Chick eyes compensate for chromatic simulations of hyperopic and myopic defocus: Evidence that the eye uses longitudinal chromatic aberration to guide eye-growth
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Abstract
Longitudinal chromatic aberration (LCA) causes short wavelengths to be focused in
front of long wavelengths. This chromatic signal is evidently used to guide ocular
accommodation. 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 alternately exposed one eye of each chick to a sine-wave grating (5 or
2 cycle/deg) simulating myopic defocus ("MY defocus": image focused in front of retina;
hence, red contrast higher than blue) and the other eye to a grating of the same spatial
frequency simulating hyperopic defocus ("HY defocus": blue contrast higher than red).
The chicks were placed in a drum with one eye covered with one grating, and then switched
to another drum with the other grating with the other eye covered. To minimize the
effects of altered eye-growth on image contrast, we studied only the earliest responses:
first, we measured changes in choroidal thickness 45 min to 1 h 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. The eyes compensated in the appropriate directions: The choroids
of the eyes exposed to the HY simulation showed significantly more thinning (less
thickening) over the course of the experiment than the choroids of the eyes exposed
to the MY simulation (all groups mean:-17 microm; 5 c/d groups: -24 microm; paired
t-test (one-tailed): p=0.0006). The rate of scleral GAG synthesis in the eye exposed
to the HY simulation was significantly greater than in the eye exposed to the MY simulation
(HY/MY ratio=1.20; one sample t-test (one-tailed): p=0.015). There was no significant
interaction between the sign of the simulated defocus and either the spatial frequency
or the presence of a +3 D lens used to compensate for the 33 cm distance of the drum.
Although previous work has shown that chromatic cues to defocus are not essential
for lens-compensation, in that chicks can compensate 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.