Continuous ambient lighting and eye growth in primates

PURPOSE. TO determine the effect of continuous light exposure on ocular gro wth and emmetropization in infant monkeys. METHODS. Nine infant rhesus monkeys were reared with the normal vivarium li ghts on continuously. The 24-hour light cycle was initiated between 1 and 4 weeks of age and maintained for 6 months. The ocular effects of continuous light were assessed by cycloplegic retinoscopy, keratometry, and A-scan ul trasonography. Longitudinal control data were obtained from 23 normal infan ts that were reared with an illumination cycle that included defined light and dark phases (either 12-hour light:12-hour dark or 8.5-hour light: 15.5 hour dark). RESULTS. In contrast to previous studies involving light-reared chickens, n o monkeys exhibited exaggerated ocular growth. There were no significant di fferences between treated and control monkeys in corneal radius, overall ey e size, or the axial dimensions of individual ocular components. At the end of the treatment period, eight of the nine experimental monkeys also exhib ited the moderate hyperopic errors(range, +1.5 to +3.4 D) that are typicall y found in normal animals. Aspects of emmetropization were, however, unusua l for three monkeys. One monkey manifested a -0.50 D myopic error that was associated with an abnormally steep cornea but had normal axial lengths. Tw o additional monkeys developed persistent axial anisometropias. CONCLUSIONS. In infant primates constant light exposure does not promote th e constellation of ocular changes (in particular corneal flattening, a decr ease in anterior chamber depth, and an increase in vitreous chamber depth) that has been observed in light-reared chickens. The slight variations from the expected developmental sequence observed in three infants may reflect individual differences. However, it is also possible that aspects of the em metropization process may not operate as effectively under constant light a s they do under an ordinary light/dark cycle.