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What are visual illusions?

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Visual illusions arise when the images perceived by the eyes and processed by the brain differ from reality. Such illusions fall into a few broad categories: optical illusions that create images in the visual system that are different from the objects that make them, physiological illusions that arise from excessive stimulation of a specific channel along the visual system pathway, and cognitive illusions that result from the brain making unconscious inferences.

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One of the best known visual illusions is the Necker Cube, a two-dimensional line-drawing of a cube. As an ambiguous line drawing, it can be interpreted as the lower left face of the cube being in front or the upper right face in front. Another illusion of this sort is the rabbit-duck drawing. Various versions exist from a simple line drawings to more highly detailed, more realistic images. The rabbit-duck image is an ambiguous drawing in that two interpretations are possible: it is both a duck facing left or a rabbit facing right, with the duck's bill becoming rabbit ears as the perceived image changes from one to the other.

Other visually ambiguous illusions in which information from the eye alone is not sufficient to explain the perception include figure-ground illusions. The best known of this is Rubin's vase-faces illusion. Depending upon what is seen as figure or ground, the image can be seen as two side-view profile or a single curvy vase. Again, it is possible to see both interpretations, but not at the same time.

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