Try out some of the experiments referred to by Professor Ramachandran.
Each eye has a blindspot. It's caused by the fact that the small area
of the retina where the optic nerve is connected to the eyeball is
not sensitive to light.
The following experiments prove the existence of the blindspot and
demonstrate how the brain can fill in the missing information. They
provide important hints about how the neural machinery of the brain
works in practice.
All experiments are taken from V.S. Ramachandran's book, Phantoms
In The Brain, published by Fourth Estate.