How do optical illusions trick the brain?

The answer

Optical illusions exploit the shortcuts and assumptions the brain uses to construct its model of visual reality from ambiguous retinal input. The brain does not passively receive images; it actively interprets them using prior experience and heuristic rules. Size illusions (like the Ponzo illusion) exploit depth perception cues: the brain interprets converging lines as distance and 'corrects' the apparent size of objects accordingly. Color illusions (like the checker shadow illusion) exploit the brain's automatic color constancy, which adjusts perceived color based on assumed lighting conditions. Motion illusions activate motion-detecting neurons with static patterns that mimic the temporal stimulation of real movement. The Hollow Face illusion shows that our expectation of convex faces is so strong that we perceive a concave mask as protruding. These illusions reveal that perception is an active constructive process, not a faithful recording of physical reality.