Limitations of the Emmetropization Process in Infant Monkeys

Summary

The purpose of this study was to determine the relationship between the effective operating range for the emmetropization process and the refractive errors that consistently produced amblyopia. Infant rhesus monkeys (Macaca mulatta, n = 46) were reared with Positive or negative lenses over one or both eyes. Retinoscopy and A-scan ultrasonography were used to evaluate the effect of optically induced changes in refractive state on ocular development. For representative monkeys, spatial contrast sensitivity functions were measured for each eye using a behavioral psychophysical paradigm. For low-powered (± 3 D), but not high-powered, anisometropic lenses (≥ 6 D), infants exhibited differential interocular axial growth rates that eliminated the lens-induced imbalance between the two eyes. Infants that failed to compensate for the induced anisometropia developed amblyopia in their non-fixating eyes. However, by combining anisometropic lenses with an alternating occlusion regimen that reduced the likelihood of amblyopia, it was possible to extend the emmetropization mechanism’s operating range. Bilateral equal-powered lenses that resulted in effective refractive errors between about −2 and +8 D predictably altered axial growth and the course of emmetropization. However, within this range, infants with high degrees of bilateral, hyperopia frequently developed anisometropia and in some cases amblyopia. Anisometropic refractive errors that exceed the effective operating range for emmetropization consistently produce amblyopia, which supports the idea that anisometropia causes amblyopia. However, factors associated with the development of early sensory and/or oculomotor anomalies also influence the ability of the eye to respond to optical defocus and can lead to anisometropia.