To most people, some of these circles look convex. The one at bottom center looks concave. In fact, all the circles are identical. The one at bottom center is just upside down. However, the brain interprets the circles with the darker part at top as concave and the darker part at bottom as convex. This is because the visual system assumes light is coming from above (the sun, an overhead light) and perceives the shape according to that model.

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