects Ethics Sub-committee of The Hong Kong Polytechnic University.
Experiments
Visual Manipulations. At 5-day post-hatching (P5), the chicks were randomly assigned to the
treatment or control group. To impose astigmatism, a crossed-cylindrical lens (PMMA, 7.6mm
base curve, 10.8mm diameter, 10.8mm optical zone; Conforma, VA, USA) of specific magni-
tude and axis was held in front of the right eye by using a Velcro mount, and the fellow eyes
were left untreated. The optical effect of crossed-cylindrical lens has been illustrated elsewhere
[54,58,59]. The opposing powers at two orthogonal meridians create no spherical power and
no astigmatic power at 45° away from the principal meridians. The minus-cylindrical axis was
carefully oriented for individual treatment groups with the palpebral fissure as a horizontal ref-
erence line [60]. During the treatment period (P5 to P12), the lens was removed daily for
cleansing; any scratched or cracked lens was replaced immediately. If the lens was found de-
tached, the data of the chick was excluded from further analysis.
The two experiments in this study determined whether and how the orientation and magni-
tude of optically imposed astigmatism altered early eye growth. In experiment A, the effects of
the astigmatic axis on eye growth were determined by randomly assigning the chicks to wear a
high magnitude (H) crossed-cylindrical lens of power +4.00DS/-8.00DC with the minus-cylin-
drical axis oriented at one of four axis orientations (45, 90, 135, and 180; n = 20 in each group).
These groups were referred to as H45, H90, H135 and H180, respectively. These four orienta-
tions were chosen for imposing with-the-rule (“WTR”, H90), against-the-rule (“ATR”, H180),
and oblique astigmatisms (H45 and H135) which are commonly found in humans. Eight age-
matched chicks received no treatment served as controls. Since we found significant effects of
axis orientation on various biometric parameters in experiment A, in experiment B we tested
the effects of magnitude on eye growth by adding two groups of chicks with a lower magnitude
(L) crossed-cylindrical power +2.00DS/-4.00DC (L90, n = 20; L180, n = 18).
Biometric measures. The details of refraction method have been described elsewhere
[61,62]. In brief, the refractive status was measured along the pupillary axis using a modified
Hartinger refractometer (Jena Coincidence Refractometer, Model 110, Carl Zeiss Meditec,
Jena, Germany) in anaesthetized chicks (isoflurane inhalation, 1.0% to 1.5% in oxygen for
rapid induction and low percentage of possible complications [63]). Although isoflurane ad-
ministration can lead to dopaminergic alteration in human [64] and drug-induced cycloplegia
in normal chick eyes [61], no significant effect on astigmatism measurements in chick has been
reported [49]. After the chick was anesthetized, the palpebral fissure was aligned horizontally,
and the lower eyelid was pulled down gently by using a lid retractor without causing any distor-
tion of the refractometer mire. Refractive errors could be varied according to the size, strength
and position of the lid retractor in conjunction with eyelid tension. Furthermore, because the
cornea does not behave in accordance with Gauss’s law of elastic dome [65], the coupling ratio
is not equal to one (i.e., the change in steep K is not the same as the change in flat K). Thus, the
effect of lid retractor could affect not only the astigmatic components, but also the spherical
components. Therefore the design and application of lid retractor should be treated with cau-
tion. Nonetheless, in practice, previous studies [49,66] have shown that the presence of lid re-
tractor produced insignificant effect on both spherical-equivalent (0.20D to 0.70D) and
astigmatism measurements. For each datum, three independent measurements were taken and
averaged using power vector analysis [58]. The seven refractive parameters (Spherical compo-
nents: spherical-equivalent, M; most hyperopic meridian, MHM; most myopic meridian,
MMM; Cylindrical components: refractive astigmatism, RA; the two vector components [58],
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 3/22
R-J0 and R-J45, and the axis) were analysed. To avoid potential effects of diurnal variations on
refractive status [67–69], all measurements were taken at approximately the same time of the
day (10:00am±1hr).
After measuring the refractive changes for a large number of astigmatism imposed chicks, a
subset of birds (n = 8) randomly assigned to each group was used for corneal topography and
eyeshape imaging. Corneal curvatures and astigmatism were measured using a custom-made
videokeratography system in alert chicks (see [70] for details). The system captured Placido-ring
images (i.e., the first Purkinje image) in multiple-shot mode and analysed the central2.80mm-di-
ameter cornea using a custom MatLab algorithm (MatLab; The MathWorks, Natick, MA. In
order to rule out the potential effect of accommodation on corneal curvature, only images ac-
quired at relaxed accommodative status were used, these images were identified from 500–1500
Placido-ring images from each eye as demonstrating the most frequently observed mean corneal
curvature [70]. The average values of the corneal curvatures along the two principal meridians
were calculated, assuming a corneal refractive index of 1.369 [71–73], from three good images
per eye at different time points. Further, seven corneal parameters (steepest curvature, SK; flattest
curvature, FK; mean curvature (average of FK and SK), MK; corneal astigmatism, CA; the two
vector components, C-J0 and C-J45; and the axis, were derived for further analyses.
Immediately after the chicks were sacrificed by carbon dioxide asphyxiation at the end of
the experiment, eyes were enucleated and eyeball profile were captured along the horizontal
and vertical meridians, by an eyeshape imaging system described previously [62]. A MatLab al-
gorithm was written to extract the following ocular dimensions by referring to the corneal
apex: the axial length (AL), ocular lengths up to 50° in 5° intervals (see Fig. 1 for illustration),
Fig 1. Illustration of eyeshape parameters. The eyeshape profiles of the horizontal meridian for the fellow eyes of a chick treated with +4.00DS/
−8.00DCx90. The profile of the treated eye (blue area) is overlaid with that of the untreated fellow eye (yellow area). Axial length, equatorial diameter and
ocular dimensions at different eccentricities (from 0° to 50° in 5° intervals), as identified by a MatLab program, are calculated with respect to the corneal apex.
Identical image analysis protocol was applied to the vertical (superior and inferior regions) eyeshape profiles.
doi:10.1371/journal.pone.0117729.g001
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 4/22
and equatorial diameters (vertical equatorial diameter, ED90; horizontal equatorial diameter,
ED180). To study the changes in posterior eyeshape in response to cylindrical lens treatment,
the inter-ocular differences in ocular dimensions between the treated/right eyes and the fellow/
left eyes (i.e., treated/right eye—fellow/left eye) were calculated first from central 0° to 50° ec-
centricity, in 5° intervals, and summated for the horizontal (ADH, differences in area along the
horizontal meridian) and vertical meridians (ADV, differences in area along the vertical merid-
ian). We used “unit area”as a general term to represent the unit for these two quantities. The
difference between ADH and ADV (ADH-ADV) was then calculated to show the meridional
difference in ocular expansion of the posterior segment. In addition, the sum of ADH and
ADV (ADH+ADV) was calculated to indicate the overall expansion of posterior globe.
Internal astigmatism. As defined previously [30,74], the internal astigmatism (IA) is the
vectorial difference after subtracting the corneal astigmatism from refractive astigmatism
[58,75,76].
Data analysis
Statistical analyses were carried out using SPSS16 (SPSS, Inc, Chicago, Illinois, USA) and Ori-
ana Version 4.01 (Kovach Computing Service). Statistical tests aimed primarily to determine
the effects of crossed-cylindrical lenses on refractive, corneal and eyeshape parameters. Com-
parisons across groups were made by one-way ANOVAs. If the one-way ANOVA revealed sig-
nificant effect, Tukey’s pairwise post hoc comparisons were used to determine which groups
were significantly different. Two-sample t-tests were used to determine the effects of the mag-
nitude of astigmatism between H and L groups. Watson-Williams F-tests [77,78] followed by
pairwise comparisons were used to determine the treatment effects on axis orientations, the
axis orientations per group were expressed as mean±angular deviation. Paired t-tests were used
for the comparisons of parameters within eyes (e.g., horizontal vs. vertical corneal curvatures)
or between treated/right and fellow/left eyes. Pearson’s correlation analyses were performed be-
tween refractive, corneal and eyeshape parameters. In all tests, significant level was set at the
95% level of confidence. Unless otherwise stated, all data were expressed in terms of inter-ocu-
lar differences (IOD) and mean±standard error (SE).
Results
Pre-treatment refractive status
At the onset of the two experiments, all refractive parameters (both spherical and cylindrical
components) were not statistically different across the treatment and control groups (one-way
ANOVA, all p0.40). The mean spherical equivalent (M) and refractive astigmatism (RA) in
each group ranged from -0.15D to +0.27D and from -0.09D to +0.18D.
Post-treatment effects
Refractive status −Effects of axis of astigmatism (Experiment A). After 1 week of treatment
(P12), there was no significant difference in the spherical equivalent or most hyperopic meridi-
an across the treatment and control groups (one-way ANOVA, both p0.11). However, com-
pared to the controls, the H45 and H90 groups developed significantly more negative most
myopic meridian (H45 = -3.18±0.61D; H90 = -3.19±0.30D; and Controls = -0.44±0.36D, one-way
ANOVA with Tukey’s post hoc tests, both p<0.05, see Table 1). More importantly, refractive
astigmatisms in the four treatment groups were all higher than those in the controls (one-
way ANOVA with Tukey’s post hoc tests, all p<0.001, see Fig. 2B). As summarized in Table 1,
the highest and lowest magnitudes of induced refractive astigmatism were found in the H90
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 5/22
group (5.51±0.26D) and the H180 group (2.84±0.44D). As shown in Table 2, the four treat-
ment groups exhibited refractive astigmatisms of different axes (Watson-Williams F-test with
pairwise comparisons, all p<0.005, see Fig. 2A); the average axes for H45, H90, H135 and
Table 1. Inter-ocular differences (treated/right eye–fellow/left eye) in refractive parameters (mean±SE) for the controls and treatment groups.
Crossed-cylindrical Lens Control
Lens Power (H) +4.00DS/−8.00DC (L) +2.00DS/−4.00DC No Lens
Axis (°) 45
a
90
a,b
135
a
180
a,b
90
b
180
b
n2020202020188
M (D) −0.94±0.64 −0.43±0.24 −0.39±0.69 +0.06±0.35 +0.29±0.17 +0.74±0.29 −0.41±0.35
MMM (D) −3.18±0.61*−3.19±0.30*−2.53±0.66 −1.36±0.44 −1.76±0.22
#
+0.14±0.27
##
−0.44±0.36
MHM (D) +1.31±0.70 +1.03±0.23 +1.75±0.74 +1.47±0.39 +2.30±0.16 +1.34±0.38 −0.42±0.34
RA (D) 4.48±0.34*** 5.51±0.26*** 4.29±0.27*** 2.84±0.44*** 4.10±0.16
#
1.34±0.22 0.03±0.22
R-J0 (D) −1.29±0.23** −2.71±0.13*** −1.17±0.17*1.22±0.25** −2.02±0.07
##
0.52±0.17 −0.01±0.11
R-J45 (D) +1.47±0.21*** +0.23±0.47 −1.71±0.13*** −0.05±0.12 0.10±0.08 −0.03±0.02
#
0.01±0.01
M = spherical-equivalent; MMM = most myopic meridian; MHM = most hyperopic meridian; RA = refractive astigmatism; R-J0 and R-J45, the two vector
components of RA. In experiment A, the comparisons across the controls and treated groups were tested by one-way ANOVA followed by Tukey’s test. In
experiment B, the comparisons between high and low magnitudes of imposed astigmatism were tested by two-sample t-tests. The levels of significant
difference are indicated by asterisk: *p0.05, ** p0.01, *** p0.001 in experiment A, and
#
p0.05,
##
p0.01,
###
p0.001 in experiment B.
a
Experiment A
b
Experiment B
doi:10.1371/journal.pone.0117729.t001
Fig 2. Refractive astigmatism induced by optically imposed astigmatism of four different orientations. (A) Distributions of inter-ocular differences in
refractive astigmatism (treated/right eye—fellow/left eye) after one weekof cylindrical lens treatment (P5-P12) for the four treatment groups (+4.00DS/-8.00DC,
n = 20 in each group) with negative cylindrical axis oriented at one of the four directions (45, 90, 135, or 180), as well as the age-matched controls (n = 8).
The effects of the axis of cylindrical lens are represented by different coloured symbols as shown in the legend. For example, in chicks treated with H90,
the +4.00DC and -4.00DC were oriented vertically and horizontally respectively; to compensate for this astigmatic error, the eyes should develop negative
cylindrical axis at 90. As shown in A, the cylindrical lenses of different axes induced compensatory astigmatism in the four treatment groups. (B) The box plots
of refractive astigmatism include the values of median (line inside the box), maximum (upper whisker), minimum(lower whisker), upper (upper border of box)
and lower quartiles (lower border of box) for the controls and treatment groups at P12. The levels of significant differences in the magnitudes of refractive
astigmatism across the treatment groups (lines above the boxes), or between treatment and controls (lines below the boxes), are indicated by asterisk:
*p0.05, *** p0.001 (Tukey’s post hoc tests).
doi:10.1371/journal.pone.0117729.g002
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 6/22
H180 were, respectively, 68±7, 84±10, 119±12, and 174±44. Further analyses of the astigmatic
components showed that R-J0s were significantly different between the controls and all treat-
ment groups (one-way ANOVA with Tukey’s post hoc tests, all p<0.05), whereas R-J45s were
significantly different only between the controls and the H45 and H135 groups (one-way
ANOVA with Tukey’s post hoc tests, both p<0.001).
Refractive status −Effects of magnitude of astigmatism (Experiment B). In the two
groups treated with 90° cylindrical axis, both the spherical equivalent and most myopic meridi-
an (two-sample t-test, both p0.018), but not most hyperopic meridian (p = 0.95), were signifi-
cantly different between the H90 and L90 groups. Also, both the refractive astigmatism and
R-J0 (two-sample t-test, both p<0.001), but not R-J45 (p = 0.36), were significantly different
between the H90 and L90 groups. In the other two groups treated with 180° cylindrical axis,
only most myopic meridian, refractive astigmatism, and R-J0 were significantly different (two-
sample t-tests, all p0.025). One-way ANOVAs (all p0.009) followed by Tukey’s post hoc
tests (all p<0.001) showed that the H90, L90, and H180 groups developed significantly higher
astigmatic components (refractive astigmatism and R-J0) than the controls. On the other hand,
the axes of induced refractive astigmatism were not significantly different in both the H and L
groups: H90 vs. L90 = 84±10 vs. 88±4; H180 vs. L180 = 174±41 vs. 172±58 (Watson-Williams
F-test, both p0.39, see Table 2 and Fig. 3A).
Table 2. Inter-ocular differences (mean±SE) in spherical equivalent (M), most myopic meridian (MMM), most hyperopic meridian (MHM),
refractive (RA), corneal (CA), and internal astigmatisms (IA) for the control group and a subset of birds from the treatment groups (remark: n = 8
in each group) with both refractive and corneal measurements.
Crossed-cylindrical Lens Control
Lens Power (H) +4.00DS/−8.00DC (L) +2.00DS/−4.00DC No lens
Axis (°) 45
a
90
a,b
135
a
180
a,b
90
b
180
b
Axis (°)
M (D) -3.08±0.88 -0.78±0.40 -3.09±0.99*0.43±0.54 0.13±0.38 0.22±0.48 -0.42±0.34
MMM (D) -5.03±0.78*** -3.22±0.54*-4.95±1.01*** -0.74±0.61 -2.12±0.42 -0.06±0.40 -0.44±0.36
MHM (D) -1.13±1.01 1.66±0.35 -1.24±0.99 1.61±0.57 2.38±0.35 0.49±0.60 -0.42±0.35
RA (D) 3.91±0.33*** 4.87±0.43*** 3.71±0.31*** 2.35±0.51** 4.51±0.16
#
0.55±0.34 0.03±0.22
CA (D) 1.81±0.24** 2.27±0.22*** 1.46±0.12 1.15±0.21 2.00±0.34
#
0.74±0.14 0.60±0.18
IA (D) 2.82±0.25*** 3.05±0.37*** 3.18±0.35*** 1.50±0.29 3.48±0.18
#
1.01±0.18 0.72±0.16
R-J0 (D) −1.34±0.15*** −2.36±0.20*** −0.96±0.22 1.19±0.24** −2.21±0.07
#
0.52±0.12 −0.01±0.11
C-J0 (D) −0.52±0.18*−0.91±0.11*** 0.04±0.16 0.51±0.12 −0.59±0.05 0.20±0.12 0.09±0.06
I-J0 (D) −0.81±0.13*−1.18±0.26** −0.80±0.20 0.67±0.17*−1.63±0.07
##
0.41±0.07
#
−0.10±0.11
R-J45 (D) 1.39±0.20*** 0.42±0.21 −1.52±0.22*** −0.24±0.13 0.23±0.15
#
−0.07±0.05
#
0.01±0.01
C-J45 (D) 0.33±0.08*−0.08±0.17 −0.51±0.11 −0.40±0.14 −0.11±0.18 −0.20±0.10 −0.16±0.07
I-J45 (D) 1.06±0.17*0.55±0.24 −1.10±0.29*** 0.16±0.07 0.34±0.20 0.13±0.11 0.17±0.07
RA (°) 68±784±10 119±12*174±44*** 88±4 172±68 70±37
CA (°) 79±11** 91+6*125±20 152±22*** 96±10 117±39 109±21
IA (°) 61±10 82±21 17±15** 15±67*82±992±109 1.4±49
Note that the astigmatic axes in the last three rows are calculated by circular statistics (mean±angular deviation) for the treated eyes only. In experiment
A, the comparisons across the controls and treated groups were tested by one-way ANOVA followed by Tukey’s test. In experiment B, the comparisons
between high and low magnitudes of imposed astigmatism were tested by two-sample t-tests. The levels of significant difference are indicated by asterisk:
*p0.05, ** p0.01, *** p0.001 in experiment A, and
#
p0.05,
##
p0.01,
###
p0.001 in experiment B. Comparisons for the astigmatic axis were
performed by Watson-Williams F-tests followed by pairwise comparison tests.
a
Experiment A
b
Experiment B
doi:10.1371/journal.pone.0117729.t002
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 7/22
Corneal curvature −Effects of axis of astigmatism. Compared to controls (0.60±0.18D),
all treatment groups except the H180 group (p = 0.095) developed significantly higher corneal
astigmatisms (one-way ANOVA with Tukey’s post hoc tests, all p<0.05). The highest and low-
est magnitudes of corneal astigmatism among the treatment groups were found in the H90
(2.27±0.22D) and H180 groups (1.15±0.21D), respectively. The average axes of corneal astig-
matism for the H45, H90, H135 and H180 groups were 79±11, 91±6, 125±20, and 152±22, re-
spectively (see Table 2). The C-J0s of the H45 and H90 groups were significantly different from
the H135, H180 groups, and the controls (One-way ANOVA with Tukey’s post hoc tests, all
p0.037). However, C-J0s were neither significant different between the H45 and H90 groups
nor among the H135, H180 and the control groups (all p0.116). On the other hand, the ef-
fects of obliquely oriented cylindrical axes were found between the H45 and H135 groups: the
C-J45s were significantly different between the H45 and H135 groups, and between the H45
and H180 groups (one-way ANOVA with Tukey’s post hoc tests, both p0.001).
Significant effects of cylindrical axis on corneal curvatures were found in the treated/right
eyes (one-way ANOVA, all p0.006) but not in the fellow/left eyes (one-way ANOVA, all
p0.241). Fig. 4 compares the steepest (top symbols) and flattest corneal curvatures (bottom
symbols) of the treated/right eyes across the control and treatment groups. As shown, both SK
and FK were much steeper in the H90 than other groups. Significant differences in SK were
found between multiple treatment groups (H90 vs. controls, H90 vs. H135, H90 vs. H180, and
H45 vs. H180; Tukey’s post hoc tests, all p0.05), whereas significant differences in FK were
found only between H90 and the other two treatment groups (H90 vs. H135; H90 vs. H180;
Tukey’s post hoc tests, both p0.05).
Corneal curvature −Effects of magnitude of astigmatism. Significant difference in C-J0
was found only between the H90 and L90 groups (two-sample t-test, p = 0.021). No significant
Fig 3. Refractive astigmatism induced by optically imposed astigmatism of two different magnitudes. (A) Distributions of inter-ocular differences in
refractive astigmatism for the four treatment groups with cylindrical lenses of two magnitudes ([H]:+4.00DS/-8.00DC and [L]:+2.00DS/-4.00DC) and two axis
orientations (H90, H180, and L90; n = 20 in each group; L180, n = 18). See caption for Fig. 2 and text for details. (B) The box plots of refractive astigmatism
include the values of median, maximum, minimum, upper and lower quartiles for each group (see Fig. 2 for details). The levels of significant difference in the
magnitudes of refractive astigmatism across the treatment groups (lines above the boxes, two-sample t-tests), or between treatment and control group (lines
below the boxes, Tukey’s post hoc tests) are indicated by asterisk: *p0.05, *** p0.001.
doi:10.1371/journal.pone.0117729.g003
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 8/22
differences in corneal astigmatism and C-J45 were found between the H and L groups of the
same axis orientations (two-sample t-tests, all p0.055).
Eyeshape profile −Axial length and equatorial diameter. In general, the cylindrical-lens-wear
produced an overall abnormal eyeshape. In the control group, no significant difference was
found in AL (RE = 9.11mm, LE = 9.07mm), ED180 (RE = 11.99mm, LE = 11.92mm) or ED90
(RE = 12.03mm, LE = 12.08mm) between the left (LE) and right (RE) eyes (paired t-tests, all
p0.191). In the treated groups, the ocular dimensions of treated eyes were significantly longer
and larger than those of their untreated fellow eyes (n = 48, treated vs. fellow, AL: 9.29±0.04mm
vs. 9.08±0.03mm; ED180: 12.26±0.04mm vs. 11.93±0.04mm; ED90: 12.22±0.05mm vs.
11.99±0.05mm, paired t-test, all p<0.001).
Eyeshape profile −Effects of axis of astigmatism. As shown in Table 3, the inter-ocular
difference in AL of the H135 group (one-way ANOVA followed by Tukey’s post hoc test,
Fig 4. Changes in corneal curvatures in response to optically imposed astigmatism. Comparisons of the steepest (top symbols) and the flattest corneal
curvatures (bottom symbols) across the controls and treatment groups at P12 (treated/right eyes data only). The levels of statistical significant difference
across the treatment groups (Tukey’s post hoc tests), are indicated by asterisk: *p0.05, ** p0.01, ***p0.001. Although both SK and FK of L90 were not
statistically different from others, they showed similar trends as those in H90.
doi:10.1371/journal.pone.0117729.g004
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 9/22
p<0.05), as well as the inter-ocular differences in ED90 of all treatment groups (one-way
ANOVA followed by Tukey’s post hoc tests, all p<0.05) were significantly larger than those of
the controls. Furthermore, inter-ocular differences in ED180 of all except the H90 treatment
groups were significantly larger than the controls.
In Fig. 5, the differences in ocular dimensions (i.e., treated/right eyes—fellow/left eyes, from
0° to 50° eccentricities in 5° intervals) towards the four peripheral regions along the vertical
and horizontal meridians are compared. We found that only the nasal regions (50° eccentrici-
ties) were significant different between the controls and all treatment groups (one-way
ANOVA with Tukey’s post hoc tests, all p0.008). To determine the effects of cylindrical
lenses on posterior ocular asymmetry, we calculated the differences in area between the fellow
eyes up to 50° eccentricity for the horizontal (ADH) and vertical meridians (ADV) were calcu-
lated. While the differences between the two meridians (ADH-ADV, Table 3) were analysed,
we found that the H180 showed significantly larger magnitudes than the H90 and control
groups (one-way ANOVA followed by Tukey’s post hoc tests, both p0.011). When the total
changes in ocular dimensions were compared (ADH+ADV, Table 3), we found that both H45
and H135 groups had significantly larger eye sizes than the controls (one-way ANOVA with
Tukey’s post hoc tests, both p0.041). In short, the H180 group showed meridional difference
in posterior globe without an alteration in eye size; both H45 and H135 groups had larger than
Table 3. Inter-ocular differences in ocular dimensions (mean±SE) related to the eye shape profile for the control and treatment groups (n = 8 in
each group).
Crossed-cylindrical Lens Control
Lens Power (H) +4.00DS/-8.00DC (L) +2.00DS/−4.00DC No lens
Axis (°) 45
a
90
a,b
135
a
180
a,b
90
b
180
b
Axis (°)
Axial / Equatorial Dimensions
AL (mm) 0.28±0.08 0.18±0.03 0.31±0.08*0.21±0.05 0.16±.0.03 0.16±0.04 0.05±0.02
ED180 (mm) 0.37±.0.04** 0.25±0.05 0.34±0.06** 0.43±0.05*** 0.30±0.07 0.29±0.06 0.08±0.06
ED90 (mm) 0.26±0.03** 0.30±0.06*** 0.22±0.08*0.31±0.03*** 0.23±0.08 0.07±0.06 −0.06±0.08
Posterior Ocular Dimensions
ADH-ADV (unit area) 0.10±0.70 −0.89±0.48 −0.32±0.47 2.35±0.61*0.42±0.68 1.86±0.73 −1.02±1.01
ADH+ADV (unit area) 12.77±2.37*7.98±1.36 12.40±3.12*10.54±1.62 9.26±1.24 8.09±1.34 3.33±1.93
Regional Differences
Nasal (unit area) 4.78±0.58*** 3.21±0.38** 5.19±0.84*** 3.93±0.45*** 3.98±0.25 3.96±0.40 0.44±0.34
Temporal (unit area) 1.65±0.73 0.33±0.42 0.85±0.88 2.52±0.50 0.87±0.31 1.02±0.30 0.72±0.90
Superior (unit area) 3.04±0.46 1.92±0.48 3.27±0.67 1.40±0.55 2.27±0.48 1.93±0.72 1.28±0.73
Inferior (unit area) 3.30±0.86 2.52±0.41 3.09±0.83 2.69±0.52 2.51±0.47 1.18±0.47 0.89±0.68
FK (D) −0.53±1.17 0.80±2.01 −0.85±0.63 −1.24±1.21 0.80±0.80 −0.18±0.97 −0.66±0.53
SK (D) 0.61±1.20 2.68±2.08*−0.15±0.56 −0.65±1.05 2.03±0.87 −0.08±1.00 −0.81±0.50
MK (D) 0.04±1.17 1.74±2.04 −0.50±0.59 −0.94±1.13 1.42±0.83 −0.13±0.96 −0.74±0.51
AL = axial length; ED180 & ED90, horizontal and vertical equatorial diameters, respectively; ADH & ADV, difference in area between the two eyes up to
50° eccentricity along the horizontal and vertical meridians, respectively. In experiment A, the comparisons across the controls and treated groups were
tested by one-way ANOVA followed by Tukey’s test. In experiment B, the comparisons between high and low magnitudes of imposed astigmatism were
tested by two-sample t-tests. The levels of significant difference are indicated by asterisk: *p0.05, ** p0.01, *** p0.001 in experiment A, and
#
p0.05,
##
p0.01,
###
p0.001 in experiment B.
a
Experiment A
b
Experiment B
doi:10.1371/journal.pone.0117729.t003
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 10 / 22
normal eye sizes but no differences in ocular expansion between these two meridians; whereas
H90 group did not show any difference from the controls in these two parameters.
Eyeshape profile −Effects of magnitude of astigmatism. Significant difference in the
ED90 was found only between the H180 and L180 groups (two-sample t-test, p = 0.002,
Table 3), no significant difference in AL or ED180 was found between the H180 and L180 or
between the H90 and L90 groups (two-sample t-tests, all p0.082).
When the differences in ocular dimensions (up to 50° eccentricity) at the four quadrants
were analysed, the H180 group was larger than the L180 group in both the temporal and
Fig 5. Regional changes in ocular dimensions for control and treatment groups. The regional differences in areas (treated/right eye–fellow/left eye, 5°
intervals) were measured across different eccentricities along vertical (superior-inferior) (A) and horizontal (nasal-temporal) meridians (B). Note that the
values at 0° showed the differences in axial length, not area. The levels of significant differences in area at eccentricity 50° between the treatment and control
groups are indicated by asterisk: ** p<0.01 (Tukey’s post hoc tests).
doi:10.1371/journal.pone.0117729.g005
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 11 / 22
inferior regions (two-sample t-tests, both p<0.05, Fig. 5A and B); but no significant differences
were found in all four regions between the H90 and L90 groups (two-sample t-tests, all
p0.115). As shown in Fig. 6A and B, there were also no significant differences in ADH-ADV,
and ADH+ADV between H180 and L180 or between H90 and L90 (two-sample t-tests, all
p0.137). However, one-way ANOVA combined with Tukey’s tests revealed the (ADH-ADV)
of the H180 group was larger than that of both the H90 and control groups (both p0.026).
Correlation analyses −Refractive, corneal, and internal astigmatisms. Data from the sub-
set of birds with both refractions and corneal topography measurements were pooled for corre-
lation analyses (n = 112, both eyes from treated and control groups). Moderate to high
correlations were found between the refractive and corneal astigmatic components (Pearson’s
correlation r = 0.78, 0.84 and 0.61 for astigmatism, J0 and J45 components respectively, all
p<0.001; Fig. 7A-C), as well as between the refractive and internal astigmatic components
(Pearson’s r = 0.94, 0.94 and 0.90 for astigmatism, J0 and J45 components respectively, all
p<0.001; Fig. 7D-F).
Correlation analyses −Spherical components and eyeshape parameters.Table 4 shows
significant correlations found between the refractive and eyeshape parameters. The most
hyperopic meridian, most myopic meridian, and spherical equivalent were significantly corre-
lated (all p<0.001) with AL (Pearson’s r = -0.61, -0.47 and -0.57, respectively) and ADH+ADV
(r = -0.40 to -0.52), but not with ED180, ED90, or ADH-ADV (all p0.22). All the three spher-
ical components were significantly correlated (Pearson’s correlations, all p0.02) with the dif-
ferences in area at superior (r = -0.36 to -0.43), inferior (r = -0.32 to -0.51), and nasal regions
(r = -0.34 to -0.49); but only the most hyperopic meridian and spherical equivalent were signif-
icantly correlated (both p = 0.03) with those at the temporal region (r = -0.30 and -0.29).
Correlation analyses −Astigmatic components and eyeshape parameters. While refrac-
tive, corneal, and internal astigmatism were all significantly correlated with ED90 (Pearson’s
Fig 6. The area differences and summations. ADH and ADV indicate the area differences (treated/right eye–fellow/left eye) along vertical and horizontal
meridians from 0° to 50° eccentricities. The difference (ADH−ADV) and the summation (ADH+ADV) of these parameters (mean±SE) are plotted in (A) and
(B), respectively. The levels of statistical significant difference across the treatment groups (Tukey’s post hoc tests), are indicated by asterisk: *p<0.05.
doi:10.1371/journal.pone.0117729.g006
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 12 / 22
r = 0.42, 0.35 and 0.32, respectively; all p0.02), only refractive and internal astigmatism were
significantly correlated (all p<0.05) with ED180 (Pearson’s r = 0.28 and 0.26). In addition,
both refractive and internal astigmatism were significantly correlated with the differences in
area at the superior, inferior and nasal regions (Pearson’s r = 0.27 to 0.45, all p0.04), as well
as ADH+ADV (Pearson’s r = 0.31 and 0.34, both p0.05). Interestingly, all the J0 components
were significantly correlated with ADH-ADV (Pearson’s correlations: R-J0: r = 0.36; C-J0:
r = 0.35; I-J0: r = 0.30; all p0.02). Furthermore, the internal astigmatism was correlated with
AL (Pearson’s r = 0.31, p = 0.02), and I-J0 was correlated with the differences in area at the
superior and inferior regions (Pearson’s correlations, both r = -0.27, both p = 0.04). On the
other hand, although C-J0 was also correlated with the difference in area at the temporal side
(Pearson’s correlation, r = 0.36, p = 0.01), corneal astigmatism was not correlated with any
other parameters (all p0.549).
Fig 7. Correlations between corneal, internal and refractive astigmatisms. Data from both eyes of all treatment and control eyes (n = 112) were included
in the correlation analyses for refractive and corneal astigmatic components (top panel, A-C), and for refractive and internal astigmatic components (bottom
panel, D-F). Internal astigmatism is derived by calculation.
doi:10.1371/journal.pone.0117729.g007
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 13 / 22
Discussion
The main findings in this study were: 1) the chick eyes developed astigmatism after wearing
crossed-cylindrical lens for a week; 2) the characteristics of the resultant astigmatism in the
treated birds were influenced by the orientation and magnitude of imposed astigmatism; 3) the
characteristics of the induced astigmatism were correlated with multiple eyeshape parameters.
Cylindrical lens wear also produced significant impacts on the corneal shape. The magni-
tudes of induced corneal astigmatism across the treatment groups varied in a similar fashion as
those of the refractive astigmatism (Table 3). In the treated eyes, both the steepest and flattest
corneal curvatures in the H90 group were significantly steeper than those of the H180 group
(Fig. 4). Furthermore, relative to their fellow untreated eyes, imposing WTR astigmatism (H90
and L90) resulted in steeper corneal curvatures whereas imposing ATR astigmatism (H180 and
L180) produced flatter corneal curvatures along both principal meridians (Fig. 4 and Table 3).
As shown in Fig. 4, imposing different astigmatic axes of high magnitudes of astigmatism ap-
peared to have a more dramatic effect on the steepest meridian: whereas significant differences
in the flattest meridians were found only in three treatment groups, significant differences in
the steepest meridians involved all treatment groups. It should be borne in mind that the resul-
tant steepest meridians were oriented at different directions across the treatment groups (e.g.,
horizontal meridian for H90 and vertical meridian for H180), the differential magnitudes and
orientations of induced astigmatism across the treatment groups suggest that the induced ocu-
lar toricity may be related to the structural anisotropy occurred regionally and/or across differ-
ent meridians. Thus, the results in young chicks showed that vision-dependent processes are
capable of altering corneal shape for compensation of imposed astigmatism. Several elucida-
tions related to the ocular structures are worthy of consideration. Firstly, the corneal collagen
fibrils are running in parallel to one another and oriented at orthogonal position to adjacent
layers with the corneal base directly connected to the ciliary muscles [79,80]. Secondly, the an-
terior segment is asymmetric at the horizontal plane, with the greatest temporal distance be-
tween equator and limbus [81]. The intermediate ciliary muscle is suggested as the depressor
Table 4. Pearson’s correlation analyses between refractive, corneal and eyeshape parameters (n = 56).
(D) AL
(mm)
EDmean
(mm)
ED180
(mm)
ED90
(mm)
ADH
(unit area)
ADV
(unit area)
ADH-ADV
(unit area)
ADH+ ADV
(unit area)
S
(unit area)
I
(unit area)
N
(unit area)
T (unit
area)
MMM −0.61*** −0.29*-- −0.44*** −0.53*** -−0.52*** −0.43*** −0.51*** −0.49*** -
MHM −0.47*** ---−0.38*** −0.38*** -−0.40*** −0.36** −0.32*−0.34** −0.30*
M−0.57*** ---−0.39*** −0.46*** -−0.45*** −0.37*** −0.45*** −0.36** −0.29*
RA - 0.43*** 0.28*0.42*** - 0.33** - 0.31*0.27*0.31*0.45*** -
CA - 0.31*- 0.35** ---- ----
IA 0.31*0.33** 0.26*0.32*0.27*0.36** - 0.34** 0.35** 0.30*0.45*** -
R-J0 - - - - - - 0.36** - ----
C-J0 - - - - - - 0.35** - - - - 0.36**
I-J0 - - - - - −0.30*0.30*-−0.27*−0.27*--
M = spherical-equivalent; MMM = most myopic meridian; MHM = most hyperopic meridian; RA = refractive astigmatism; R-J0 and R-J45, the two vector
components of RA; AL = axial length; EDmean = average of ED180 and ED90; ED180 & ED90, horizontal and vertical equatorial diameters, respectively;
ADH & ADV, difference in area between the two eyes up to 50° eccentricity along the horizontal and vertical meridians, respectively; T, N, I and S =
difference in area between the two eyes up to 50° eccentricity at temporal, nasal, inferior and superior regions, respectively. The levels of significant
difference between treatment and control groups are indicated by asterisk: *p0.05, ** p0.01, *** p0.001.
doi:10.1371/journal.pone.0117729.t004
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 14 / 22
corneae, with its greatest effect occurring temporally [82]. By contrast, the shortest ciliary mus-
cle fibers, absence of intermediate ciliary muscle and poorly developed scleral venous sinus are
found in the nasal quadrant. Thirdly, the overlapping patterns of the scleral ossicles (Gallus gal-
lus: 14 ossicles, type D (1,9;6,10) pattern [83]) allow the chick cornea to alter its toricity during
accommodation [84,85]. The ossicle numbers 1 (inferior) and number 9 (superior), “+”ele-
ments (on top of the others), are located nearly at the vertical meridian (i.e., the axis meridian);
while the ossicle numbers 6 and 10, “-”elements (under the others), provide a buffer for the
movement of the axis meridian. In combination with our previous findings of corneal accom-
modations [70], we speculated that the cornea could respond to imposed astigmatism, for ex-
ample, the H90 group by contraction of the ciliary muscles (i.e., positive corneal
accommodation), enhanced by the specific pattern of ossicles, to create an against-the-rule cor-
neal profile (ossicles number 1&9 move forward, while number 6&10 move backward) for as-
tigmatic compensation. Therefore, the cornea showed comparatively steeper curvatures in
both principal meridians at the end of treatment, especially at the horizontal meridian due to
ciliary asymmetry. As shown in Fig. 4and 8, no significant differences in the equatorial diame-
ters and posterior globe between the H90, L90 groups and the controls were found. Perhaps,
the scleral ossicles might have sufficient flexibility to counteract the positive accommodation.
The reverse is the case; the cornea compensates for the imposed ATR astigmatism, the corneal
curvatures are relatively flatter as a result of negative corneal accommodation. However, the
magnitude of negative corneal accommodation is smaller than that of positive corneal accom-
modation [70] that may be due to the limitation of the ossicles, and/or the cornea structure. Ex-
cessive negative accommodation might stretch the cornea and sclera which become flatter,
along the horizontal meridian in particular. As a consequence, the magnitudes of induced
astigmatism in both the H180 and L180 groups were significantly lower than the others. The
corneal fibril arrangement, ciliary muscle asymmetry and the ossicle pattern provide flexibility
for cornea to alter its profile, but also limitation for astigmatism compensation at the same
time. Therefore full compensation was found in the L90 group, but not found in the H90 group
even the magnitude of imposed astigmatism was doubled.
The refractive astigmatism found in this study was correlated moderately with the corneal
astigmatism (r = 0.61 to 0.84; Fig. 7A-C) and strongly with the internal astigmatism (r = 0.90
to 0.94; Fig. 7D-F). In terms of the magnitude of refractive astigmatism, corneal astigmatism
contributed to about 40% (30% to 52%) across the treatment group (Table 2). However, when
the two astigmatic components (J0 and J45) were considered, the components of internal astig-
matism contributed a larger proportion about 60% (51% to 70%) to the refractive astigmatism
than those of corneal astigmatism in most of the treatment groups (Table 2). Our results
showed that the internal astigmatism not only contributed to the induced astigmatism but also
correlated with multiple eyeshape parameters including the axial length, equatorial diameters,
meridional (ADH and ADV) and regional changes in ocular dimensions (see Table 4). More
importantly, similar to R-J0 and C-J0, the I-J0 was also correlated with the ADH-ADV. These
results suggest that the differential changes at the posterior eye segment might have altered the
normal balance of internal refractive components across different meridians and contributed
to ocular toricity. Although no correlations between refractive errors and thickness of retina,
choroid or sclera in chicks have been reported [86], recent clinical findings suggested that in-
ternal astigmatism may correlate with retinal topography [87]. Also, several studies [86,88,89]
found increase in choroidal thickness in defocus-induced hyperopic chick eyes, whereas only
equatorial choroidal thickening was found in those treated with plus-cylindrical lens (plano/
+10.00DS) [48]. Such regional choroidal expansion was also demonstrated in the partial form-
deprived eyes followed by unrestricted vision [90]. How the retinal topography change with
imposed astigmatism in chick is still open to question. It should be noted that the internal
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 15 / 22
astigmatism in our study was derived from calculation; neither the curvature nor the dimen-
sion of internal ocular components was measured. Thus, it remains unclear what were the
structural correlates for these internal toricities. In this respect, it has been suggested that the
internal astigmatism could be due to posterior corneal astigmatism [91], the variation of refrac-
tive index in crystalline lens [92], the tilting and/or decentration of the crystalline lens with re-
spect to the visual axis [93]. In addition, since the crystalline lens is located inside the eyeball
[18,94], the nasal-temporal asymmetric ocular expansions as observed in this study (see Fig. 5)
might have influenced the on-axis refractive status. The reasons for the eyes treated with im-
posed astigmatism of different magnitudes and axes resulted in the nasal ocular expansion re-
mains unclear. One possibility is that the optic nerve head and pecten at the inferior-temporal
quadrant of the posterior globe may restrict potential eye growth at the temporal region [95].
Another possibility is the higher ganglion cell density on the nasal retina where might be more
sensitive to visual manipulation than that on the temporal side [96,97]. Future works using the
latest imaging technology may help to determine the origins of the internal astigmatism.
Unlike previous studies which showed a slight myopic shift (minus-cylindrical lenses, in
chicks [45,49,51–53]) and hyperopic shift (plus-cylindrical lenses, in chicks [48,49,53]; or
crossed-cylindrical lenses, in chicks [54] and monkeys [55,57]), our chicks did not show a sig-
nificant shift in spherical-equivalent refractive error (Table 1). Instead, in addition to the in-
duced astigmatism, we found that the imposed astigmatism altered the eyeshape parameters
and multiple eyeshape parameters were correlated with both spherical (M, MHM, and MMM)
and astigmatic components (Table 4). However, the eyeshape parameters that were correlated
with spherical components do not necessarily also correlated with astigmatic components and
vice versa. First, whereas all spherical components (i.e., M, MHM, and MMM) were negatively
correlated with axial length, only internal astigmatism was positively correlated with axial
length. Second, whereas the spherical components only correlated with the ADH+ADV, the
three J0 components only correlated with ADH-ADV. Third, whereas all spherical compo-
nents were correlated with ADV and ADH, only RA and I-J0 were correlated with ADV or
ADH. Fourth, whereas most myopic meridian was negatively correlated with the average equa-
torial diameter, the refractive, corneal and internal astigmatisms were positively correlated
with nearly all equatorial dimensions. Thus, the different eyeshape parametric changes associ-
ated with spherical and astigmatic components as observed in this study cautious the use of
conventional measure such as ocular axial length when characterizing the impacts of changes
on the posterior eye segment in the development of astigmatism.
Another interesting finding from this study is the differential effects of imposing WTR and
ATR astigmatism on eyeshape parameters. As summarized in Fig. 8, imposing WTR astigma-
tism (H90 and L90) produced significantly steeper horizontal corneal curvature than vertical
curvature; imposing ATR astigmatism (H180 and L180) produced flatter horizontal corneal
curvature (H180 and L180, only H180 reached statistical significance). In contrast, imposing
ATR astigmatism produced significantly greater posterior ocular expansions in the horizontal
than the vertical meridian, but this effect was not observed in the groups treated with WTR
astigmatism. The horizontal equatorial diameter was also significantly larger in one of the
ATR-treated groups (L180) but not in any of the two WTR-treated groups. Thus, optically im-
posed WTR and ATR astigmatisms appear to have stronger effects on, respectively, the anterior
and posterior eye shapes. However, even with these contrasting effects on the different seg-
ments of the eyeball, only the most ametropic meridian in H45 and H90 was significantly dif-
ferent from controls, the spherical equivalent refractive error was not significantly different
between the controls and any of the treatment groups (Table 1). These results suggest that, at
least within the range that we tested, the ocular parametric changes in response to astigmatic
error cues may be quite specific regionally and probably independent from those observed
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 16 / 22
under form deprivation and spherical defocus ([41,98]). It is possible that these differential ef-
fects of specific astigmatic cues on the individual ocular dimensions had rendered the relatively
lower correlations between the refractive changes and axial length (Table 4). As postulated in
previous studies, the mechanisms controlling the growths of anterior chamber and vitreous
chamber, [99,100], as well as those regulating the growths of equatorial diameter and axial
length, [101] could be independent from each other.
After one week of cylindrical lens treatment, virtually all treated eyes developed astigmatism
and the amount of induced astigmatism varied dependent on the axis orientation and magni-
tude of astigmatism imposed by the lenses (Fig. 2and 3). The highest and lowest magnitudes of
induced astigmatism were found, respectively, in the treatment groups that experienced a week
of WTR (H90: 5.51±0.26D) and ATR astigmatisms (L180: 1.34±0.22D). Only the L90 group
developed a magnitude (4.10±0.16D) and an axis (88±4) that appeared to compensate fully for
the 4.00DC imposed astigmatism (Table 1). In contrast to our findings, several earlier studies
using higher magnitudes of optically imposed astigmatism (10.00DC to 16.00DC, usually
plano-cylindrical lenses) did not show clear compensatory astigmatic changes in chicks
(Schmid & Wildsoet, 1997; Laskowski & Howland, 1996; Phillips and Collins, 2000; Thibos,
2001). However, in the pioneer study [48] that employed similar paradigm (P0 or P2 birds
worn plano/+10.00DC or plano/-9.00DC for 7 days) as ours, partial compensations in refrac-
tive astigmatism were found when the plano/+10.00DC lenses were oriented at 135 (3.75±0.63D,
the highest) or 45 axes (1.00±0.38D, the lowest). One possibility for this discrepancy across
studies is that the visual signals and its effects imposed by the high-powered cylindrical lenses
(i.e., 10.00DC) might have approached the operating limits of the sensory mechanism and/or
the structural correlates. It is worth noting that we used relatively lower magnitudes of cylindri-
cal lenses (8.00DC and 4.00DC) and each principal powered meridian only imposed either
4.00D or 2.00D of defocus. Even with these lower powers of cylindrical lenses, the chicks only
compensated partially in most of our treatment groups. Many biometric parameters were not
significantly different between the H and L groups suggest that even the H lenses might have
approached the limits of the operating mechanisms. Another possibility is the starting age in
different experiments. Since hatchling chicks typically exhibit significant amounts of natural
astigmatism [49], wearing cylindrical lenses immediately after hatching might have confounded
Fig 8. Effects of imposing WTR and ATR astigmatisms on eyeshape parameters. Comparisons of the effects of imposing WTR (H90& L90) and
ATR (H180 & L180) astigmatism on the horizontal and vertical meridians of corneal radius of curvature, equatorial diameter, and posterior ocular expansion.
As illustrated in the schematic diagram, the comparisons were made for corneal curvatures of horizontal vs. vertical meridians; equatorial diameters
of the horizontal (ED180) vs. vertical directions (ED90); and the difference in area up to 50° eccentricity of the horizontal(ADH) vs. vertical meridians
(ADV). The table on the right summarizes the results of comparisons, the “+”and “−”signs indicate a significantly higher (horizontal>vertical) and lower
(horizontal<vertical) values respectively, the “ns”represents no significant difference. The levels of significant difference, using paired t-test, are indicated by
asterisk: *p0.05, **p0.01, *** p0.001.
doi:10.1371/journal.pone.0117729.g008
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 17 / 22
the visual error signals used for regulating refractive development. Furthermore, age-depen-
dent anatomical changes were also noted in normal post-hatched chicks with respects to
corneal flattening [72,73] and the orientation of collagen circumscribing the central cornea
[102]. Thus, the differences in experimental methodology and paradigm may have been the
possible reasons for the discrepancy reported in these studies. Regardless, the current study,
which included a large number of animals and biometric measures from anterior to posterior
ocular segment, demonstrates that chicks are capable of compensating for astigmatic error
signals and the regulatory mechanism is sensitive to the axis orientation and magnitude of
imposed astigmatism.
In conclusion, the current study extends our understanding of astigmatic eye growth in
chick and provides new insights into the effects of optically imposed astigmatism on corneal
and eyeshape parameters.
Supporting Information
S1 Dataset. Raw data from individual birds are presented in four excel worksheets: 1). Re-
fraction, both spherical and cylindrical refractive data of both eyes at P5 and P12 of treatment
and control groups are included; 2). All, including data of refraction, corneal topography and
eyeshape parameters (axial length and equatorial diameters) of both eyes at P5 and P12 (n = 8
per group); 3 & 4). RE and LE, ocular length at different eccentricities (from -60° to +60°)
along horizontal and vertical meridians of right eye and left eye, respectively.
(XLS)
Acknowledgments
We would like to express our sincere thanks to Drs. Jeremy Guggenheim, David Troilo, and
Frank Schaeffel for their many constructive comments and helpful suggestions for an earlier
version of this manuscript.
Author Contributions
Conceived and designed the experiments: CHGC CSK. Performed the experiments: CHGC.
Analyzed the data: CHGC. Contributed reagents/materials/analysis tools: CSK. Wrote the
paper: CHGC CSK. Designed the hardware of VKS: CHGC CSK. Designed the hardware of
Eyeshape System: CHGC.
References
1. Read SA, Collins MJ, Carney LG (2007) A review of astigmatism and its possible genesis. Clinical
and Experimental Optometry 90. PMID: 17958574
2. Young TL, Metlapally R, Shay AE (2007) Complex trait genetics of refractiveerror. Arch Ophthalmol
125: 38–48. PMID: 17210850
3. Cox MJ (2010) Astigmatism. In: Darlene AD, editor. Encyclopedia of the Eye. Oxford: Academic
Press. pp. 135–145.
4. Kee CS (2013) Astigmatism and its role in emmetropization. Exp Eye Res 114: 89–95. doi: 10.1016/j.
exer.2013.04.020 PMID: 23644224
5. Dobson V, Miller JM, Clifford-Donaldson CE, Harvey EM (2008) Association between anisometropia,
amblyopia, and reduced stereoacuity in a school-aged population with a high prevalence of astigma-
tism. Invest Ophthalmol Vis Sci 49.10: 4427–4436. doi: 10.1167/iovs.08-1985 PMID: 18539935
6. Gwiazda JE, Mohindra I, Brill S, Held R (1985) Infant astigmatism and meridional amblyopia. Vision
Res 25: 1269–1276. PMID: 4072007
7. Abrahamsson M, Sjöstrand J (2003) Astigmatic axis and amblyopia in childhood. Acta Ophthalmol
Scand 81.1: 33–37. PMID: 12631016
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 18 / 22
8. Mohindra I, Held R, Gwiazda JE, Brill S (1978) Astigmatism in infants. Science 202: 329–331. PMID:
694539
9. Fulton AB, Hansen RM, Petersen RA (1982) The relation of myopia and astigmatism in developing
eyes. Ophthalmology 89: 298–302. PMID: 7099549
10. Ehrlich DL, Atkinson J, Braddick O, Bobier WR, Durden K (1995) Reduction of infant myopia: A longi-
tudinal cycloplegic study. Vision Res 35: 1313–1324. PMID: 7610592
11. Kaye SB, Patterson A (1997) Association between total astigmatism and myopia. J Cataract Refract
Surg. 23: 1496–1502. PMID: 9456407
12. Howland HC, Atkinson J, Braddick O, French J (1978) Infant astigmatism measured by photorefrac-
tion. Science 202: 331–333. PMID: 694540
13. Harvey EM, Dobson V, Miller JM, Schwiegerling J, Clifford-Donaldson CE, et al. (2011) Prevalence of
corneal astigmatism in Tohono O'odham Native American children 6 months to 8 years of age. Invest
Ophthalmol Vis Sci 52: 4350–4355. doi: 10.1167/iovs.10-6061 PMID: 21460261
14. Harvey EM, Dobson V, Miller JM (2006) Prevalence of high astigmatism, eyeglass wear, and poor vi-
sual acuity among Native American grade school children. Optom Vis Sci 83: 206–212. PMID:
16614575
15. Miller JM, Dobson VM, Harvey EM, Sherrill DL (2000) Astigmatism and amblyopia among Native
American children (AANAC): Design and methods. Ophthalmic Epidemiol 7: 187–207. PMID:
11035554
16. Dobson V, Miller JM, Harvey EM, Mohan KM (2003) Amblyopia in astigmatic preschool children. Vi-
sion Res, 43: 1081–1090. PMID: 12676249
17. Lansche RK (1966) Asthenopia caused by "against-the-rule" astigmatism. Headache: The Journal of
Head and Face Pain 6: 147–151.
18. Jonas JB, Kling F, Gründler AE (1997) Optic disc shape, corneal astigmatism, and amblyopia. Oph-
thalmology. 104: 1934–1937. PMID: 9373129
19. Vongphanit J, Mitchell P, Wang JJ (2002) Population prevalence of tilted optic disks and the relation-
ship of this sign to refractive error. American Journal of Ophthalmology 133: 679–685. PMID:
11992866
20. Bozkurt B, Irkec M, Gedik S, Orhan M, Erdener U (2002) Topographical analysis of corneal astigma-
tism in patients with tilted-disc syndrome. Cornea 21: 458–462. PMID: 12072719
21. Flitcroft DI, Adams GGW, Robson AG, Holder GE (2005) Retinal dysfunction and refractive errors: An
electrophysiological study of children. British Journal of Ophthalmology 89: 484–488. PMID:
15774929
22. Hammond CJ, Snieder H, Gilbert CE, Spector TD (2001) Genes and environment in refractive error:
The twin eye study. Invest Ophthalmol Vis Sci 42: 1232–1236. PMID: 11328732
23. Clementi M, Angi M, Forabosco P, Di Gianantonio E, Tenconi R (1998) Inheritance of astigmatism:
Evidence for a major autosomal dominant locus. The American Journal of Human Genetics 63: 825–
830. PMID: 9718344
24. Abraham JE, Volovick JB (1972) Preliminary navajo optometric study. J Am Optom Assoc 43: 1257–
1260.
25. Cowen L, Bobier WR (2003) The pattern of astigmatism in a Canadian preschool population. Invest
Ophthalmol Vis Sci 44: 4593–4600. PMID: 14507908
26. Mandalos AT, Peios DK, Mavracanas A, Golias VA, Megalou KG, et al. (2002) Prevalence of astigma-
tism among students in northern Greece. Eur J Ophthalmol 12: 1–4. PMID: 11936436
27. Shih YF, Hsiao C, Tung YL, Lin LK, Chen CJ, et al. (2004) The prevalence of astigmatism in Taiwan
schoolchildren. Optometry & Vision Science 81: 94–98. doi: 10.1111/opo.12195 PMID: 25604956
28. Montés-Micó R (2000) Astigmatism in infancy and childhood. J Pediatr Ophthalmol Strabismus 37:
349–353. PMID: 11392409
29. Fan SP, Rao SK, Cheung YY, Islam M, Chew S, et al. (2004) Astigmatism in Chinese preschool chil-
dren: Prevalence, change, and effect on refractive development. Br J Ophthalmol 88: 938–941.
PMID: 15205242
30. Lyle WM, Grosvenor T, Dean KC (1972) Corneal astigmatism in Amerind children. Am J Optom Arch
Am Acad Optom 49: 517–524. PMID: 4504449
31. Saunders H (1981) Age‐Dependence of Human refractive errors. Ophthalmic and Physiological Op-
tics 1: 159–174. PMID: 7342008
32. Fledelius HC, Stubgaard M (1986) Changes in refraction and corneal curvature during growth and
adult life. A cross-sectional study. Acta Ophthalmol (Copenh) 64: 487–91. PMID: 3492854
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 19 / 22
33. Dobson V, Harvey EM, Miller JM (2007) Spherical equivalent refractive error in preschool children
from a population with a high prevalence of astigmatism. Optom Vis Sci 84: 124–130. PMID:
17299342
34. Goss DA (1999) Refractive error changes in mixed astigmatism. Ophthalmic and Physiological Optics
19: 438–440. PMID: 10768025
35. Wallman J, Winawer J (2004) Homeostasis of eye growth and the question of myopia. Neuron, 43:
447–468. PMID: 15312645
36. Benavente-Perez A, Nour A, Troilo D (2012) The effect of simultaneous negative and positive defocus
on eye growth and development of refractive state in marmosets. Invest Ophthalmol Vis Sci 53:
6479–6487. PMID: 22918633
37. Hung LF, Crawford ML, Smith IEL (1995) Spectacle lenses alter eye growth and the refractive status
of young monkeys. Nat Med 1: 761–765. PMID: 7585177
38. Norton TT, Siegwart JT, Amedo AO (2006) Effectiveness of hyperopic defocus, minimal defocus, or
myopic defocus in competition with a myopiagenic stimulus in tree shrew eyes. Invest Ophthalmol Vis
Sci 47: 4687–4699. PMID: 17065475
39. Sivak JG, Barrie DL, Callender MG, Doughty MJ, Seltner RL, et al. (1990) Optical causes of experi-
mental myopia. Ciba Found Symp 155: 160–72; discussion 172–7. PMID: 2088675
40. Shen W, Sivak JG (2007) Eyes of a lower vertebrate are susceptible to the visual environment. Invest
Ophthalmol Vis Sci 48: 4829–4837. PMID: 17898310
41. Wallman J, Turkel J, Trachtman J (1978) Extreme myopia produced by modest change in early visual
experience. Science. 29: 1249–1251.
42. Troilo D, Nickla DL, Wildsoet CF (2000) Form deprivation myopia in mature common marmosets (Cal-
lithrix Jacchus). Invest Ophthalmol Vis Sci 41: 2043–2049. PMID: 10892841
43. Smith IEL, Harwerth RS, Crawford ML, Von Noorden GK (1987) Observations on the effects of form
deprivation on the refractive status of the monkey. Invest Ophthalmol Vis Sci 28: 1236–1245. PMID:
3610541
44. Marsh-Tootle WL, Norton TT (1989) Refractive and structural measures of lid-suture myopia in tree
shrew. Invest Ophthalmol Vis Sci 30: 2245–2257. PMID: 2676897
45. Irving EL, Sivak JG, Callender MG (1992) Refractive plasticity of the developing chick eye. Ophthal-
mic Physiol Opt 12: 448–456. PMID: 1293533
46. Kisilak M, Hunter J, Huang L, Campbell MC, Irving EL (2008) In chicks wearing high powered negative
lenses, spherical refraction is compensated and oblique astigmatism is induced. Journal of Modern
Optics 55: 611–623.
47. Smith IEL (1998) Spectacle lenses and emmetropization: The role of optical defocus in regulating ocu-
lar development. Optometry & Vision Science 75: 388–398. doi: 10.1111/opo.12195 PMID:
25604956
48. Irving EL, Callender MG, Sivak JG (1995) Inducing ametropias in hatchling chicks by defocus—Aper-
ture effects and cylindrical lenses. Vision Res, 35: 1165–1174. PMID: 7610578
49. Schmid KL, Wildsoet CF (1997) Natural and imposed astigmatism and their relation to emmetropiza-
tion in the chick. Experimental Eye Research, 64: 837–847. PMID: 9245915
50. Thomas S, Schaeffel F (2000) Developmental compensation of imposed astigmatism is not initiated
by astigmatic accommodation in chickens. Vision Res, 40: 3553–3558. PMID: 11116159
51. Laskowski FH, Howland HC (1996) Effect of experimentally stimulated astigmatism on eye growth
and refractive development in chicks. Invest Ophthalmol Vis Sci 37 (Suppl.) S687.
52. Phillips J, Collins A (2000) Reponse of the chick eye to simple hyperopic astigmatic defocus. In: Pro-
ceedings of the VIII international conference on Myopia (pp 162–165) Boston, MA: 162–165.
53. Thibos LN, Cheng X, Phillips J, Collins A (2001) Astigmatic deprivation of chicks produces myopia,
but not astigmatism. Invest Ophthalmol Vis Sci (ARVO Suppl.) 42: S58.
54. McLean RC, Wallman J (2003) Severe astigmatic blur does not interfere with spectacle lens compen-
sation. Invest Ophthalmol Vis Sci 44: 449–457. PMID: 12556368
55. Smith IEL, Huang JC, Hung LF (1998) Cylindrical spectacle lenses alter emmetropization and pro-
duce astigmatism in young monkeys. In: Anonymous Myopia Updates.: Springer. pp. 336–343.
56. Kee CS, Hung LF, Qiao-Grider Y, Roorda A, Smith IEL (2004) Effects of optically imposed astigma-
tism on emmetropization in infant monkeys. Invest Ophthalmol Vis Sci 45: 1647–1659. PMID:
15161822
57. Huang J, Hung LF, Smith IEL (1997) Astigmatism is associated with the development of axial hyper-
opia in young monkeys. Invest Ophthalmol Vis Sci 38 (Suppl.): S546.
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 20 / 22
58. Thibos LN, Wheeler W, Horner D (1997) Power vectors: An application of fourier analysis to the de-
scription and statistical analysis of refractive error. Optom Vis Sci 74: 367–375. PMID: 9255814
59. Raasch TW (1995) Spherocylindrical refractive errors and visual acuity. Optometry & Vision Science
72: 272–275. doi: 10.1111/opo.12195 PMID: 25604956
60. Irving EL, Callender MG, Sivak JG (1991) Inducing myopia, hyperopia, and astigmatism in chicks. Op-
tometry and vision science 68: 364–368. PMID: 1852398
61. Wallman J, Adams J (1987) Developmental aspects of experimental myopia in chicks: Susceptibility,
recovery and relation to emmetropization. Vision Res 27: 1139–1163. PMID: 3660666
62. Chu CH, Deng L, Kee CS (2012) Effects of hemiretinal form deprivation on central refractive develop-
ment and posterior eye shape in chicks. Vision Res 55: 24–31. doi: 10.1016/j.visres.2011.12.011
PMID: 22245708
63. Furtado KS, Andrade FO (2013) Comparison of the beneficial and adverse effects of inhaled and in-
jectable anaesthetics: A mini-review. OA Anaesthetics 1(2): 20–25.
64. Votaw JR, Byas-Smith MG, Voll R, Halkar R, Goodman MM (2004) Isoflurane alters the amount of do-
pamine transporter expressed on the plasma membrane in humans. Anesthesiology 101: 1128–
1135. PMID: 15505448
65. Sokol JA, Tammaro YR, Haji SA, Brodie SE, Asbell PA (2005) Conductive keratoplasty and the cou-
pling phenomenon. Eye Contact Lens 31: 111–116. PMID: 15894877
66. Kee CS, Deng L (2008) Astigmatism associated with experimentally induced myopia or hyperopia in
chickens. Invest Ophthalmol Vis Sci 49: 858–867. doi: 10.1167/iovs.06-1370 PMID: 18326703
67. Campbell MC, Bunghardt K, Kisilak M, Irving EL (2008) Diurnal rhythms of refractive error compo-
nents in normal chick. Journal of Vision 8: 48–48.
68. Nickla DL (2006) The phase relationships between the diurnal rhythms in axial length and choroidal
thickness and the association with ocular growth rate in chicks. Journal of Comparative PhysiologyA:
Neuroethology, Sensory, Neural, and Behavioral Physiology 192: 399–407. PMID: 16380843
69. Johnson CA, Lytle G, Troilo D, Nickla DL (2004) Chick eyes show A diurnal rhythm in refractive error.
Invest Ophthalmol Vis Sci 45: 4295.
70. Chu CH, Zhou YJ, Zheng YP, Kee CS (2014) Bi-directional corneal accommodation in alert chicks
with experimentally induced astigmatism. Vision Res 98: 26–34. doi: 10.1016/j.visres.2014.03.002
PMID: 24637152
71. Mandelman T, Sivak JG (1983) Longitudinal chromatic aberration of the vertebrate eye. Vision Res
23: 1555–1559. PMID: 6666057
72. Irving EL, Sivak JG, Curry TA, Callender MG (1996) Chick eye optics: Zero to fourteen days. Journal
of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 179:
185–194.
73. Avila N, McFadden S (2010) A detailed paraxial schematic eye for the white leghorn chick. Journal of
Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 196: 825–
840. doi: 10.1007/s00359-010-0562-0 PMID: 20665035
74. Duke-Elder SS, Abrams D (1970) System of ophthalmology: Ophthalmic optics and refraction; by Sir
Stewart Duke-Elder and David Abrams.: Kimpton. PMID: 25077213
75. Alpins N (2001) Astigmatism analysis by the Alpins method. J Cataract Refract Surg 27: 31–49.
PMID: 11165856
76. Alpins N (1993) A new method of analyzing vectors for changes in astigmatism. J Cataract Refract
Surg 19: 524–533. PMID: 8355160
77. Batschelet E (1981) Circular statistics in biology.: Academic Press London. PMID: 25121329
78. Mahan RP (1991) Circular statistical methods: Applications in spatial and temporal performance anal-
ysis. Circular Statistic Al Methods: Applications in Spatial and Temporal Performance Analysis. 16.
79. Coulombre AJ (1964) Problems in corneal morphogenesis. Adv Morphog 4: 81–109. PMID: 5331708
80. Trelstad RL, Coulombre AJ (1971) Morphogenesis of the collagenous stroma in the chick cornea. J
Cell Biol 50: 840–858. PMID: 4329158
81. Murphy CJ, Glasser A, Howland HC (1995) The anatomy of the ciliary region of the chicken eye. In-
vest Ophthalmol Vis Sci 36: 889–896. PMID: 7706037
82. Lord RD Jr (1956) A comparative study of the eyes of some falconiform and passeriform birds. Am
Midl Nat: 325–344.
83. de Queiroz K, Good DA (1988) The scleral ossicles of opisthocomus and their phylogenetic signifi-
cance. Auk: 29–35.
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 21 / 22
84. Walls GL (1942) The vertebrate eye and its adaptive radiation.: The Cranbrook Press, bloomfield
Hills, Michigan. PMID: 20784611
85. Glasser A, Troilo D, Howland HC (1994) The mechanism of corneal accommodation in chicks. Vision
Res 34: 1549–1566. PMID: 7941363
86. Beresford JA, Crewther SG, Kiely PM, Crewther DP (2001) Comparison of refractive state and circum-
ferential morphology of retina, choroid, and sclera in chick models of experimentally induced ametro-
pia. Optom Vis Sci 78: 40–49. PMID: 11233334
87. Oh IK, Oh J, Yang KS, Lee KH, Kim SW, et al. (2014) Retinal topography of myopic eyes: A spectral-
domain optical coherence tomography study. Invest Ophthalmol Vis Sci 55: 4313–4319. doi: 10.
1167/iovs.14–14277 PMID: 24917143
88. Wildsoet CF, Wallman J (1995) Choroidal and scleral mechanisms of compensation for spectacle
lenses in chicks. Vision Res 35: 1175–1194. PMID: 7610579
89. Nickla DL, Sharda V, Troilo D (2005) Temporal integration characteristics of the axial and choroidal re-
sponses to myopic defocus induced by prior form deprivation versus positive spectacle lens wear in
chickens. Optom Vis Sci 82(40: 318–327.
90. Wallman J, Wildsoet CF, Xu AM, Gottlieb MD, Nickla DL, et al. (1995) Moving the retina: Choroidal
modulation of refractive state. Vision Res, 35: 37–50. PMID: 7839608
91. Piñero DP, Ruiz-Fortes P, Pérez-Cambrodí RJ, Mateo V, Artola A (2014) Ocular residual astigmatism
and topographic disparity vector indexes in normal healthy eyes. Contact Lens and Anterior Eye 37:
49–54. doi: 10.1016/j.clae.2013.07.006 PMID: 23910507
92. Birkenfeld J, de Castro A, Ortiz S, Pascual D, Marcos S (2013) Contribution of the gradient refractive
index and shape to the crystalline lens spherical aberration and astigmatism. Vision Res 86: 27–34.
doi: 10.1016/j.visres.2013.04.004 PMID: 23597582
93. Duke-Elder S (1993) Duke-elder's practice of refraction. Edinburgh; New York: Churchill Livingstone.
PMID: 25590127
94. Gündüz A, Evereklioglu C, Er H, Hepsen IF (2002) Lenticular astigmatism in tilted disc syndrome.
Journal of Cataract & Refractive Surgery 28: 1836–1840. doi: 10.3928/01913913-20150114-03
PMID: 25608281
95. Nalbach HO, Wolf-Oberhollenzer F, Remy M (1993) Ch.2. exploring the image. In: Zeigler HP, Bischof
HJ, editors. Vision, brain, and behavior in birds. United States: MIT Press.
96. Chen Y, Wang Z, Shibata H, Naito J (2004) Quantitative analysis of cells in the ganglion cell layer of
the chick retina: Developmental changes in cell density and cell size. Anat Histol Embryol 33: 161–
167. PMID: 15144285
97. Chen YX, Naito J (2000) Regional specialization of ganglion cell layer of the chick retina. The Journal
of Veterinary Medical Science 62: 53–57. PMID: 10676890
98. Schaeffel F, Troilo D, Wallman J, Howland HC (1990) Developing eyes that lack accommodation
grow to compensate for imposed defocus. Vis Neurosci 4: 177–183. PMID: 2271446
99. Wildsoet CF, Pettigrew JD (1988) Experimental myopia and anomalous eye growth patterns unaffect-
ed by optic nerve section in chickens: Evidence for local control of eye growth. Clin Vis Sci 3: 99–107.
100. Barutchu A, Crewther SG, Crewther DP (2002) Effects of optical defocus and spatial contrast on ante-
rior chamber depth in chicks. Clin Experiment Ophthalmol 30: 217–220. PMID: 12010218
101. Fischer AJ, Ritchey ER, Scott MA, Wynne A (2008) Bullwhip neurons in the retina regulate the size
and shape of the eye. Dev Biol 317: 196–212. doi: 10.1016/j.ydbio.2008.02.023 PMID: 18358467
102. Boote C, Hayes S, Young RD, Kamma-Lorger CS, Hocking PM, et al. (2009) Ultrastructural changes
in the retinopathy, globe enlarged (rge) chick cornea. J Struct Biol 166: 195–204 doi: 10.1016/j.jsb.
2009.01.009 PMID: 19258040
Experimentally Induced Astigmatism in Chicks
PLOS ONE | DOI:10.1371/journal.pone.0117729 February 12, 2015 22 / 22
