Inducing myopia, hyperopia, and astigmatism in chicks.

Myopia and hyperopia have been produced in chicks by applying specially designed convex and concave soft contact lenses to the eyes of newly hatched birds. After 2 weeks of wear, the eyes develop refractive states equivalent in sign and amount (+8 and -10 D) to the lens used. However, the lenses produce an artificial hyperopic shift during the first week of wear due to corneal flattening. We have developed a new approach involving the use of goggles with hard convex and concave contact lens inserts placed between the frontal and lateral visual fields. Myopia and hyperopia (+10 and -10 D) can be produced within days (4 days for hyperopia and 7 days for myopia) if the defocus is applied from the day of hatching. We can also produce significant amounts of astigmatism (1 to 5 D) axis at 90 degrees and 180 degrees by using cylindrical contact lens inserts. Although these last results are preliminary, they suggest that accommodation is not likely involved at this stage of refractive development because we do not believe that the accommodative mechanism can cope with cylindrical defocus. All spherical refractive errors produced using the goggle system appear to result from alterations in vitreous chamber depth.