The Effects of Low Intensity Ambient Lighting on Refractive Development in Rhesus Monkeys (Macacca mulatta)

PURPOSE: Elevated ambient lighting levels protected animals from certain forms of experimental myopia, suggesting that alterations in ambient lighting levels influence refractive development. The purpose of the studies reported in this dissertation was to evaluate the extent to which low ambient lighting influences refractive development in primates and to determine whether and how low ambient lighting levels cause myopias. METHODS: Infant rhesus monkeys (Macaca mulatta) were reared under reduced or “dim” ambient lighting (~50 lux) with either unrestricted vision or one of the monocular lens treatments that induces experimental anisometropias. The development of their refractive errors, corneal powers, and ocular axial dimensions was measured longitudinally and compared to those in monkeys reared under typical laboratory lighting (“normal” light) with the same visual conditions. Their choroidal thickness changes were also longitudinally measured to reflect the activity of refractive regulation. RESULTS: The results showed that (1) dim light did not produce myopia in monkeys reared with unrestricted vision, but increased the variability in refractive error and reduced the likelihood of successful emmetropization; (2) dim light did not increase nor reduce the magnitude of form-deprivation myopia (FDM), but interfered with the refractive development after the discontinuation of form-deprivation and reduced the probability of recovery form FDM; (3) dim light reduced the probability of lens-induced compensating changes, increased the variability in refractive development, and reduced the degree of compensating anisometropias. All refractive observations were associated with alterations in vitreous chamber depth. The failures in developing the anticipated vision-induced anisometropias were associated with an absence of vision-induced relative choroidal thickness changes. CONCLUSIONS: Dim light is not necessarily myopiagenic; however, extended exposure to dim light could cause myopia through reductions in the efficacy of visual mechanisms that normally regulate refractive development.