SCIENCE PHOTO LIBRARY SCIENCE PHOTO LIBRARY
Poggendorff illusion. The two red lines crossing the blue bar are colinear. Many people see them as being offset. There are still no satisfactory explanations as to why this should be the case. One theory, borne out by experimental evidence and with a neurological basis, is that the brain has a conflict between the two edges - the edge of the rectangle and the line. Lateral inhibitors in the eye diminish both lines when they are viewed close together, causing the brain to consistently overestimate acute angles.
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