We have developed a lightweight plastic goggle with rigid contact lens inserts that can be applied to the eyes of newly hatched chicks to explore the range and accuracy of the developmental mechanism that responds to retinal defocus. Convex and concave lenses of 5, 10, 15, 20 and +30D were applied to one eye on the day of hatching. The chick eye responds accurately to defocus between -10 and +I5D, although hyperopia develops more rapidly than myopia. Beyond this range there is first a levelling off of the response and then a decrease. The resulting refractive errors are caused mainly by increases and decreases in a.xial length, although high levels of hyperopia are associated with corneal flattening. If + 10 D defocusing lenses are applied nine days after hatching the resulting myopia and hyperopia are equal to about 80% of the inducing power. After one week of inducing myopia and hyperopia with ± !0 D lenses, the inducing lenses were reversed. In this case, the refractive error did not reach the power of the second lens after another week of wear. Instead, astigmatism in varying amounts (0-12 D) was produced, being greater when reversal was from plus to minus. Finally, astigmatism can also be produced by applying 9 D toric inducing lenses on the day of hatching. The astigmatism produced varies from 2 to 6 D, and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be , primarily due to corneal toricity. Furthermore, the greatest magnitude of astigmatism was produced when the piano meridian of the inducing lens was placed 45" from the line of the palpebral fissure.

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This study investigated the relationship between retinal dopamine and lens induced refractive errors in chicks by high performance liquid chromatography with ultraviolet detection (HPLC-UV). After two weeks of lens wear, the chick eyes treated with +10D lenses were hyperopic (+8.29 +/- 0.43D), while the eyes treated with -10D lenses were myopic (-11.69 +/- 0.74D). At the same time, in myopic eyes the level of retinal dopamine and its metabolite 3,4-dihydroxy-phenylacetic acid (DOPAC) were reduced compared to control eyes, while in hyperopic eyes the level of retinal dopamine and DOPAC were increased as compared with control eyes. Therefore, retinal dopamine may participate in the development of lens induced refractive errors in chicks.

Light-weight translucent plastic goggles with convex or concave rigid contact lens inserts were applied unilaterally to the eyes of young chicks. Convex and concave cylindrical lenses produced astigmatic refractive errors. The magnitude of the induced astigmatism was less than that of the inducing lens and varied with axis orientation. Decreased aperture size or interruption of the defocus resulted in a decreased response to refractive defocus. Slit apertures and spherical defocus produced variable amounts of myopia, hyperopia and astigmatism. Choroidal changes (increased thickness) were observed only in birds developing hyperopia or recovering from myopia.

Ocular dimensions and refractive state data for chicks 0 to 14 days of age were obtained from 234 untreated control eyes of birds treated unilaterally in previous work involving various defocussing lenses and/or translucent goggles. Refractive state and corneal curvatures were measured in vivo by retinoscopy and ophthalmometry respectively. Intraocular dimensions were measured by A-scan ultrasonography, after which the eyes were removed, weighed and measured. In some cases (n = 52) intraocular dimensions and lens curvatures were obtained from frozen sections of enucleated eyes. The hyperopia of hatchling chicks (+6.5 +/- 4.0 D) initially decreases rapidly and then more gradually to +2.0 +/- 0.5 D by 16 days. The distribution of refractive errors is very broad at Day 0, but becomes leptokurtotic, with a slight myopic skew, by Day 14. Corneal radius is constant for the first four days, possible as a result of pre-hatching lid pressure, and then increases linearly, as do all lens dimensions, axial diameter and equatorial diameter. Schematic eyes were developed for Days 0, 7, and 14.

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