Role of Accommodation and Developmental Aspects of Experimental Myopia in Chicks
We have previously shown that extreme axial myopia (often more than 40 D) could be produced by raising chicks with their field of view restricted to the frontal visual field.
Since animals raised with their vision restricted to the lateral visual field do not become myopic, it is the specific visual experience that is important. Since chicks use the frontal visual field for near vision and the lateral for distance vision, it is possible that the amount of near vision is the relevant variable.
We have now completed a series of developmental studies on this experimental myopia. The results can be summarized as follows: (1) The myopia can be reversed, in most subjects, by removing the visual field restriction. (2) This recovery process is extremely rapid for young animals, but much slower for older animals. (3) The experimental myopia develops very rapidly, being substantial at one week of age and maximal at two weeks. (4) The myopia shows some spontaneous reversal even with the visual field restrictors still in place. This result seems consistent with a hypothesis implicating accommodation in the etiology of myopia, since severely myopic animals would presumably have little need to accommodate, and thus the myopia would decrease. (6) Normal chicks have rather variable refractions at hatching. These change in the direction of emmetropia over approximately the same time period that the myopia develops in the experimental chicks.
Taken together, these studies argue that although this experimental myopia, and the recovery from it, can both occur over a substantial period of the animals development, the extent of susceptibility to both processes changes greatly as the animal develops. The similarity in the developmental time course of the refractive changes in normal and myopic chicks argues that the experimental myopia may be the result of the same developmental regulatory mechanisms that normally cause the eye to grow toward correct refraction.
Longitudinal studies of the anatomical changes associated with the experimental myopia and recovery from it are in progress.
KeywordsTree Shrew Corneal Curvature Ciliary Ganglion Corneal Power Refractive Change
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