How Is Emmetropization Controlled? Results of Research on Experimental Myopia

Summary

The mechanisms by which emmetropia might be reached in growing eyes is discussed. Although emmetropia might result from normal developmental processes causing eyes to be a certain shape, the preponderance of evidence, especially from the accurate compensation for varying degrees of hyperopic and myopic defocus imposed by spectacle lenses, argues that the visual responses of the eye guide it toward emmetropia by means of a feedback mechanism. Little is known about the details of this control of eye growth, or what the error signal might be that indicates whether the eye is myopic or hyperopic, but there are clues that restrict the possibilities. The hypothesis that compensation is accomplished simply by the rate of ocular elongation being determined by the average amount of blurred or sharp vision, making lens compensation a special case of deprivation myopia, is not supported by most evidence available at present.