Early abnormal visual experience induces strabismus in infant monkeys

We measured ocular alignment in the horizontal direction for 17 monkeys reared under deprivation paradigms that involved monocular defocus, monocular occlusion and optically corrected aphakia coupled with continuous or partial occlusion of the fellow eye. Alignment was measured at 3 and 7 months with a photographic corneal light reflex method. Results showed that a majority of the monkeys in each paradigm developed strabismus following deprivation rearing, the common factor being early abnormal visual experience. Results also indicated a trend in which many of the deviations seen at 3 months of age were exotropic while all of the animals with deviations at 7 months of age were esotropic. These results on deprivation-induced strabismus, which are the first reported in monkeys, are consistent with previous findings in cats and humans, providing further evidence that deprivation affects not only sensory, but motor systems as well. These findings provide evidence that infant monkeys are a good model for studies of the possible relationships between amblyopia and strabismus that are often noted in children with early visual deprivation. Furthermore, it raises the prospect that some of the findings in previous animal studies that have been attributed to the direct effects of deprivation may actually be secondary to the induced misalignment.