Purpose. Chicks emmetropize accurately to experimentally induced myopic and hyperopic defocus. The authors investigated the emmetropization response when a specific proportion of the retina was exposed to myopic defocus while the remainder was exposed to (competing) hyperopic defocus. Methods. Normal chicks (14-15 days old) were fitted monocularly with a "lens-cone" device that exposed a specific proportion of the available visual field to a high-contrast grating under 10 diopters (D) of myopic defocus (with accommodation relaxed) in a series of patches. The remainder of the visual field (adjacent patches) viewed a grating under 10 D of hyperopic defocus. Groups of chicks wore a lens-cone device designed to provide a "spatial ratio" (relative proportion of visual field area) of 100:0, 50:50, 40:60, 33:67, 25:75, or 0:100 myopic versus hyperopic defocus. On-axis ocular refraction and axial ocular component dimensions were assessed after 3 and 6 days of cone wear. Results. Interocular differences in refraction (mean ± SD) at day 6 were as follows: +10.4 ± 2.5 D, +7.6 ± 3.6 D, +5.9 ± 3.7 D, +1.6 ± 2.6 D, -2.4 ± 2.7 D, and -8.9 ± 2.6 D for spatial ratios of 100:0, 50:50, 40:60. 33:67, 25:75, and 0:100 respectively. The corresponding interocular vitreous chamber depths were as follows: -515 ± 135 μm, -447 ± 137 μm, -253 ± 220 μm, -105 ± 252 μm, 230 ± 218 μm, and 592 ± 161 μm. The refraction and biometry results for the 33:67 and 25:75 groups were significantly different from those of the single defocus control groups. Conclusions. In chicks, the on-axis emmetropization response was weighted according to the spatial ratio. Thus, as the proportion of retinal area receiving myopic defocus increased relative to that receiving hyperopic defocus, the degree of myopic eye growth was reduced.
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