Local ocular compensation for imposed local refractive error.

Chicks were raised in a low-ceiling environment to find out if their eye growth could compensate for locally imposed hyperopic refractive errors. These chicks became selectively more myopic in the upper visual field than chicks raised in a high-ceiling environment. The vitreous chamber in the low-ceiling birds showed a selective elongation in the ventral region that was not seen in the eyes of the high-ceiling birds. This morphological difference was small, but probably adequate to account for the additional myopia in the low-ceiling birds. These results are consistent with the idea of a visually mediated growth mechanism regulating the local refractive state across the entire visual field so that it matches the customary viewing conditions. Such a mechanism might account for the finding of Fitzke, Hayes, Hodos, Holden and Low (Journal of Physiology, London, 369, 33-44, 1985) that the refractive errors in the lower field are exactly appropriate for focusing the image of the ground on the retina.