Jacob G. Sivak

Refractive plasticity of the developing chick eye.

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 + 15D. 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 axial 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 ±10 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.

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° and 180° 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.

Contribution of the cornea and lens to the spherical aberration of the eye

Abstract Spherical aberration of the whole eye of the crystalline lensin vivo and of aphakic eyes, has been measured. The results showed that the aberration of the lens does not systematically neutralize that of the cornea or vice-versa.

Spherical aberration of the crystalline lens

Split beams of varying separations from a helium-neon laser were directed through the crystalline lenses of a number of vertebrates. Photographs of the focal effects indicate the extent to which the refractive index variation of the lens and lens shape control spherical aberration. Of the fish studied, only rock bass lenses are relatively free of spherical aberration. Both goldfish and yellow perch exhibit substantial amounts of positive spherical aberration. Varying amounts of negative spherical aberration are present in frog, juvenile duck, dog and rat lenses. Positive and negative spherical aberration is found in human and cat lenses while cow, pig, lamb and rabbit lenses are almost free of aberration.

Experimentally induced myopia in chicks: Morphometric and biochemical analysis during the first 14 days after hatching

Application of a translucent goggle over the chick eye on the first day after hatching led to the development of myopia. By the 14th day, the mean refractive error was about -10.0 D. Significant increases in axial and equatorial diameters were observed when the treated eyes were compared with untreated contralateral eyes. The lens did not appear to be affected, either optically or biochemically. A temporal study showed that changes were evident within 2 days of goggle application, and were significantly established 5 days later. Total soluble protein concentrations of the treated and untreated eyes were not significantly different, nor were the dry weights of the sclera and cornea. The enlargement of the eyeball that was observed in the experimental induction of myopia seems due to an increase in fluid within the eye. The data are consistent with the view that refractive properties of the chick eye are dependent upon the clarity of the visual image and modulation of these features occurs after hatching.

Chromatic dispersion of the ocular media.

Measurements of chromatic dispersion of aqueous and vitreous humors, cornea and lens of the eye are sparse and incomplete. The wavelength variation in refractive index of the ocular media of cow, pig, frog (Rana pipiens), chicken, rock bass (Ambloplites rupestris), albino rat and cat as well as human lenses was determined by means of Abbe and Pulfrich refractometry. While the humors are somewhat less dispersive than water, the cornea is more dispersive at short wavelengths. In general, the lens is significantly more dispersive than water with dispersion increasing asymptotically at the blue end of the spectrum. The exaggerated dispersion taking place at short wavelengths should be taken into account in calculations of chromatic aberration.

Inducing ametropias in hatchling chicks by defocus-aperture effects and cylindrical lenses

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.

Retinal dopamine and lens-induced refractive errors in chicks.

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.

Longitudinal chromatic aberration of the vertebrate eye

A recent study involving Abbe and Pulfrich refractometry analyses the dispersion of the human lens and the ocular media of a number of vertebrates. In general, the lens and, to a lesser extent, the cornea, are more dispersive than expected at wavelengths below 500 nm. The dispersion findings of this study were used in conjunction with reduced eye parameters of a number of vertebrates to calculate the longitudinal chromatic aberration of rock bass, frog, chicken, rat, cat, pig, cow, and human eyes. The calculated chromatic aberration of the human eye is greater than values reported earlier, because of the exaggerated dispersion of the lens at short wavelengths. While the values calculated for the additional species studies may be larger in some instances than expected, presumably due to lens dispersion as well, chromatic aberration is not large enough to account for the hyperopia found by retinoscopic study of small eyes.

Mechanisms of accommodation in the bird eye

SummaryKeratoscopic study of corneal curvature before and after accommodation in two common bird species failed to provide evidence of a corneal accommodative mechanism. Accommodative changes in refractive state measured retinoscopically are presumably brought about by the effect of ciliary muscle contraction on lens curvature. However, retinoscopic and freeze-sectioning study of accommodation in diving ducks supports the long suspected existence of an iris accommodative mechanism capable of producing dramatic changes in lens curvature. This mechanism is believed to be a means of compensating for the refractive loss of the cornea in water.