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F. J. Rucker, J. Wallman; Sign of defocus can be derived from relative cone contrasts. Journal of Vision 2010;10(15):73. doi: 10.1167/10.15.73.
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The effect of defocus on the cone contrast of each of the three cone-types is such that the contrast of all three types of cones falls in parallel for myopic defocus (image in front of photoreceptors), but the contrasts of the three cones change differentially for hyperopic defocus (image behind photoreceptors). We hypothesize that these changes in chromatic and luminance contrast might be used to discern the sign of defocus. Thus, if changes in focus caused large changes in the chromatic contrast of the retinal image the eye could infer that it was on the hyperopic side of focus, whereas if there were large changes in the luminance contrast of the retinal image it could infer it was on the myopic side.
We exposed chicks for three days to sinusoidally modulated light, with red/green components either in-phase or counterphase, thereby causing modulation in either luminance or chromatic contrast. We also had illuminants that were primarily luminance-modulated, with a small amount of chromatic contrast, or chromatically modulated with a small amount of luminance contrast.
As our hypothesis predicted, eyes exposed to luminance flicker became more hyperopic (the opposite direction from normal emmetropization) and reduced their rate of elongation, while those exposed to chromatic flicker became slightly myopic and increased their rate of elongation. Chicks exposed to combinations of luminance and chromatic flicker showed intermediate responses.
We conclude that the eye might be able to discern the sign of defocus by detecting changes in chromatic and luminance contrast.
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