Ehrenstein illusion - illusory contours forming circles. Created in 1954 by the psychologist Walter Ehrenstein, this illusion demonstrates the false perception of areas of increased brightness. Crosses are apparently joined by bright white circles. In fact no such circles exist. It is still a source of some debate whether this is due to the brain's attempts to interpolate fragmentary information and infer boundaries, or to the visual cortex pre- processing information before it reaches the conscious parts of the brain.
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