Avian models of experimental myopia: environmental factors in the regulation of eye growth.

During normal development a precise match between the length of the path of light within the eye and the refractive state of the eye's optical elements is required for the retinal image to remain sharply in focus. Recent studies have shown that this match between eye growth and refractive state is partially under the control of environmental factors. Investigations in several species of ground-foraging birds have shown the existence of an optical mechanism that maintains the retinal image of the ground in focus on the upper retina without a change in accommodation, when the bird is standing erect. Moreover, as a young bird grows, the refractive state changes to maintain a sharp focus of the retinal image of the ground. These findings suggest that the quality of the retinal image may be an important regulator of the matching of refractive state to growth of the ocular globe. We have found in young, rapidly growing birds that the application of optical devices that degrade the quality of the retinal image interferes with the normal regulation of eye growth and results in excessive growth and severe myopia. Additional environmental variables that affect eye growth include nutritional deficiencies of calcium and vitamin D and elevated ocular temperature.