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Note that there appear to be edges between adjacent areas of different color. For example, at the edge between A and B, it appears that A is a bit lighter as it nears B, and the edge near B looks darker than the rest of B. Please believe it that each rectangle is uniform in the physical stimulus. What you are seeing are illusory edges that exaggerate the differences. These edges are known as Mach bands, and they can be explained by the theory that adjacent areas of the retina inhibit each other. That explains why people perceive sharp edges around objects, even though the retinal image is blurred.
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