Illusions: Mind the Gap; April 2005; Scientific American Mind; by Vilayanur R. Ramachandran and Diane Rogers-Ramachandran; 2 Page(s)

Our perception of the world depends, to a surprising degree, on intelligent guesswork by the brain. An oval-shaped white image exciting your retina could be produced by an egg, a perfectly circular, flat tilted disk, or an infinite number of intermediate shapes each angled to the right degree. Yet your brain "homes in" instantly on the correct answer. It does this by using certain unconscious assumptions about the statistics of the natural world--suppositions that can be revealed by visual illusions.

The manner in which the brain deals with inexplicable gaps in the retinal image--a process called filling in--provides a striking example of this principle. You can demonstrate this using the blind spot of your eye.