Temporal constraints on experimental emmetropization in infant monkeys

Abstract

Purpose: To characterize the temporal integration properties of the emmetropization process, the authors investigated the effects of brief daily interruptions of lens wear on the ocular compensation for negative lenses in infant rhesus monkeys.

Methods: Eighteen monkeys wore -3 D lenses binocularly starting from approximately 3 weeks of age. Six of these monkeys wore the lenses continuously. For the other animals, the -3 D lenses were removed for four 15-minute periods each day. During these periods, the monkeys viewed through either zero-power lenses (n = 6) or +4.5 D lenses (n = 6). Three monkeys reared with binocular plano lenses and 16 monkeys reared normally served as controls. Refractive development was assessed by cycloplegic retinoscopy and A-scan ultrasonography.

Results: As expected, the group of animals that wore the -3 D lenses continuously exhibited clear evidence of compensating axial myopia. These predictable myopic changes were mostly eliminated by the brief, daily periods of viewing through plano lenses. Interestingly, brief periods of viewing through +4.5 D lenses produced weaker protective effects.

Conclusions: Brief periods of unrestricted vision can prevent the axial myopia normally produced by long daily periods of imposed hyperopic defocus. Thus, the temporal integration properties of the emmetropization process normally reduce the likelihood that transient periods of hyperopic defocus will cause myopia.

Figure 1

Longitudinal changes in the spherical-equivalent refractive errors of the right eyes for infant monkeys in the −3.0 D (A), the −3.0 D/plano (B), and the −3.0 D/+4.5 D lens groups (C). Thin solid lines: data from the right eyes of control monkeys.

Figure 2

Left: longitudinal changes in the spherical-equivalent refractive errors for the different treatment groups. Growth curves were generated using a locally weighted, nonlinear-smoothing algorithm. Right: spherical-equivalent refractive errors for treated animals at the end of the treatment period and for control animals at equivalent ages. Filled bars: Mean ± SD. Different symbols represent individuals in the different treatment groups.

Figure 3

Left: longitudinal changes in vitreous chamber depth for the treatment groups. Growth curves were generated using a locally weighted, nonlinear-smoothing algorithm. Right: spherical-equivalent refractive error for each monkey is plotted as a function of vitreous chamber depth. Monkeys from different treatment groups are represented by different symbols, as shown in the legend. Solid line: linear regression values.