Download Optically induced anisometropia in kittens.

)ptically induced anisometropia in kittens
Gregory W. Maguire, Earl L. Smith III, Ronald S. Harwerth,
and M. L. J. Crawford
sometropia was simulated in 11 kittens during the critical period of visual system developit by securing a high-powered minus lens in front of one eye. Behavioral determinations of
nocular grating acuity indicated that the induced refractive error resulted in severe
blyopia in the defocused eye. Microelectrode recordings in striate cortex revealed that the
.cits in visual acuity were paralleled by a decrease in binocularly innervated neurons and a
<-ked shift in ocular dominance toward the nondeprived eye. "Position of paralysis" estimates
ealed that all the anisometropic kittens exhibited anomalous interocular alignments that
~e indicative of esotropia. There was also a significant reduction in the cross-sectional areas
lorsal lateral geniculate nucleus (LGNd) neurons in laminae innervated by the deprived eye.
I week period of binocular recovery initiated when three of the anisometropic kittens were
)roximately 16 weeks of age resulted in a partial recovery of visual acuity; however, there
? little or no evidence for recovery of cortical binocularity, cortical ocular dominance, LGNd
iron cell size, or interocular alignment. In general the visual system alterations produced by
induced anisometropia appear to be qualitatively similar to the anomalies associated with
longed unilateral lid suture. (INVEST OPHTHALMOL VIS SCI 23:253-264, 1982.)
Key words: anisometropia, amblyopia, lateral geniculate nucleus, striate cortex,
ocular dominance, visual acuity, kitten, interocular alignment, esotropia
teral form deprivation surgically proy lid closure is the most commonly
procedure to induce functional
>ia in experimental animals. Lid clogenerally considered to mimic the
of the most severe type of amblyopia
the human population, i.e., amblymopsia,1 and has been shown to have
ial anatomic, 2 " 5 behavioral,6"10 and
ysiologic11"14 consequences on the
ing visual system. A number of other
College of Optometry, University of Houston,
spartment of Ophthalmology, University of
lealth Science Center, and the Sensory Sciienter, Houston, Tex.
1 by Research Grants EY01120, EY01139, and
1 from the National Eye Institute.
1 for publication May 29, 1981.
equests: E. L. Smith, III, College of Op, University of Houston-Central Campus,
i, Tex. 77004.
experimental manipulations have been used
in attempts to simulate the etiology of the
more common forms of functional amblyopia.
However, despite the fact that anisometropia
is considered the most frequent cause of
functional amblyopia in humans, 15 the consequences of an artificially induced anisometropia have not been extensively studied. In
addition to its obvious clinical significance,
evaluating the effects of an optically induced
anisometropia would provide information
concerning alterations produced by a controlled amount of form deprivation in the absence of any light deprivation.
There have been three previous studies of
the effects of an induced anisometropia on
the developing visual systems of experimental animals, von Noorden and Crawford16 induced an extreme anisometropia in young
rhesus monkeys by surgically removing the
crystalline lens of one eye and subsequently
found a shift in cortical eye dominance and a
52/080253+12$01.20/0 © 1982 Assoc. for Res. in Vis. and Ophthal., Inc.
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Maguire et al.
reduction in dorsal lateral geniculate nucleus
(LGNd) cell size. In kittens, Eggers and
Blakemore17 optically induced anisometropia
with ophthalmic lenses, and Ikeda and Tremain18 mimicked the unilateral defocus associated with anisometropia by chronic atropinization. Again, both teams of investigators
found a shift in cortical eye dominance, but in
addition they reported a reduction in the
spatial resolving capacity of neurons in the
geniculostriate pathway. Although these studies revealed a possible neurophysiologic basis
for anisometropic amblyopia, the presence of
an amblyopia was not verified behaviorally in
these studies. Therefore one of the primary
purposes of the present study was to determine behaviorally the effects of induced anisometropia on the kittens' visual acuity. In
addition, we evaluated the effects of a habitually defocused retinal image on a number
of visual system characteristics (interocular
alignment, cell size of LGNd neurons, cortical ocular dominance, and recovery of visual
system function with subsequent normal visual experience) that have been extensively
studied in lid-sutured kittens.
Methods
Animals. Fourteen kittens, born in an isolated
colony, were used in the experiments. Anisometropia was optically induced in 11 of the kittens by a
previously described technique. 19 In brief, the kittens were dark-reared from the time of eye opening until 4 weeks of age. From 4 to 12 weeks of age
the kittens were allowed 2 to 3 hr of visual experience each day but only while wearing goggles that
held a zero-powered lens over the left eye and a
high-powered negative spherical lens over the
right eye. The dioptric power of the negative lens
was constant for a given animal but varied between — 10 and —16 diopters for the group. The
lenses were 25 mm in diameter and were fitted
into the goggles so that the optical centers coincided with the geometric centers of the lens rings.
The goggles held the lenses at a vertex distance of
approximately 12 mm. Viewing through the minus
lenses optically simulated a large hypermetropic
refractive error so that in order for the treated eye
to obtain a clear retinal image, the kittens would
have been required to alter their state of accommodation to compensate for the minus lens as well
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Invest. Ophthalmol. Vis. Sci.
August 1982
as for any naturally occurring refractive error. Kittens are moderately hyperopic at the start of the
critical period20 and are generally believed to have
a limited amplitude of accommodation (=4 D). 21
Moreover, it has been reported that kittens reared
in this manner posture their accommodation for
the eye without the minus lens. 17 Therefore, since
the powers of the goggle lenses exceeded the amplitude of accommodation and the depth of focus
calculated by the procedure outlined by Green
et al. 22 (pupil size was measured directly from the
experimental subjects; visual acuity estimates were
taken from the data of Mitchell et al.23), the rearing procedures utilized resulted in a habitually defocused retinal image in the treated eye throughout the rearing period.
Three kittens were used as controls, two of
which were reared in a normally lighted environment. The third control was fitted with goggles
that held zero-powered lenses over both eyes and
was reared in a manner identical to that of the
anisometropic kittens.
Behavioral assessment of visual acuity. A modification of the jumping-stand technique described
by Mitchell et al.23> 24 was used to determine the
monocular grating acuity for all 14 animals. Shaping procedures were initiated at 12 weeks of age to
train the kittens to jump from a raised platform
onto the trapdoor with the positive stimulus (a 19
by 19 cm laminated photograph of a square-wave
grating). The grating targets had a Michelson contrast of 0.60 and the same space average mean
luminance (75 cd/m2) as the negative stimulus (a
homogeneous gray target). When the animals
made a correct response and jumped to the side of
the apparatus with the grating pattern, they were
given a food reward. However, if the kittens
jumped to the side with the homogeneous gray
target (an incorrect response), the food reward was
withheld and on some trials the weight of the kittens caused the trapdoor to open and the animals
fell a short distance to the floor. For successive
trials, the positions of the positive and negative
stimuli were interchanged in a pseudorandom
fashion.
The initial training was carried out under binocular viewing conditions (goggles with zeropowered lenses over both eyes). To test monocular
performance, one eye was occluded by covering
one of the lens wells with a piece of black tape.
Once the kittens had reached a criterion performance under the monocular viewing conditions, a modified staircase procedure was initiated
to obtain an estimate of the animals' grating
Volume 23
Number 2
acuities. Criterion performance was considered to
be at least 90% correct responses over a block of 20
trials with a low spatial-frequency grating. For the
untreated eyes of the anisometropic kittens and for
both eyes of the control kittens, criterion performance was usually established with a 1 cy/deg
grating at a 57 cm viewing distance. To determine
grating acuity, the staircase was started with the 1
cy/deg target. If the kitten responded correctly on
at least six out of a block of eight trials, the spatial
frequency of the grating was increased 0.5 cy/deg
by substituting a finer grating. If the animal did
not respond correctly to at least six out of the eight
trials, the spatial frequency was decreased by 1.0
cy/deg and a new series was begun. However,
because many of the anisometropic kittens behaved as if they were functionally blind when
forced to use their deprived eyes, estimates of visual performance for some of the kittens were ascertained by a slightly modified procedure. If,
after repeated testing, the kitten was unable to
reach criterion performance with the deprived eye
for the lowest spatial-frequency grating (0.25
cy/deg at 10 cm), they were trained to discriminate between a solid black target and a solid white
target (i.e., a simple light-dark discrimination). It
was hoped that this extra practice would help the
experimental animals to learn the grating discrimination. All the anisometropic kittens were
able to reach criterion performance on the blackwhite discrimination; nevertheless, even with
subsequent training some of the kittens were unable to discriminate a grating pattern from a uniform gray target.
After the initial behavioral testing was completed, three of the anisometropic kittens were
placed in a normally lighted environment and allowed normal binocular viewing. The monocular
grating acuities of these three experimental animals were assessed periodically to determine
whether visual acuity would, improve if normal
viewing conditions were restored. The remaining
experimental kittens continued to receive only 2
to 3 hr of visual experience each day through the
anisometropic goggles until they were prepared
for the neurophysiologic experiments.
Assessment of ocular dominance and interocular alignment. The kittens were prepared for
single-unit recording in the striate cortex between
21 and 24 weeks of age. The surgical procedures,
methods of single-unit recording, and determination of interocular alignment have been described
previously.25' 26 Anesthesia was induced with an
intraperitoneal injection of sodium pentobarbital
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Anisometropic amblyopia in kittens 255
(Nembutal, 40 mg/kg). After completion of the
surgical procedures, the kittens were immobilized
with gallamine triethiodide (Flaxedil, a loading
dose of 10 mg/kg followed by an intravenous infusion of 10 mg/kg/hr) and respirated with room air.
Throughout the recording session, heart rate and
core temperature were continuously monitored
and all wound margins and pressure points were
periodically infiltrated with lidocaine HC1 (Xylocaine). Cycloplegia was produced by the topical
application of 1% atropine sulfate, and the nictitating membranes were retracted with 10% phenylephrine hydrochloride. The eyes, fitted with the
appropriately powered contact lenses, were focused on a rear projection screen 1 m in front of
the animal.
Neural activity was recorded from cortical units
with Epoxylite-insulated, tungsten microelectrodes by means of standard extracellular procedures. To minimize sampling bias, the electrodes
were directed at an oblique angle down the medial
bank of the lateral gyrus in the left visual cortex.
Hand-held stimuli were used to map the receptive
field properties of isolated cortical neurons. The
receptive fields were plotted separately for the
two eyes, with the primary emphasis being given
to the determination of ocular dominance. Each
neuron was classified according to the seven-category ocular dominance scheme described by
Hubel and Wiesel.27
Estimates of interocular alignment were obtained by determining the directions of gaze the
eyes assumed 1 hr after the induction of paralysis,
i.e., the "position of paralysis. "7> 28 The orientations of the eyes were specified by plotting the
projections of the optic discs onto the rear projection screen according to the procedures for
mapping retinal landmarks outlined by Pettigrew
et al.29 The linear distance between the centers of
the two optic discs was measured as an estimate of
eye alignment. Optic disc separations that fell
outside the range for kittens reared under normal viewing conditions were considered to reflect
anomalous interocular alignments. Small optic disc
separations indicated a convergent deviation and
conversely, large separations indicated a divergent
deviation.
LGNd measurements. At the end of the recording session, the animals were sacrificed with a lethal injection of sodium pentobarbital and were
perfused through the heart with 0.9% saline followed by an equal mixture of 2% paraformaldehyde and 2% glutaraldehyde in a cacodylate buffer. A block of tissue containing both LGNd nu-
256
Invest. Ophthalmol. Vis. Sci.
August 1982
Maguire et al.
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Fig. 1. Frequency-of-seeing curves (percentage of correct responses vs. spatial frequency of
the grating target) for four experimental kittens. Data are shown for the left (circles) and right
(squares) eyes. Performance for the treated eye of the - 1 5 D anisometropic kitten (C) on the
black-white discrimination (B-W) is represented by the hexagon. Threshold visual acuity is
defined as the spatial frequency for which the kitten responded correctly to the grating on 75%
of the trials (arrows).
clei were embedded in celloidin, cut coronally in
26 jum thick sections, and stained with cresyl violet. Sections taken for measurement were from the
approximate area of the nucleus receiving projections from the central retina. The measurement
procedure consisted of selecting 50 cells from each
lamina A and Al based on a sharply outlined nucleolus and lack of overlay with other cells. To avoid
sample bias associated with systematic changes in
cell-size within a given lamina, the sampling
procedures described by Hickey et al.5 were
used. Each cell was photographed at a magnification of X1000 projected with a photographic enlarger, and the outline of the cell body was traced
on white paper. Cross-sectional areas were measured with an electronic planimeter. The cell photographs taken from each lamina were given a letter code, which was withheld from the persons
doing the area measurements.
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Results
The effects of the induced anisometropia
on the kittens' visual acuities are shown in
Fig. 1, where frequency-of-seeing curves for
a normal kitten (Fig. 1, A), the goggle-reared
control animal (Fig. 1, B), and two of the
anisometropic kittens (Fig. 1, C and D) are
illustrated. Each data point represents the
percentage of correct responses for a given
spatial frequency stimulus over a minimum of
40 trials. For the normal and goggle-reared
control kittens, performance for the left- and
right-eye viewing conditions was quite similar. With a 75% correct criteria, the threshold grating acuities (indicated by the arrows
on the abscissa) for both eyes of the control
animals are within the range of visual acuities
Volume 23
Number 2
Anisometropic amblyopia in kittens
RECOVERY
TREATMENT
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Fig. 2. A, Visual acuity, defined as the highest spatial frequency target for which the animal
responded correctly on at least six of the block of eight trials, plotted as a function of time in
days for the treated (squares) and untreated eyes (circles). Day zero represents the day on
which visual acuity testing began (~12 weeks of age). At day 25 the kitten was placed in a
normally lighted environment. B-W, Performance on the simple black-white discrimination.
B, Frequency-of-seeing curves for the nondeprived (circles) and deprived eyes (squares) after
25 days of normal visual experience. The threshold visual acuities, using a 75% correct response criterion, are indicated by the arrows.
obtained for control kittens in a previous
study,26 but probably due to protocol differences (e.g., lower stimulus contrasts and
luminances) they are lower than the visual
acuities reported by Mitchell et al.23> 24 for
normal age-matched kittens. The grating acuities determined for the untreated eyes of the
anisometropic kittens were not significantly
different from those of the control animals;
however, substantial differences in resolving
capacity were noted between the treated and
untreated eyes of all the anisometropic kittens. In fact, seven of the 11 anisometropic
kittens were unable to reach criterion performance on the grating acuity task when
viewing with the treated eye even though all
these animals were able to make a simple
black-white discrimination (Fig. 1, C). The
highest visual acuity determined for the
treated eye of an anisometropic kitten (Fig.
1, D) was still almost an octave lower than the
acuity for its fellow untreated eye.
In a previous study, Smith et al.19 demonstrated that the form deprivation associated
with an optically induced anisometropia re-
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sulted in an experimental myopia similar to
that produced by lid suture and corneal opacification. To ensure that the differences in
visual acuity noted for the deprived and nondeprived eyes of the anisometropic kittens
were not simply the result of a difference in
refractive error between the two eyes, the
position of the jumping platform was adjusted
so that the grating targets were always within
the far point of the eye being tested.
All three of the anisometropic kittens that
had been allowed normal visual experience
after the initial behavioral testing demonstrated a partial recovery in the visual acuity
of the treated eye. Fig. 2, A, illustrates the
increase in the resolving capacity associated
with the recovery period for one of the
anisometropic kittens. This particular animal was unable to reach the criterion performance on the grating acuity task prior to
the recovery period. However, after several
days in the normally lighted environment, an
improvement in the visual acuity of the deprived eye was evident. The visual acuity of
the deprived eye appeared to stabilize by the
Invest. Ophthalmol. Vis. Sci.
August 1982
258 Maguire et al.
70
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30
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10
0
10
ICONTROLI
11
12
13
14
15
16
AMOUNT OF INDUCED ANISOMETROPIA IDIOPTERSI
Fig. 3. Optic disc separation (cm) plotted as a function of the degree of optically induced anisometropia in diopters. Circles, Control animals; triangles, anisometropic kittens; open triangles, three
anisometropic kittens that had been allowed a recovery period; dashed lines, limits of optic disc
separations found in normal kittens in a previous
study.26
end of the first week of recovery. Fig. 2, B,
shows the frequency-of-seeing curves compiled during the final three testing sessions.
Although there was no systematic improvement of visual acuity in the nondeprived eye
throughout the recovery period, the originally defocused eye obtained a resolving capacity approximately one octave below the
acuity of the nondeprived eye. Similar improvements in performance were noted in the
deprived eyes of the other two anisometropic
kittens that had been allowed a recovery period in a normally lighted environment.
Interocular alignment. Anomalous posi-
tions of paralysis were observed in virtually
all the anisometropic kittens, including the
treated kittens that had been allowed a recovery period prior to the electrophysiologic
experiments. Fig. 3 shows the optic disc separation plotted as a function of the rearing
condition for each subject. The optic disc
separations for the control animals were between 54 and 60 cm, which is within the
range found for six control animals in a previous study26 (Fig. 3, dashed lines). The optic
disc separations for the anisometropic kittens
(triangles) were all smaller than the smallest
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value for the control group, indicating that
after anesthesia and paralysis the eyes of
the anisometropic kittens were significantly
more convergent than the controls (MannWhitney-Wilcoxon, p = 0.002). For the deprived kittens as a group there was no relationship between the degree of induced anisometropia and the magnitude of the optic
disc separation (coefficient of linear regression = 0.13).
Assuming a constant 15° horizontal separation between the area centralis and the center of the optic disc for each eye across all
subjects,28 the 58 cm optic disc separation
found for the goggle-reared control indicates
that the visual axes of this animal were diverged by 2.34°. In contrast, a 35.2 cm separation, the mean for the anisometropic kittens, indicates that the visual axes were
over-converged with respect to the rear projection screen by 10.04°, suggesting that the
"average" anisometropic kitten manifested an
esotropia of approximately 12°. Casual inspection of the anisometropic kittens prior to
surgery, specifically of the positions of the
corneal reflexes produced by a distant light
source, suggested that the treated kittens
were indeed esotropic in the alert-awake
state.
Cortical ocular dominance. Ocular dominance histograms compiled for the control
kittens, the anisometropic kittens, and anisometropic kittens that had undergone binocular recovery are shown in Fig. 4, A, B, and C,
respectively. All the cortical units studied
had receptive fields located within 10° of the
area centralis. Because no systematic differences in ocular dominance were observed as
a function of receptive-field eccentricity, data
for all receptive-field eccentricities were
pooled. Likewise, since the results for the
goggle-reared control were similar to those of
the normally reared kittens, the data for all
three control animals were pooled.
In comparison with the ocular dominance
histograms for the control kittens, the distributions for the anisometropic kittens showed
a decrease in the encounter rate for neurons
that were strongly excited by both eyes (categories 3, 4, and 5), an increase in the propor-
Volume 23
Number 2
Anisometropic amblyopia in kittens 259
A. CONTROL
KITTENS
B. ANISOMETROPIC KITTENS
C. RECOVERY
KITTENS
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IDEPRIVEDI
OCULAR DOMINANCE
Fig. 4. Ocular dominance histograms of striate neurons for the three control kittens (A), the
eight anisometropic kittens (B), and the three anisometropic kittens who later received normal
visual stimulation (C). Categories 1 and 7 represent monocular cells stimulated by the contralateral and ipsilateral eyes, respectively. Varying degrees of influence are represented by
numbers 2 through 6. Category 4 represents binocular cells driven equally by both eyes. Since
all electrode penetrations were in the left hemisphere, the contralateral categories represent
the originally defocused eye. N, Number of units sampled in each group.
Table I. Mean cell sizes of LGNd neurons from five anisometropic kittens, two of
which were allowed a recovery period (—13R)*
Cross-sectional areas (fim2) for
deprived laminae (AL + A1R)
Cross-sectional areas (fim2) for
normal laminae (AR + A1L)
Rearing condition
(diopters of
anisometropia)
X-total
X-10 largest
X-10 smallest
-10
-11
-15
-13R
-13R
239
238
151
255
253
410
399
256
476
325
112
148
77
138
143
X-total
189
199
119
181
174
X-10 largest
X-10 smallest
322
365
230
307
269
101
120
56
109
87
X-total = mean size for all of the cells in the two laminae; X-10 largest = mean value for the 10 largest cells in the two given laminae;
X-10 smallest = mean of the 10 smallest cells in the given laminae.
*Each value is the mean for the neurons sampled from either the normal laminae (lamina A on the right side plus lamina Al on the left
side) or the deprived laminae (lamina A on the left side plus lamina Al on the right side). In all animals, the means for the 10 smallest,
10 largest, and total population of cells sampled in the deprived laminae were significantly smaller than the corresponding means for
the normal laminae (Mann-Whitney-Wilcoxon, p < 0.05 for each animal).
tion of monocularly excited units, and a substantial shift in ocular dominance toward the
untreated eye. The shift in ocular dominance
toward the normal (ipsilateral) eye was observed despite the bias introduced by restricting all the electrode penetrations to the
left cortex, contralateral to the defocused
eye. The ocular dominance distribution for
the three anisometropic kittens that had undergone binocular recovery is similar to the
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histogram for the anisometropic kittens that
had not been allowed a recovery period. The
similarity indicates that the recovery period
did not result in a reversal of the effects of the
optically induced anisometropia.
LGNd cell size. The mean cross-sectional
area of the 100 cells sampled in the deprived
and nondeprived LGNd laminae of five
anisometropic kittens, two of which had undergone binocular recovery, are summarized
Invest. Ophthalmol.
260 Maguire et at.
150 ANISOMETROPIA
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August
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13D ANISOMETROPIA t RECOVERY
CROSS-SECTIONAL AREA (um2)
Fig. 5. LGNd cell size measurements for layers (A
and Al) innervated by the deprived (squares) and
nondeprived eyes (circles) of two anisometropic
kittens, one of which was allowed a recovery period (B). The arrows along the abscissa indicate the
sample mean size for the two laminae groups, and
D max indicates the maximum difference in cell
size between the two samples.
in Table I. In efforts to determine whether
the induced anisometropia had a uniform effect on the size of all LGNd neurons, the
means for the 10 largest and 10 smallest cells
in the deprived and nondeprived laminae are
also given. In addition, the cumulative proportion of cells within the deprived and nondeprived laminae were plotted as a function
of cross-sectional area for each anisometropic
kitten. Typical cumulative proportion curves
for the deprived and nondeprived laminae for
two of the anisometropic kittens are illustrated in Fig. 5. By this plotting method, the
shape of the function for a random sample
drawn from a normally distributed population would approximate an ogive. Should a
uniform decrease in cell size occur as a result
of the induced anisometropia, the function
for the deprived laminae would be shifted
horizontally from the function for the nondeprived laminae but would remain parallel
to it. If, in contrast, the larger cells in the
LGNd were selectively affected by the in-
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1982
duced anisometropia, the curves for the deprived and nondeprived laminae would not
be parallel but would differ by the greatest
amount in the region of the functions corresponding to the larger cell sizes. Conversely,
the function for the deprived and nondeprived laminae would differ primarily in regions representing the smaller cells if the
habitually defocused image had selectively
altered the size of the smaller cells.
Table I shows that in all five anisometropic
kittens, including the binocular-recovery animals, the mean cross-sectional areas of cells
in the deprived laminae were significantly
smaller than cells in the nondeprived laminae
(Kolmorogov-Smirnov, p < 0.01 for each animal). No systematic differences were evident
between the results obtained for the binocular-recovery animals as a group compared
with the anisometropic kittens that had not
been allowed a recovery period. A comparison
of the means for the 10 largest and 10 smallest cells in the deprived and nondeprived
laminae suggests that in all animals, both the
largest and smallest cells encountered in the
deprived laminae were smaller than their
counterparts in the nondeprived laminae.
Moreover, the cumulative proportion curves
compiled for the deprived laminae (see Fig.
5) appear to parallel the functions for the
nondeprived laminae, although the deprived
laminae functions are shifted horizontally in
the direction of smaller cross-sectional areas.
Discussion
These results indicate that the effects of an
induced anisometropia of the magnitude
employed in the present study are qualitatively similar to the visual system alterations
produced by prolonged unilateral lid closure.
Either rearing strategy results in permanent
deficits in spatial resolving capacity, anomalous interocular alignments, a decrease in
the encounter rate for binocularly excitable
striate neurons, a shift in cortical ocular dominance toward the untreated eye, and a reduction in cell size of LGNd neurons primarily innervated by the deprived eye.
The consistently lower visual acuities demonstrated behaviorally by the experimental
Volume 23
Number 2
kittens when they were forced to use their
habitually defocused eyes indicates that the
induced anisometropia produced amblyopia
in the treated eye. The severity of the deficit
varied from apparent blindness (seven of 11
anisometropic kittens were initially unable to
reach criterion performance on the grating
acuity task) to an acuity level in the deprived
eye approximately one octave below that in
the untreated eye. These initial deficits, however, can be partially reversed, since the
three anisometropic kittens that had been allowed normal visual experience after the period of deprivation showed a relatively fast
but incomplete improvement in the visual
acuity of the treated eye. Giffin and Mitchell8
have reported that prolonged monocular lid
closure imposed from birth also results in apparent blindness and that subsequent periods
of recovery lead to a limited improvement in
measurable visual acuity. In comparison, the
rate of recovery of vision from lid suture is
much slower and the final ratio of visual acuities between the treated and untreated eyes
is lower than that for the anisometropic kittens. The relatively milder deficits and the
faster rates of recovery noted for the anisometropic kittens probably reflect differences in
the degree of visual deprivation afforded by
the two rearing strategies as well as the substantially lower total hours of anomalous visual
experience associated with the lens rearing
procedures (2 to 3 hr/day vs. 16 hr/day for
lid-sutured kittens).
Although it is difficult to compare directly
the behavioral data of the present study with
either the electrophysiologically determined
contrast sensitivity functions obtained for striate cortex neurons by Eggers and Blakemore17
or the spatial resolving power of LGNd neurons by Ikeda and Tremain,18 the results from
these studies appear to be in agreement concerning the effects of a habitually defocused
image on the spatial resolving capacity of the
treated eye. For instance, Eggers and Blakemore17 found that when the stimuli were presented to the treated eye of a kitten reared
with 12 D anisometropia, the highest spatial
frequency cutoffs determined for the population of striate neurons were approximately
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Anisometropic amblyopia in kittens 261
one octave lower than the highest cutoffs
measured for the untreated eye. In the present study, the highest visual acuities measured for the treated eyes of the anisometropic
kittens were also approximately one octave
lower than the visual acuities of the untreated
eyes. In both studies, a lower spatial resolving capacity was found for the treated eye of
every anisometropic kitten.
With respect to the effects of form deprivation on interocular alignment, several investigators have found interocular misalignments in kittens monocularly deprived of
form vision by lid suture.7* 30> 31 There is,
however, some disagreement between the
present study and that of Eggers and Blakemore17 concerning interocular alignment in
anisometropic kittens. Whereas we observed
that virtually all of the lens-reared kittens
manifested anomalous positions of paralysis
indicative of esotropia, Eggers and Blakemore17 reported that the eyes of their experimental kittens did not deviate from normal
in the paralyzed state. This apparent discrepancy can most likely be attributed to differences in the duration of the treatment period
and the age at which the kittens were prepared for electrophysiologic recording. The
anisometropic kittens of Eggers and Blakemore17 received a maximum of 80 hr of visual
experience through the goggles and were
prepared for electrophysiologic recording between the ages of 9 and 17 weeks. In comparison, our lens-reared kittens received a minimum of 240 hr of abnormal visual experience
and were at least 21 weeks of age at the time
interocular alignment was assessed. In support of this explanation, von Grunau,32 using
a corneal reflex technique, showed that the
magnitude of the anomalous interocular alignment resulting from abnormal visual experience reflects an age-dependent maturational
process and that the maximum deviation from
normal is not reached until the experimental
kittens are approximately 19 weeks old.
Therefore it is likely that Eggers and Blakemore17 failed to observe interocular misalignments simply because their anisometropic kittens were sacrificed before they
manifested an interocular deviation that
262 Maguire et al.
would be apparent from estimates of the position of paralysis.
The alterations in cortical ocular dominance observed in the anisometropic kittens
are similar to the well-established cortical effects of monocular lid suture and are in support of the competitive interaction hypothesis (i.e., "binocular competition") regarding
the effects of environmental manipulations
on the binocular innervation of striate neurons.33' 34 That is, procedures that prevent
concordant binocular stimulation during maturation and that put one eye at a "competitive disadvantage " (i.e., any type of unilateral
form deprivation) result in a breakdown of
cortical binocularity and a shift in ocular
dominance to the "normal" or nondeprived
eye. The degree of shift in ocular dominance
demonstrated by the anisometropic kittens is
not as severe as that reported in kittens lid
sutured for comparable periods of time, but
considering the disparity in the total amount
of anomalous visual experience and the degree of form deprivation associated with the
two rearing strategies, the observed differences are not surprising. However, despite
the fact that our anisometropic kittens received substantially greater total periods of
anomalous visual experience than those studies by Eggers and Blakemore,17 there is very
good agreement between the two studies regarding the proportion of binocularly excited
neurons and the degree of cortical dominance
demonstrated by the normal and defocused
eyes. For example, Eggers and Blakemore17
reported that 57% and 13% of the neurons
encountered were driven exclusively by the
nondeprived and defocused eyes, respectively, with the remaining 30% excited by
both eyes. In the present study, the percentages of neurons excited binocularly, by
the nondeprived eye and by the defocused
eye, were 33%, 55%, and 12%, respectively.
With respect to the efficacy of recovery
periods in reversing the physiologic effects of
monocular lid-suture, a number of investigators have shown that both forced usage of
the originally deprived eye and simple binocular recovery are effective if initiated
within the first 2 to 3 months of life.13' 14 In
general, little or no reversal of the effects of
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Invest. Ophthalmol. Vis. Sci.
August 1982
monocular lid suture is evident, however,
when the recovery period is delayed until the
kittens are 14 to 16 weeks of age.13' 14> 35 The
results of the present study are in agreement
with these earlier findings, since no recovery
of binocularity or increase in the proportion
of cortical neurons excited by the originally
defocused eye was noted in the three anisometropic kittens that had been allowed a 6
week binocular recovery period beginning at
approximately 16 weeks of age. In light of the
fact that the lens-reared kittens developed
esotropia and unequal refractive errors (true
anisometropia, see Smith et al.19), the failure
to observe a significant change in cortical ocular dominance after the recovery period was
not unexpected.
The cell size measurements of LGNd neurons indicate that the cross-sectional areas of
cells in the binocular portion of laminae receiving projections from the defocused eye
were smaller than those in the nondeprived
laminae. The mean differences in cell size
observed in the anisometropic kittens (27%
to 31%) are within the range of cell size differences reported recently for kittens monocularly lid sutured for comparable periods of
time (20% to 42%).5 The effects of induced
anisometropia on the cumulative proportion
functions for cell sizes within the sampled
populations (Fig. 5) indicate that the functions for the deprived laminae were similar in
shape but shifted in a relatively parallel fashion from the functions compiled for the nondeprived laminae. This parallel shift in the
cumulative proportion functions, together
with the observations that differences existed
between both the 10 largest and 10 smallest
cells sampled from the deprived and nondeprived laminae, suggests that all the cells
in the deprived laminae, large and small
alike, were reduced in size as a result of the
induced anisometropia. However, as Hickey
et al.5 have discussed, a second interpretation of these data is plausible. Specifically,
similar results would be expected if the induced anisometropia had selectively altered
the size of the large cell subpopulation to the
extent that after deprivation the "large cells"
were actually smaller than the smallest cells
within the normal "small cell" subpopulation.
Volume 23
Number 2
Regardless of which interpretation is correct,
Hickey et al.5 have demonstrated that monocular lid suture produced differences in cell
size between the 10 largest and 10 smallest
cells sampled from the deprived and nondeprived laminae, and therefore the results
for the lens-reared kittens are in agreement
with those from monocularly lid-sutured
kittens.
The differences in cell size obtained for the
deprived and nondeprived LGNd laminae of
the anisometropic kittens that had been allowed a postdeprivation recovery period
were not significantly different from those of
the other anisometropic kittens. Again, analogous findings have been reported for monocularly lid-sutured kittens. 4 In general, recovery periods initiated after an animal had
undergone monocular lid suture for the first
14 weeks of life are ineffective in producing a
significant degree of morphologic recovery in
the LGN. 4
The qualitative similarity of the effects of
the optically induced anisometropia and monocular lid suture supports the conclusion
formulated by several investigators16' 35 that
form deprivation per se is the causative factor
that initiates the visual system anomalies observed in lid-sutured animals. The quantitative differences resulting from the two rearing strategies can probably be attributed to
differences in the degree of form deprivation
and the total amount of anomalous visual experience. It has been shown that varying the
duration of lid suture produces various degrees of visual system alterations. Since the
basic effects of anisometropia and lid suture
appear analogous, it is tempting to suggest
that observations obtained from lid-sutured
kittens could be generalized to all anisometropic kittens. It must be kept in mind, however, that the degree of anisometropia induced in the present study was quite large. It
will be interesting to see if the same parallels
are found with less severe anisometropia.
Note added in proof
Subsequent to the submission of this
paper, Boothe et al. (INVEST OPHTHALMOL VIS
SCI 22:228, 1982) have reported a reduction
in monocular visual acuity of monkeys after
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Anisometropic amblyopia in kittens
263
long-term monocular atropinization. Additionally, Ikeda and Tremain (Trans Ophthalmol Soc UK 100:450, 1980) have reported
that long-term atropinization results in a reduction of resolving capacity of retinal ganglion cells associated with the defocused
eye(s).
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