Although we did not find any evidence for the macrophthalmous
commonly found in light-reared chickens, several observations in
individual monkeys suggest that continuous light exposure may have
affected the efficiency of emmetropization. First, one light-reared
monkey (ROG7,
Fig. 5 ) briefly manifested a low myopic refractive error
(−0.5 D at 3 months of age) and thereafter exhibited essentially
emmetropic refractive errors that were clearly below the levels of
hyperopia found in normal monkeys. This is potentially significant
because myopia is very rare in young monkeys.
20 In rhesus
monkeys, the emmetropization process is largely complete by 4 or 5
months of age.
5 20 At these ages none of the 23 normal
monkeys that we followed longitudinally had hyperopic errors below+
0.87 D of hyperopia. However, it is important to note that this myopic
animal did not exhibit an increase in either overall axial length or
vitreous chamber depth, the most common ocular changes associated with
myopia. Instead, this animal’s refractive error was associated with a
relatively steep cornea (filled triangles,
Fig. 2 ). For example, at 124
days of age, when this animal showed the largest relative myopic error,
its corneal radius of curvature was shorter than that for any of the
age-matched, control monkeys. Second, beginning at approximately 100
days of age, monkeys XAV and PAR developed persistent axial
anisometropias. XAV’s anisometropia, which was larger than any
interocular refractive-error difference observed in normal monkeys (up
to 1.62 D, see
Figs. 4 and 6 ), was maintained for the remainder of the treatment period but
eventually did resolve several months after the animal was returned to
a normal lighting cycle. PAR’s anisometropia was relatively small
(between 94 and 171 days; average, 0.74 D; range, 0.5–0.94 D), but it
also persisted for the rest of the light-rearing period. The
significance of these findings is not clear cut. Control animals
occasionally exhibit anisometropias of approximately 1 D; however,
as shown in
Figure 6 these interocular differences in refractive error
are typically transient and regress between measurement sessions. It is
also important to note that of the nine light-reared monkeys, XAV had
the highest degree of hyperopia at the start of the treatment period.
In this respect, we have previously observed that infant monkeys
frequently develop significant anisometropias in response to high
degrees of optically imposed hyperopia or when they are recovering from
high degrees of experimentally induced hyperopia.
5 26 Consequently, it is possible that XAV’s anisometropia was due to its
high initial hyperopic refractive error rather than exposure to
continuous light.