Retinal-image degradation produces ocular enlargement in chicks

Two groups of 3-day-old broiler chicks were fitted over the right eye with plastic goggles that degraded the retinal image by producing spherical aberration, reduction of contrast, and blurring of edges. In one group of chicks, the goggles were hemispheric and affected the entire visual field. In the second group, the goggles only affected the frontal visual field. In both groups, the left eye was used as a control. A third group of chicks served as an untreated control group. After 3 weeks, the chicks were killed, the eyes removed, hemisected, and measured in the axial and equatorial planes. Because the avian eye is flattened in the equatorial plane, both sets of measurements are necessary in order to represent changes in its size and shape. The axial length of a laterally located avian eye is related to distance vision. The equatorial length is related to vision in the frontal field, which is used for near vision. For each group of chicks, the difference between the right eye and the left eye was compared. The treated eyes of the full- goggle group were significantly larger than their untreated eyes in both the axial and equatorial dimensions. The treated eyes of the frontal- goggle group differed only in the equatorial dimension. These results suggest that the increased equatorial length found after retinal-image degradation may serve as an animal model of myopia.