When you see a red ball rolling across the floor, the ball's redness, roundness and motion appear to be unified and inseparably bound together as features of the ball. But neurophysiological evidence indicates that visual features such as colour, shape and motion are processed in separate regions of the brain. Here we describe an illusion that exploits this separation, causing colour and motion to be recombined incorrectly while a stable stimulus is being viewed continuously. The illusion is seen instimuli containing two sheets of random dots, wher e one sheet is moving up and one is moving down. The sheets contain dots of two colours such that the central and peripheral portions of the stimuli combine colour and motion in opposite fashions (Fig. la). On the upward-moving sheet, dots in the centre are red and dots in the periphery are green. On the downward-moving sheet, dots in the centre are green and dots in the periphery are red. (For movie, see supplementary information.)
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