Effects of brief periods of unrestricted vision on the development of form-deprivation myopia in monkeys

PURPOSE. To characterize the temporal integration properties of the mechanisms responsible for form-deprivation myopia (FDM), the effects of brief daily periods of unrestricted vision on the degree of FDM were investigated in infant monkeys. METHODS. Starting at approximately 3 weeks of age, unilateral form deprivation was produced in 24 infant rhesus monkeys by securing a diffuser spectacle lens in front of one eye and a clear, zero-powered lens in front of the fellow eye. During the treatment period (17 ± 2 weeks), six infants wore the diffuser lenses continuously. In the other experimental infants, the diffuser lenses were removed each day and replaced with clear, zero-powered lenses for 1 (n = 7), 2 (n = 7), or 4 hours (n = 4). Refractive development was assessed by retinoscopy and A-scan ultrasonography. Control data were obtained from 11 normal infants and 3 infants reared with zero-powered lenses over both eyes. RESULTS. The degree of FDM varied significantly with the duration of unrestricted vision. Continuous form deprivation produced -5.2 ± 3.6 D of relative axial myopia. However, 1 hour of unrestricted vision was sufficient to reduce the degree of axial FDM by more than 50% (-1.7 ± 3.2 D). The infants that were allowed 4 hours of unrestricted vision exhibited only -0.4 ± 0.5 D of FDM. CONCLUSIONS. As observed in chickens and tree shrews, relatively long periods of form deprivation can be counterbalanced by quite short periods of unrestricted vision. These results indicate that the processes or signals that promote axial elongation in monkeys are comparatively weak or easily overridden by factors that slow ocular growth.