Darkness Causes Myopia in Visually Experienced Tree Shrews

Studies of animal models of human juvenile-onset myopia have shown that the axial length of the eye is actively matched to the eye’s optical power through a visual feedback mechanism (for recent reviews, see Refs. 1-5). When normal tree shrews open their eyes at about 3 weeks of age, they are over 20 D hyperopic, and the axial length is approximately 85% of the adult value.6 As the vitreous chamber depth increases rapidly over the first 15 days of visual experience (VE) the hyperopia decreases and, by 60 days of VE, the eyes stabilize close to emmetropia.7 This process is visually guided. If a concave (minus-power) lens is placed in front of an eye that has nearly completed the emmetropization process, the emmetropization mechanism causes the eye to increase its elongation rate until the retina is moved to the appropriate location so that it again receives focused images. After “minus lens compensation” occurs, the elongation rate returns to normal for that stage of postnatal development.8 Another way to induce axial elongation and myopia is to use a translucent diffuser (form deprivation) to eliminate high-spatial-frequency images and drastically reduce the contrast of all retinal images. In this instance, the axial elongation rate continues to be elevated if the diffuser is left in place because, in contrast to the situation with a minus lens, there is never a signal to indicate that the retina has reached a shifted focal plane.8,9 Form deprivation is ineffective in very young tree shrews, less than approximately 15 days of VE, possibly because the retinal signaling mechanisms are immature.6,9,10 After myopia has been induced, if the diffuser or minus lens is then removed, a “recovery” process begins; the eye remains nearly the same length but the optics mature, moving focused images back to the retina.8,10,11 This process has been reported to be visually guided (Amedo AO et al. IOVS 2005;46:ARVO E-Abstract 1977).12,13 However, McFadden et al.14 reported recently that, after 3 days in complete darkness, guinea pigs recovered from myopia induced either with form deprivation or a minus lens. This raised the possibility that recovery may not require visual cues and might be guided by another signal, related in some way to the eye’s being elongated beyond its normal axial length. The effect of removing all visual stimuli by placing young animals in the dark has been found to affect refractive development in some species more than in others. Raising monkeys in the dark for many months had no apparent effect on refractive development in one study.15 In another study in which shorter periods were used, darkness prevented normal emmetropization, so that the animals generally remained hyperopic.16 In tree shrews, a long-duration study (McKanna JA et al. IOVS 1983;24:ARVO Abstract 56, p 226) in which dark rearing began before eye opening and continued to adulthood (6-8 months) found no effect of darkness on refraction and axial lengths. In chicks, however, rearing from near hatching in the dark has consistently produced elongated, hyperopic eyes.17-22 The increased axial length is due to vitreous chamber elongation; the hyperopia occurs because there is substantial flattening of the cornea. In the present study, the effect of darkness was examined after tree shrews had nearly completed the emmetropization process in the standard lighting of an animal colony. Their refractive state was measured for several days to establish that their refractions were stabilizing near emmetropia before they were placed in darkness for 10 days and then remeasured. In addition, a group of animals was made myopic in one eye by wearing a monocular -5-D lens for 11 days. They were then placed into darkness for 10 days to learn whether recovery would occur without visual guidance.