Optical causes of experimental myopia

Experiments in which chicks are reared wearing a translucent goggle, or some similar device designed to degrade the retinal image, usually result in the induction of a significant degree of myopia. The induced myopia is due to enlargement of the vitreous chamber of the eye. Despite extensive change in the size, shape and refractive index distribution of the crystalline lens, its refractive power is static in the embryonic and early chick eye. The contribution to myopia of the cornea is uncertain; some studies have indicated either an increase or a decrease in corneal radius of curvature while others found no change. The fact that experimental myopia can be produced in chicks even when the optic nerve has been cut or when the degraded retinal image is restricted to one sector of the eye suggests that accommodation is not involved. Nevertheless, when the retinal image is degraded by being defocused with convex or concave lenses, myopia (concave lenses) or hyperopia (convex lenses) results. Chicks wearing concave or convex soft contact lenses from the day of hatching develop refractive states equal to the lens power (+8 and -10 diopters) within one week. The ability of the eye to vary its refractive development according to the sign of defocus suggests a role for accommodation. However, study of the ciliary muscle and accommodative apparatus of myopic and emmetropic chick eyes does not reveal any morphological differences that might indicate that the myopic eye had experienced an increased level of accommodation.