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Darkness Causes Myopia in Visually Experienced Tree Shrews

From the Department of Vision Sciences, University of Alabama at Birmingham, Birmingham, Alabama.

PURPOSE. To examine the effect of a period of continuous darkness on the refractive state and vitreous chamber depth of normal light-reared juvenile tree shrew eyes, and to learn whether eyes that developed myopia in response to monocular minus-lens wear will recover in darkness.

METHODS. Starting at 16 days of visual experience (VE), the refractive state of five dark-treatment tree shrews was measured daily to confirm that it was stable and nearly emmetropic. After corneal and ocular component dimension measures, the animals were placed into continuous darkness for 10 days. On removal of the animals from darkness, corneal and ocular component measures were repeated, and daily refractive measures were resumed. The refractive state of the dark-treatment group was compared with that of a normal-lighting group (n = 5) that received standard colony lighting throughout the measurement period. Five dark-recovery animals wore a monocular –5-D lens for 11 days to induce myopia before they were placed into continuous darkness for 10 days.

RESULTS. The animals in the normal-lighting group completed the emmetropization process, stabilizing at approximately (mean ± SEM) 0.7 ± 0.3 D of hyperopia (noncycloplegic refraction, corrected for the small eye artifact) at 60 days of VE. Dark-treatment group eyes shifted toward myopia (mean ± SEM, –4.3 ± 0.5 D) in the dark. The vitreous chamber became elongated by 0.09 ± 0.02 mm relative to normal eyes. Corneal power showed a small, near-normal decrease (1.4 ± 0.3 D). Four of five myopic eyes in the dark-recovery group became more myopic (–2.2 ± 0.9D) in darkness, and all the fellow control eyes shifted toward myopia (–2.8 ± 0.5 D).

CONCLUSIONS. Maintaining emmetropia is an active process. After eyes have achieved emmetropia or have compensated for a minus lens, continued visual guidance is necessary to maintain a match between the axial length and the focal plane or for recovery to occur. Absence of light is myopiagenic in tree shrews that have developed with normal diurnal lighting. This result contrasts with the apparent absence of a darkness effect in tree shrews reared in the dark from before normal eye opening.

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