See Inside October 2005  The Reality of Illusory Contours  How
can an imaginary square look more real than a box with actual
lines?  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 “hal