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Stare at the red dot on this woman’s nose for thirty seconds. Then, look over at a blank white space (a piece of paper or an empty browser tab will do). Did you see the “correct” version of the image? Here’s how it works: stare long enough at an object and the eye’s photoreceptors (particularly the color-sensitive cone cells) lose sensitivity from overstimulation. Divert the eyes to a blank space, and the surrounding cone cells send out a much stronger signal. The brain interprets this discrepancy as looking at the precise opposite colors, in what is known as a ‘negative afterimage.’ Nifty, no?

From Yahoo