Visual illusions are fun, but do they have a practical application?

Indeed they do. Architects since the ancient Greeks have taken into account perceptual error when designing buildings. But of even greater importance is their application to areas where accurate visual perception is essential to people's safety. The safe operation of many vehicles -- from cars to trains to airplanes -- relies on people's accurate perceptions of lines and figures.

If all sensation is cut off, as it is in a sensory deprivation tank, do you still perceive?

People who have spent time in a sensory deprivation tank often report losing perception of their body and the passage of time. However, they still experience conscious thought.