Effects of form deprivation on peripheral refractions and ocular shape in infant rhesus monkeys (Macaca mulatta).

Studies of refractive development, particularly those concerned with the role of vision in the genesis of common refractive errors, have historically focused on the refractive status at the fovea (for a review, see Ref. 1). However, in addition to the increase in radial astigmatism with eccentricity, spherical-equivalent refractive error can vary substantially across the visual field.1–12 The magnitude of these peripheral refractive errors is often sufficient to degrade peripheral visual performance.13,14 There is additional interest in peripheral refraction because peripheral refractive errors may contribute to the development of common axial refractive errors.15–20 In particular, the pattern of peripheral refractive errors may play a role in the onset and progression of myopia. For example, in comparison with children and adults with emmetropia and hyperopia, those with myopia at the fovea typically demonstrate less myopia or more hyperopia in the periphery,4,5,21 and the magnitude of this relative peripheral hyperopia increases with the degree of axial myopia.22,23 It is possible that the association between peripheral hyperopia and central myopia is causal because relative peripheral hyperopia often precedes the onset or progression of central myopia in humans15,17,18 and because experimentally imposed peripheral hyperopic defocus can produce central axial myopia in infant monkeys (Smith EL III, et al. IOVS 2007;48:ARVO E-Abstract 1533). Although peripheral refractions are dependent on anterior segment optics and geometry, variations in the pattern of peripheral refraction as a function of central refractive error are thought to primarily reflect differences in the shape of the posterior globe.23–25 Despite some inconsistencies between studies1 and possible meridional variations within eyes,23 it appears that subjects with myopia exhibit relative hyperopia in the periphery because the posterior poles of their eyes are relatively prolate, whereas the relative peripheral myopia in those with axial hyperopia occurs because their eyes are relatively oblate (Gilmartin B, et al. IOVS 2007;48:ARVO E-Abstract 1215).23 Littleis known about why subjects with myopia and those with hyperopia have differently shaped eyes or how these differences develop over time. It has been argued that some of these differences in posterior globe shape are associated with absolute differences in axial length and are probably a consequence of a preprogrammed genetic growth process.26 Shape differences may also arise during eye growth as a result of mechanical factors21,23,27,28 and/or regional variations in the mechanical properties of the sclera.29,30 It is also possible, however, that differences in posterior globe shape come about as a consequence of selective visual experience operating through local retinal mechanisms10,22,23 (Flitcroft DI. IOVS 2006;47:ARVO E-Abstract 4778) and/or of regional variations in the number, density, or effectiveness of vision-dependent regulatory mechanisms.31,32 Visual experience can have dramatic effects on central refractive errors, and it is reasonable to expect that changes in the eye's optical and axial dimensions responsible for changes in central refraction also affect peripheral refraction. In this respect, it has recently been reported that the more myopic eyes of marmosets with vision-induced anisometropia exhibit relative hyperopia in the temporal field, but not in the nasal field (Totonelly KC, et al. IOVS 2008;49:ARVO E-Abstract 3589). The aims of this investigation were to determine whether the pattern of peripheral refraction and the shape of the posterior globe are influenced by visual experience in young rhesus monkeys and whether the patterns of peripheral refractive errors in macaques with experimentally induced central refractive errors are similar to those observed in humans.