Ocular Refractive Development Affects Skull (Orbital) Development

Abstract

Experiments carried out over the last two decades have shown that it is possible to induce refractive errors in the eyes of young animals by distorting early visual experience. Because of their precocial nature and the fact that they grow and develop rapidly, domestic chicks (Gallus gallus domesticus) have been used extensively in this research. Earlier work involved depriving the developing eye of clear form vision, either by suturing the eyelids together or by mounting a translucent occluder over one eye (Lauber and Oishi, 1987; Pickett-Seltner et al., 1988). This treatment invariably leads to an axial elongation of the eye and myopia (near-sightedness). More recently, it has been shown that it is possible to induce both myopia and hyperopia (far-sightedness) in chicks by defocussing the retinal image of the developing eye with convex and concave spectacle lenses (Schaeffel et al., 1988). The range of refractive errors induced was extended through the use of lightweight contact lenses mounted over the treated eye (Irving et al., 1992). Basically, a concave lens simulates the condition of hyperopia, causing a compensatory increase in eye growth with the result that the eye is myopic when the lens is removed. The opposite occurs (ie. a slowing of eye growth) with convex lenses. Experiments have shown that newly hatched chicks will compensate accurately (within 4 to 7 days) to defocus of between -10 and +15 diopters (Irving et al., 1992). Astigmatic refractive states can also be induced with cylindrical defocussing lenses (Irving et al., 1995). The astigmatic effect displays a meridional sensitivity and is due to induced corneal astigmatism, possibly coupled with some lenticular astigmatism. While these studies have addressed changes in refractive state and dimensions of the eye, virtually no attention has been directed to the effects of these changes on the surrounding orbital tissues, notably the bones of the orbit. In the following report, we demonstrate that, in the chick, ocular growth and development is coupled to growth and development of the orbits.