Experimental myopia in chicks: ocular refraction by electroretinography.

Application of devices that degrade the retinal image has been reported to produce enlargement of the ocular globe in young domestic chicks. Two such device types (domes and arches) were applied to 3-day-old chicks. The domes affected the entire visual field whereas the arches affected only the lateral field. A third group wore a thin circumorbital ring to control for possible mechanical impediments to growth. Untreated control chicks comprised a fourth group. At ages ranging from 3 to 7 wk, the chicks were refracted in their lateral visual fields with a Maxwellian view optometer based on Scheiner's principle, which yields an objective assessment of the refractive state of the photoreceptor image plane. One to seven measurements were taken from each of 48 urethane-anesthetized chicks. These indicated that the mean refractive states of the untreated eyes and the ring eyes were -0.20 D and -0.19 D, respectively, which did not differ significantly from emmetropia. In contrast, the mean refractive states of the arch eyes and the dome eyes were -4.11 D and -14.88 D, respectively, which differed significantly from emmetropia and from each other. The results indicate that early retinal image degradation can result in the relatively rapid development of a substantial myopia in these experimental animals.