Spectacle lens compensation in the pigmented guinea pig
Marcus H.C. Howletta,b,*, Sally A. McFaddena
aSchool of Psychology, Faculty of Science and Information Technology, The University of Newcastle, Australia
bSchool of Biomedical Sciences, Faculty of Health, The University of Newcastle, Australia
a r t i c l ei n f o
Received 16 September 2008
Received in revised form 10 October 2008
a b s t r a c t
When a young growing eye wears a negative or positive spectacle lens, the eye compensates for the
imposed defocus by accelerating or slowing its elongation rate so that the eye becomes emmetropic with
the lens in place. Such spectacle lens compensation has been shown in chicks, tree-shrews, marmosets
and rhesus monkeys. We have developed a model of emmetropisation using the guinea pig in order to
establish a rapid and easy mammalian model. Guinea pigs were raised with a +4D, +2D, 0D (plano),
?2D or ?4D lens worn in front of one eye for 10 days or a +4D on one eye and a 0D on the fellow eye
for 5 days or no lens on either eye (littermate controls). Refractive error and ocular distances were mea-
sured at the end of these periods. The difference in refractive error between the eyes was linearly related
to the lens-power worn. A significant compensatory response to a +4D lens occurred after only 5 days and
near full compensation occurred after 10 days when the effective imposed refractive error was between
0D and 8D of hyperopia. Eyes wearing plano lenses were slightly more myopic than their fellow eyes
(?1.7D) but showed no difference in ocular length. Relative to the plano group, plus and minus lenses
induced relative hyperopic or myopic differences between the two eyes, inhibited or accelerated their
ocular growth, and expanded or decreased the relative thickness of the choroid, respectively. In individ-
ual animals, the difference between the eyes in vitreous chamber depth and choroid thickness reached
±100 and ±40 lm, respectively, and was significantly correlated with the induced refractive differences.
Although eyes responded differentially to plus and minus lenses, the plus lenses generally corrected the
hyperopia present in these young animals. The effective refractive error induced by the lenses ranged
between ?2D of myopic defocus to +10D of hyperopic defocus with the lens in place, and compensation
was highly linear between 0D and 8D of effective hyperopic defocus, beyond which the compensation
was reduced. We conclude that in the guinea pig, ocular growth and refractive error are visually regulated
in a bidirectional manner to plus and minus lenses, but that the eye responds in a graded manner to
imposed effective hyperopic defocus.
Crown Copyright ? 2008 Published by Elsevier Ltd. All rights reserved.
If defocus is imposed on a growing eye by a spectacle lens, the
rate of ocular elongation and emmetropisation is modified, so that
the eye eventually becomes emmetropic with the lens in place.
When hyperopic defocus is imposed with a negative lens, the eye
elongates more rapidly and becomes relatively myopic (when
measured without the lens in place). Conversely, when myopic
defocus is imposed with a positive lens, the eye decreases its rate
of ocular elongation and becomes hyperopic relative to untreated
eyes (Fig. 1). This phenomenon is known as spectacle lens compen-
sation. Compensation to both plus and minus spectacle lenses was
first shown in the chick (Schaeffel, Glasser, & Howland, 1988); and
subsequently in the tree shrew (Siegwart & Norton, 1993); rhesus
monkey (Hung, Crawford, & Smith, 1995; Smith & Hung, 1999) and
marmoset (Graham & Judge, 1999). Preliminary reports suggest
that the guinea pig also compensates for spectacle lenses (McFad-
den, Howlett, & Mertz, 2004; McFadden & Wallman, 1995).
The chick eye compensates to an extraordinary range of lens
powers from ?10D to +15D (Irving, Sivak, & Callender, 1992) while
other species studied compensate to a comparably smaller range,
particularly for plus lenses (macaque: ?3D to +3D, Hung et al.,
1995; Smith & Hung, 1999; Smith, Hung, & Harwerth, 1999; mar-
mosets: ?8D to <+4D, Graham & Judge, 1999; tree shrew: ?10D to
+4D, Metlapally & McBrien, 2008). The magnitude of the ocular
change within these ranges is well matched to compensate for
the effective power of the imposed defocus.
In the chick eye, the initial compensatory response to plus or
minus lenses involves a rapid thickening or thinning of the choroid,
respectively, which repositions the photoreceptor plane to par-
tially compensate for the imposed defocus (Wildsoet & Wallman,
1995). During +15D lens-wear the choroid can thicken 2.6-fold,
expanding as much as 300 lm (Wildsoet & Wallman, 1995) which
can account for up to 9D of change in the refractive error. After sev-
0042-6989/$ - see front matter Crown Copyright ? 2008 Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Address: School of Psychology, Faculty of Science and
Information Technology, The University of Newcastle, Australia.
E-mail address: firstname.lastname@example.org (M.H.C. Howlett).
Vision Research 49 (2009) 219–227
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eral days, the choroidal response dissipates, and is substituted by a
slower compensatory change in ocular length. In other species
studied, bidirectional changes in the thickness of the choroid have
also been found to precede ocular length changes, but they are sig-
nificantly smaller in magnitude. In macaque monkeys wearing a
plus lens on one eye and a minus lens over the other eye, the max-
imum difference in the thickness of the choroid was 40–50 lm,
equivalent to only 0.5D of the refractive error disparity (12D)
and accounted for less than 15% of the compensatory anisometro-
pia (Hung, Wallman, & Smith, 2000). In the tree shrew, the choroid
thins by 15 lm after five days of ?5D lens-wear (Gentle & McBrien,
1999), accounting for 0.7D, or 11% of the refractive error difference
between the lens-wearing and fellow eye. Choroidal thickening
associated with eyes recovering from myopic defocus arising from
previous form deprivation is also much larger in chicks (+400 lm,
Wallman et al., 1995) compared to tree shrews (+10 lm, Gentle &
McBrien, 1999), marmosets (+50 lm, Troilo, Nickla, & Wildsoet,
2000), macaques (+23lm, Hung et al., 2000; Qiao-Grider, Hung,
Kee, Ramamirtham, & Smith, 2004) or guinea pigs (+18 lm, How-
lett & McFadden, 2006).
choroids, a difference in the magnitude of the choroidal response
might be expected. In particular, most of the choroidal volume of
the chick consists of a dilated lymphatic system, presumably due
to fluid accumulation when the eye experiences myopic defocus
of the primate occupy a much smaller proportion of the choroid
(Hung et al., 2000). In the current study, we sought to determine
the magnitude of the response of the guinea pig eye to low powered
spectacle lenses, and to determine the nature of the choroidal re-
sponse. The guinea pig retina, like the avian retina is also avascular.
It is reported here, that spectacle lenses altered the ocular
development and choroidal thickness of the guinea pig eye in a
manner dependent upon both the sign and the magnitude of the
imposed lens power. Some of this work has been previously pre-
sented in abstract form (Howlett & McFadden, 2002; McFadden
& Howlett, 2002).
2.1. Animals and housing
Fifty-six guinea pigs (Cavia porcellus, pigmented, tricoloured)
were reared and housed with their mothers and littermates as pre-
viously described (Howlett & McFadden, 2007; McFadden et al.,
2004). In brief, animals were housed in opaque hard plastic boxes
(65 ? 45 ? 20 cm) with wire mesh lids which allowed unrestricted
vision to the room ceiling with the exception of a small opaque
section (38 ? 18 cm) located at the rear of each lid. The lighting
was provided by ceiling fluorescent lights with a 12/12 hour day/
night cycle. All procedures were approved by the University of
Newcastle under Australian legislative requirements and were in
accordance with NIH Guidelines.
Guinea pigs were raised from 2 to 3 days of age with a +4D
(n = 8), +2D (n = 6), 0D (n = 11) (plano), ?2D (n = 6), or ?4D
(n = 12) lens worn on one eye for 10 days (Experiment 1, monocu-
lar lens-wear) or with a +4D on the left eye and 0D on the right eye
for 5 days (n = 7, Experiment 2, binocular lens-wear) or no lens on
either eye (age-matched controls, n = 6). The age that lenses were
worn was during the most rapid period of emmetropisation (How-
lett & McFadden, 2007). Refractive error and axial parameters were
measured in both eyes after the lens-wear period (at 12–13 days of
age in Exp. 1 and the age-matched controls, and at 7 days of age in
Exp. 2). Additionally, in thirty guinea pigs in Experiment 1 (n = 6
for each lens group) the refractive error of both eyes was also mea-
sured immediately prior to lens-wear.
2.2.1. Lenses and their application
Concave lenses made of polymethylmethacrylate (diameter,
12mm; optic zone, 10.5–11.5mm; back optic radii, 8mm) were
worn in front of the eye with the distance from the cornea to the
lens apex being approximately 5mm. The effective power (Fe) of
the +4D, +2D, ?2D and ?4D lenses at the cornea was +4.08,
+2.02, ?1.98 and ?3.92D, respectively (approximated as Fe= F/
(1 ? d ? F) where F is the nominal lens power in D, and d is the dis-
tance of the lens from the corneal vertex in m). For convenience,
lens power is referred to in terms of the nominal rather than the
effective power of the lenses. Lenses were attached using Velcro?,
two arcs of which were glued above and below the eye (Fig. 2A)
while the animal was briefly anaesthetised with halothane (induc-
tion: 5%, maintenance: 1–2%, oxygen flow rate: 1 L/min). The
following day, lenses attached to a ring backed with Velcro, were
attached onto the matching arcs (Fig. 2B). Lenses were worn con-
tinuously except when they were removed for cleaning which took
up to 2 min, 3 times/day. During cleaning animals were placed in
the dark. Soft tape was applied to the back foot ipsilateral to the
lens-wearing eye to reduce damage to the lens from scratching.
2.2.2. Refractive error
3 drops of 1% cyclopentolate hydrochloride (CyclogylTM, Alcon).
izontal and vertical meridians (see Fig 1 in Howlett & McFadden,
12 days, 0.69D at 30 days of age, Howlett & McFadden, 2007).
2.2.3. Ocular dimensions
The dimensions of the eye on the optic axis were measured
using ultrasound (20 MHz) in anaesthetised guinea pigs (1–2% Hal-
Fig. 2. Lens attachment. (A) Lenses were attached to arcs made of velcro?(white
arrows). (B) Lens with matching velcro?base in place over the eye.
Fig. 1. Spectacle lens compensation. (A) The eye expands to compensate for a
negative lens, and (B) reduces its rate of growth to compensate for a positive lens.
M.H.C. Howlett, S.A. McFadden/Vision Research 49 (2009) 219–227
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