Dissertation

The visual control of eye growth in chicks

Troilo, David Bruce

1989

Overview

The growth of the vertebrate eye achieves a close match between the power of the optics and the length of the eye with the result that images are focused on the retina (emmetropia). The possibility that vision is required for the feedback regulation of eye growth was studied experimentally using domestic chicks (Gallus domesticus) as subjects. The approach was to produce either myopia (nearsightedness) or hyperopia (farsightedness) by manipulating the chick's visual experience just after hatching. After discontinuing the manipulations, the ability of the eye to correct the refractive errors was determined in intact chicks, and in chicks with a severed optic nerve (to test the role of the brain in eye growth regulation) or with lesions of the Edinger-Westphal nucleus (to test the role of ocular accommodation in eye growth regulation). Chicks recovered quickly from induced myopia or hyperopia mainly by adjusting the growth of their vitreous chambers--growth decreased in eyes correcting for myopia and increased in eyes correcting for hyperopia. Because the hyperopic eyes were already larger than normal controls these results indicate that refractive error, rather than eye size per se, guides the growth of the eye toward emmetropia. This growth compensating for induced refractive errors was found in chicks despite either optic-nerve-section or Edinger-Westphal lesion. The treatments differed in that Edinger-Westphal-lesioned chicks attained emmetropia, whereas optic-nerve-sectioned eyes did not, but instead overshot emmetropia and reversed the sign of the initial refractive error. Together these results suggest that: (1) The eye itself can sense the sign of a refractive error and adjust growth accordingly. (2) Accommodation is not necessary for emmetropization. (3) For emmetropia to be achieved an intact optic nerve is required, suggesting that visual pathways in the brain are involved. (4) Adjustment of refractive state by local ocular growth mechanism is probably different from that of the brain-mediated mechanism. Other experiments presented in this dissertation describe the role of cornea and lens in accommodation in chicks, and the local nature of myopia produced by partial deprivation of the visual field.

• Publisher: ProQuest Dissertations & Theses
• Subject: Biology / Health / Health sciences / Neurology / Neurosciences / Veterinary services
• ISBN: 9798207012964