Human vision recognizes the direction of a human, an animal, and objects in translational motion, even when they are displayed in a still position on a screen as filmed by a panning camera and with the background erased. Because there is no clue to relative motion between the object and the background, the recognition relies on a facing direction and/or movements of its internal parts like limbs. Such high-level object-based motion representation is capable of affecting lower-level motion perception. An ambiguous motion pattern is inserted to the screen behind the translating object. Then the pattern appears moving in a direction opposite to that which the object implies. This is called the backscroll illusion, and psychophysical studies were conducted to investigate phenomenal aspects with the hypothesis that the illusion reflects a strategy the visual system adopts in everyday circumstances. The backscroll illusion convincingly demonstrates that natural images contain visual illusions.
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