The relationship between image degradation and myopia in the mammalian eye

Background: In all species studied, myopia develops if the eye is deprived of detailed vision during development (form deprivation myopia). However, different degrees of spatial image deprivation produce different effects and have not been described in the mammalian eye. Therefore, the effect of image degradation on guinea pig emmetropisation was investigated. Methods: Eighty-one guinea pigs wore a treatment on one eye from 6 to 13 days of age. There were four treatments: a translucent diffuser (no lines or edges were visible through the diffuser); one of five Bangerter foils (BF: 0.8, 0.6, 0.4, 0.2, light perception only), which differed in their cut-off spatial frequencies; a 'ring mount' control with no filter; or one of two neutral density filters that reduced luminance only (ND, optical density grades 0.1 and 0.6). Refractive error and ocular elongation were measured after seven days of treatment. Results: The extent of induced myopia and ocular growth were related to the amount of image degradation (mean difference between the treated and untreated eyes changed in a graded manner -7.0 D to -0.2 D and from 85 mu m to seven mu m respectively, for spatial frequency cut-offs between zero and 24 cycles per degree). Corresponding reductions in luminance from ND filters did not increase eye growth and caused significantly less myopia than the BFs that caused a similar luminance decrement. The greatest myopia occurred when no or limited spatial information was available to the eye, but moderate myopia still occurred with spatial frequency cut-offs of six and 12 cycles per degree, well beyond the visual acuity range of guinea pigs. Conclusion: Excessive ocular growth and myopia are most robust when induced by spatial frequency reductions within the visual acuity range but can also be induced beyond this. Either the mechanism of ocular growth can detect supra-threshold spatial frequencies, possibly due to aliasing, or it is sensitive to small amounts of contrast degradation.