Abstract
Purpose: :
We have previously shown that simulations of the effect of longitudinal chromatic aberration (LCA) with hyperopic and myopic defocus can cause compensatory ocular changes (Rucker and Wallman, 2009). Under severe astigmatic blur (Jackson Cross Cylinders, JCC), refractive compensation for imposed myopic or hyperopic defocus was unimpaired under white light but impaired under monochromatic light. We now investigate the time course of the effect of monochromatic light and JCC on choroidal responses to imposed defocus.
Methods: :
: Chicks wore lenses that presented astigmatic defocus (+5/-5 D crossed cylinders) together with +3 D of spherical defocus over one eye (Positive X-cyl lens) and astigmatic defocus (+4/-4 D crossed cylinders) together with -2 D over the other eye (Negative X-cyl lens) under red or green monochromatic light, or under white light. Other chicks wore lenses that imposed only spherical defocus (+3 D on one eye, -3.5 D on the other). We measured choroid thickness by high-frequency ultrasound at 3 hrs, 24 hrs and 3 days.
Results: :
Compensatory choroidal responses were transient under monochromatic light. With positive lenses, choroids had thickened by 3 hours (Red Sph: 29 μm; Red X-cyl: 14 μm; Green Sph: 52 μm; Green X-cyl: 90 μm). With negative lenses, thinning peaked at 24 hours (Red Sph: -65 μm; Red X-cyl: -65 μm; Green Sph: -38 μm).After 3 days normal compensatory responses were found with astigmatic defocus under white light. Choroids thickened in eyes wearing positive lenses (98 μm) and thinned (-51 μm) in eyes wearing negative lenses (p< 0.001). In contrast, under monochromatic light with astigmatic defocus, choroids responded inappropriately in that they thinned with positive lenses (Red X-cyl: -32μm; Green X-cyl: -1 μm), as well as with negative lenses (Red X-cyl: -51 μm; Green X-cyl: -17 μm; difference between positive and negative lenses was not significant).
Conclusions: :
The normal compensatory choroidal response was present under astigmatic defocus with white light but not with monochromatic light. These results suggest that when other visual cues are impaired by astigmatic defocus, the eye relies on chromatic cues to decode the sign of defocus.
Keywords: myopia • choroid • color vision