Stimulus Requirements for the Decoding of Myopic and
Hyperopic Defocus under Single and Competing
Defocus Conditions in the Chicken
Sigrid Diether1and Christine F. Wildsoet2
PURPOSE. The bidirectional nature of emmetropization, as ob-
served in young chicks, implies that eyes are able to distinguish
between myopic and hyperopic focusing errors. In the current
study the spatial frequency and contrast dependence of this
process were investigated in an experimental paradigm that
allowed strict control over both parameters of the retinal
image. Also investigated was the influence of accommodation.
METHODS. Defocusing stimuli were presented through lens-
cone devices with attached targets. These devices were mon-
ocularly applied to 5-day-old chickens for 4 days. Defocus
conditions included: (1) 7 D of myopic defocus, (2) 7 D of
hyperopic defocus, and (3) a combination of the two. Two
high contrast target designs, a spatially rich, striped Maltese
cross (target 1) and a standard Maltese cross (target 2) were
used, except in some experiments where target contrast or
spatial frequency content was further manipulated. To test the
role of accommodation, the treated eye of some chicks under-
went ciliary nerve section before attachment of the device.
Refractive error (RE) was measured by retinoscopy and axial
ocular dimensions measured by A-scan ultrasonography, both
in chicks under anesthesia.
RESULTS. With imposed myopic defocus and high contrast,
target 1 elicited significantly better compensation than did
target 2. With imposed hyperopic defocus, both targets elicited
near normal compensatory responses. Reducing image con-
trast to 32% for target 2 and to 16% for target 1 precluded
compensation for myopic defocus, inducing myopia instead.
The low-pass–filtered target also induced myopia, irrespective
of the sign of imposed defocus. With competing defocus and
intact accommodation, target 1 induced a transient hyperopic
growth response, whereas myopia was consistently observed
with target 2. When accommodation was rendered inactive,
both targets induced myopia under these competitive condi-
CONCLUSIONS. Compensation to myopic defocus is critically
dependent on the inclusion of middle to high spatial frequen-
cies in the stimulus and has a spatial frequency–dependent
threshold contrast requirement. With competing myopic and
hyperopic defocus, the former transiently dominates the latter
as a determinant of ocular growth, provided that the stimulus
conditions include sufficient middle to high spatial frequency
information and that accommodation cues are available. (In-
vest Ophthalmol Vis Sci. 2005;46:2242–2252) DOI:10.1167/
cess referred to as emmetropization. Animal studies have pro-
vided convincing evidence that emmetropization is an active,
visually guided process. For example, the chick eye rapidly
compensates for refractive (focusing) errors (RE) imposed by
spectacle lenses by appropriately adjusting its choroidal thick-
ness1and axial length.1,2Specifically, hyperopic defocus (im-
posed by negative lenses) induces choroidal thinning and ac-
celerated eye elongation, whereas myopic defocus (imposed
by positive lenses) induces choroidal thickening and inhibits
eye elongation. This emmetropization process also exists in
mammals, including monkeys,3,4although the range of com-
pensation is reduced relative to that in chickens (rhesus mon-
keys, ?3 to ?6 D5; chickens, ?10 to ?15 D6).
That eyes can detect and appropriately respond to both
myopic and hyperopic defocus implies that they are able to
distinguish the sign of the imposed defocus. However, we are
still far from understanding how this is accomplished, even in
terms of which features of the defocused retinal image are used
by the eye to decode this sign information. Because of its
relevance to myopia control—insights into this sign detection
problem may allow control through manipulation of the visual
environment—we targeted the stimulus requirements for em-
metropization in the present study.
Of relevant, already published studies, most relate to normal
developmental emmetropization. Specifically, form depriva-
tion experiments indicate that normal developmental em-
metropization has both spatial frequency and contrast require-
ments. The devices used in such experiments (e.g., frosted,
translucent diffusers), typically show low-pass filter character-
istics, eliminating moderate to high spatial frequency informa-
tion as well as reducing image contrast. The net result is the
derailment of emmetropization, with increased axial elonga-
tion leading to myopia.7,8That eyes can recover from this
induced myopia when normal vision is restored at a sufficiently
early age represents a more direct example of emmetropiza-
tion. Predictably, this recovery process can be prevented in
chicks by low-pass filtering of the visual image.9In another
relevant study, form-deprived chicks were exposed daily to
brief periods of “normal vision”10; manipulation of the spatial
frequency information available during these exposures
showed intermediate spatial frequencies (0.86 cyc/deg) to be
more effective than either higher or lower frequencies in
preventing the development of myopia. This spatial frequency
dependence of emmetropization is similar to that reported for
accommodation, another ocular focusing mechanism.11,12
A limitation of the experimental paradigms used in the cited
studies is the need to restrain the animal during visual manip-
ulation. This imposes constraints on the duration of exposure.
In the present study, we made use of a cone-shaped imaging
system (lens-cone device) that allows strict and sustained con-
uring postnatal development, the length of the vertebrate
eye becomes matched to its focal length through a pro-
Hospital Tu ¨bingen, Tu ¨bingen, Germany; and the2School of Optome-
try, University of California, Berkeley, California.
Supported by German Research Council (DFG) Grant DI 834/1-1
and National Eye Institute Grant EY012392.
Submitted for publication October 8, 2004; revised March 8, 2005;
accepted March 13, 2005.
Disclosure: S. Diether, None; C.F. Wildsoet, None
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be marked “advertise-
ment” in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Corresponding author: Christine F. Wildsoet, School of Optome-
try, 588 Minor Hall, University of California, Berkeley, CA 94720-2020;
1Section of Neurobiology of the Eye, University Eye
Investigative Ophthalmology & Visual Science, July 2005, Vol. 46, No. 7
Copyright © Association for Research in Vision and Ophthalmology
trol over the visual information presented as well as retinal
image defocus.13,14We took advantage of its flexibility in
allowing visual information to be presented in one or more
planes, at different levels of defocus, with control over both
spatial frequency and contrast. For some of these conditions,
we also added ciliary nerve lesioning surgery by way of testing
the influence of accommodation.
The present study builds on the primary result of a recently
published study by one of the authors indicating that compen-
sation in response to defocused stimuli is directed by the
imposed optical vergence in the absence of other cues to
distance.14In the present study, we investigated the effect of
manipulating the spatial frequency and contrast content of
retinal images on the eye’s growth response to both single and
competing defocus stimuli. We find that compensation for
myopic defocus has both spatial frequency and contrast re-
quirements. The inclusion of the competing defocus stimuli
was intended to simulate better the conditions encountered in
the natural environment and follows up on another study
involving chicks wearing multifocal lenses that imposed defo-
cus stimuli of the opposite sign; hyperopia was observed when
accommodation was left intact, but myopia occurred when
accommodation was prevented surgically (Wildsoet CF, et al.
IOVS 2000;41:ARVO Abstract 3930). We report herein similar
changes in the response bias under competing defocus condi-
tions when accommodation was eliminated.
Animals and Rearing
White-Leghorn chickens (Gallus domesticus) were used in this study,
obtained as 1-day-old hatchlings from a commercial hatchery (Privett
Hatchery, Portales, NM). They were reared under diurnal lighting
conditions (12-hour light–dark cycle) with access to food and water ad
libitum. The experimental treatments conformed to the ARVO State-
ment for the Use of Animals in Ophthalmic and Vision Research and
were approved by the Animal Care and Use Committee of the Univer-
sity of California-Berkeley.
Table 1 provides a summary of the specific details of each of the
treatment conditions tested, including imposed defocus information,
whether accommodation was left intact, and the number of birds
assigned to each treatment. More details about the design of the cone
devices and attached targets are provided in the following sections. In
all cases, the devices were applied monocularly to 5-day-old chicks for
4 days. The untreated contralateral eyes served as controls.
Lens Cone Devices. The design of the lens-cone devices has
been described in detail.13,14The cones were made from white, trans-
lucent polyethylene and were attached by Velcro ring supports. They
were designed to provide a field of view of approximately 45°. In a
pilot experiment comparing three different fields of view (approxi-
mately 30°, 45°, and 60°), the 45° field size was the best compromise
between the need to avoid peripheral form deprivation on the one
hand, and to provide uniform defocus across the target plane on the
other. Accordingly, cones providing a field of view of approximately
45° were used in all subsequent experiments. Each cone was fitted
with a positive lens (modified human PMMA contact lenses) at its
proximal end and a target at its distal end. To avoid any visual expe-
rience beyond the target plane, each target was backed with an opaque
white sheet that spanned the distal opening. The imposed defocus
conditions were manipulated by varying either the lens power that
determined the location of the focal plane and/or the target distance.
In all, three defocus conditions were tested: 7 D of myopic defocus
(Fig. 1A; target located beyond the focal plane of the ?40 D imaging
lens), 7 D of hyperopic defocus (Fig. 1B; target located within the focal
plane of the ?30 D lens), and a combination of myopic and hyperopic
defocus (Fig. 1C, two targets located beyond and within the focal plane
of a ?40 D lens). We regularly monitored the chicks to ensure that the
devices remained properly attached throughout each experiment.
Also, to avoid unintended deprivation effects, we cleaned the imaging
TABLE 1. Summary of Treatment Groups
Imposed DefocusType of Target
Myopic defocus (?7D) Target 1 100
Hyperopic defocus (?7D)
Competing defocus (?7D)Target 1 at both positions14 ? 9*
Target 2 at both positions 100
Target 3 (myopic defocus) and
target 1 (hyperopic defocus)
Data include the defocus imposed by the lens-cone device used, type of Maltese cross target used
striped (target 1), standard (target 2), low-pass filtered striped (target 3), or low-pass filtered standard
(target 4), target contrast, intact or paralyzed accommodation, and number of birds in each group.
* This experiment was performed twice, as a direct comparison experiment (same batch and identical
conditions) to two other competing defocus experiments, presenting (1) target 2 at both positions and (2)
target 3 in myopic defocus and target 1 in hyperopic defocus.
IOVS, July 2005, Vol. 46, No. 7
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