Dark-rearing interference with emmetropization in the rhesus monkey.

Dark rearing has been shown to protect against the development of lid-suture myopia in monkeys and tree shrews. Dark-reared monkeys and cats, with or without lid suture, are significantly hyperopic in comparison to light-reared controls. The time course of refractive change during dark rearing has only been systematically investigated in chicks, with hyperopia increasing from 14 to 42 days after hatching. Longitudinal refractions of dark-reared monkeys have not been reported previously. Five infant rhesus monkeys were dark reared with their mothers from the first day of life until 58 to 161 days of age. Cycloplegic retinoscopies were performed at 2-week intervals and were compared with cross-sectional data from 18 normal monkeys at ages 1 to 81 days. The normal monkeys typically had hyperopic refractions from +4 to +8 diopters at birth with an average refraction of +2.8 diopters between 30 and 81 days of age, compared with an average refraction of +5.3 diopters between 30 and 81 days of age for the monkeys raised in darkness (difference significant at P less than 0.05). Three of the dark-reared animals retained an average of 7.0 diopters of hyperopia. Darkness thus slowed or interrupted the normal loss of hyperopia in three of the five experimental subjects, and may be useful for creating model hyperopic animals on the order of +5 to +8 diopters.