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Close one eye, and somewhere in front of you is a hole wide enough to swallow a head across a room. You will not find it: your brain paints over it. Every test here runs on your own eyes, right on your screen: the blind spot and filling in, the fovea and the jumps of your eyes, top-down and bottom-up perception, shape from shading, lightness, depth, and why you cannot tickle yourself.
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