Temporal constraints on experimental emmetropization in infant monkeys.

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.