Iris Pigmentation and Visual-Geometric Illusions

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Abstract
Blur or degrading of the retinal image has been shown to be a factor in the formation of visual-geometric illusions where intersecting line elements are present. Light irises allow more scattered light within the eye, which results in more image degradation than found in dark-eyed subjects. Measurements on 755 observers show that illusion magnitude varies as a function of iris pigmentation for a configuration with intersecting line elements (Müller — Lyer illusion), but not for a configuration devoid of such features (Ebbinghaus illusion).