Illusions: Transparently Obvious; April/May 2008; Scientific American Mind; by Vilayanur S. Ramachandran and Diane Rogers-Ramachandran; 3 Page(s)

Our ability to perceive visual scenes effortlessly depends on intelligent deployment of built-in knowledge about the external world. The key word here is "intelligent," which raises the questions: Just how smart is the visual system? What is its IQ? For example, does the visual system know the laws of physics? Does it use inductive logic only (as many suspect), or can it perform deductions as well? How does it deal with paradoxes, conflicts or incomplete information? How adaptable is it?

Some insight into perceptual intelligence comes from the study of transparency, a phenomenon explored by Gestalt psychologist Fabio Metelli. He first drew attention to the fact that compelling illusions of transparency can be produced by using relatively simple displays.