We found that the eyes of adolescent marmosets remain susceptible
to form deprivation; the vitreous chamber becomes longer than normal
and myopia develops.
Figure 2 shows the refractive errors and vitreous chamber depth in both the
experimental (black bars) and control (white bars) eyes before (pre)
and at the end (post) of the deprivation period. Refractive error and
vitreous chamber depth were not significantly different between the
experimental and control eyes before the deprivation. After the period
of deprivation, the refractive errors (
Fig. 2 , top) were significantly
more myopic in the experimental versus the control eyes (mean ±
SE; −4.29 ± 1.2 versus −1.29 ± 1.2 diopters [D]; paired
t-test,
P < 0.05). This myopia is axial in
nature: The vitreous chamber depth of the experimental eye becomes
significantly larger than its fellow control eye over the period of
deprivation (
Fig. 2 , bottom: 7.34 ± 0.11 versus 7.16 ± 0.08
mm;
P < 0.05). The form deprivation did not produce
any significant change in corneal radius of curvature (3.49 ±
0.03 versus 3.47 ± 0.06 mm;
P = 0.67), anterior
chamber depth (1.86 ± 0.03 versus 1.86 ± 0.02 mm;
P = 0.76), lens thickness (1.82 ± 0.04 versus
1.81 ± 0.03 mm;
P = 0.74), or choroid thickness
(136 ± 12 versus 138 ± 4 μm;
P = 0.83).
All the individual animals deprived of form vision in this study showed
a susceptibility to form deprivation myopia, but they differed in the
magnitude of their response. To indicate the variability in the
refractive errors and axial growth responses between animals,
Figure 3 shows the change in both the experimental and the control eyes for the
five individual subjects plotted as arrows; the tails represent the
measurement at the onset of deprivation, and the heads indicate the
measurement at the end of deprivation. Although there is some
variability in the magnitude of the response, in all cases the
form-deprived eyes were more myopic at the end of the deprivation
period relative to the beginning, and became more myopic relative to
their untreated fellow eyes (
Fig. 3 , top). Similarly, the vitreous
chamber depths of the treated eyes increased more than those of their
fellow eyes (
Fig. 3 , bottom) over the period of deprivation. Note that
at this age there is still some elongation in the untreated fellow
eyes, but the changes are significantly smaller than those for the
experimental eyes (paired
t-test,
P <
0.05).
Although form deprivation in our adolescent marmosets reliably produced
significant axial elongation and myopia, the magnitude of the effect
was reduced compared with similarly deprived younger marmosets
(Figs. 4 and 5) . In the older adolescent animals the change in refractive error at the
end of the deprivation resulted in approximately half as much myopia as
seen in younger animals, although this difference did not reach
statistical significance (
Fig. 4 , top: −3.00 ± 1.49 D, older,
versus −7.35 ± 1.94 D, younger; unpaired
t-test,
P = 0.11). Similarly, the mean interocular difference
in vitreous chamber depth in the older group is significantly smaller
than in the younger group (
Fig. 4 , bottom: 0.18 ± 0.06 mm, older,
versus 0.68 ± 0.13 mm, younger; unpaired
t-test,
P < 0.01). Although the reduced myopia can be at least
partially explained by the fact that refractive error corresponding to
a given change in axial length is reduced in larger eyes due to optical
scaling effects,
14 16 this does not explain the reduced
amount of elongation in the experimental eyes of older animals. The
reduced response to form deprivation in older marmosets occurs despite
these animals having been deprived for a longer duration than the
younger animals (means, 108 versus 83 days). Thus, the age-related
decrease in response might be even greater than suggested by our data.
It has previously been reported that juvenile marmosets do not recover
from the form deprivation myopia induced by lid suture.
7 We found in the present study that animals deprived later in life
similarly do not show recovery from the induced myopia
(Fig. 5) . After
the end of lid suture the treated eyes of both the younger and older
groups of marmosets remained longer than their fellow controls (
Fig. 5 ,
bottom) and, as a consequence, the myopia persisted (
Fig. 5 , top). The
interocular differences at lid opening did not differ significantly
from the interocular differences at the last measurement taken (
Fig. 5 ,
bar insets).
Figure 5 also again shows the relatively reduced response
to lid suture in the older marmosets (right) compared with the younger
ones (left).