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F. J. Rucker, A. R. Garzon, J. Wallman; Longitudinal Chromatic Aberration May Guide Eye Growth in Chicks. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1037. doi: https://doi.org/.
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Longitudinal chromatic aberration (LCA) causes short wavelengths to be focused in front of long wavelengths. Ocular accommodation evidently uses this chromatic signal because simulating alternating myopic and hyperopic blur by manipulating the relative contrast of the red, green and blue components of a spatial sine-wave grating causes accommodation to follow the simulated defocus. We asked whether chick eyes exposed to static gratings simulating the chromatic effects of myopic or hyperopic defocus would "compensate" for the simulated defocus.
We exposed one eye of each chick to a 5 cycle/deg grating simulating myopic defocus (image focused in front of retina; hence, red contrast high, blue contrast low) and the other eye to a grating simulating hyperopic defocus (blue contrast high, red contrast low) by placing the chick in one drum with one eye covered and then in the other drum with the other eye covered. To minimize real defocus from the drum, all eyes wore +3 D lenses. To minimize the effects of altered eye-growth, we studied only the earliest responses: first, we measured changes in choroidal thickness 45 min after one 15-min episode in the drum, then we measured glycosaminoglycans (GAG) synthesis in sclera and choroid (by the incorporation of labeled sulfate in tissue culture) after a day of four 30 min episodes in the drum.
After one episode of viewing the gratings, choroidal thickness increased twice as much in the eye viewing the myopia-simulating grating than in the eye viewing the hyperopic one (55 vs 25 µm change; p<0.05; greater in 10 of 13 birds). In contrast, synthesis of scleral GAGs was 49% higher in the eye viewing the hyperopic grating than the myopic one (p<0.05). Both changes were in the compensatory direction. Curiously, although eyes exposed to chromatically identical, but coarser (2 cycles/deg), gratings showed no differential changes in choroidal thickness, they too showed a 49% difference between the eyes in synthesis of scleral GAGs, but the difference was not significant.
Although previous work has shown that chromatic cues to defocus are not essential for lens-compensation because chicks can compensate even in monochromatic light, our evidence implies that the eye may be able to infer whether the eye is myopic or hyperopic from the different chromatic contrasts that result from different signs of defocus.
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