Mathematical model of emmetropization in the chicken.

Recently a number of observations were made on refractive development in chickens and monkeys under various experimental treatments. Degradation of the retinal image by use of occluders produces myopia, although with a high variability in the individual refractions. This kind of image degradation myopia can be induced in the absence of accommodation and with the optic nerve sectioned. In chickens, recovery occurs both in normal birds and in operated birds in which accommodation has been eliminated by lesions in the Edinger Westphal nucleus. In addition, it was shown that, in the chicken eye, growth compensates for an imposed refractive error induced by spectacle lenses. A compensatory change in growth can also be induced in the absence of accommodation. Thus the role of accommodation in the refractive development must be explained. Here we develop a minimal model for regulation of eye growth that provides a possible explanation for all the above observations. A major conclusion is that the presence of two independent feedback loops must be assumed, one dependent on accommodation and one on a local mechanism detecting blur in the retina.