Illusions: The Reality of Illusory Contours; October 2005; Scientific American Mind; by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran; 2 Page(s)

Computers can calculate at staggering speed, but they cannot match the human visual system's uncanny ability to assemble a coherent picture from ambiguous fragments in an image. The brain seems to home in effortlessly on the correct interpretation by using built-in knowledge of the statistics of the world to eliminate improbable solutions.

This problem-solving aspect of perception is strikingly illustrated in (a) by the famous illusory rectangle of the late Italian psychologist Gaetano Kanizsa and Richard L. Gregory, now emeritus professor of neuropsychology at the University of Bristol in England. Your brain regards it as highly unlikely that some malicious scientist has deliberately aligned four Pacmen in this manner and instead interprets it parsimoniously as a white opaque rectangle partially covering four black disks in the background. Remarkably, you even fill in, or "hallucinate," the edges of the phantom rectangle. The main goal of vision, it would seem, is to segment the scene to discover object boundaries so that you can identify and respond to them.