an idle mind is the devil's picture-show

Your brain hates nothing so much as nothing, to the point that it will fill the nothing with anything just to have something there. Or something:

The late historian Lord Dacre of Glanton, formerly Hugh Trevor-Roper, was unusual among [Charles Bonnet Syndrome] patients in that he talked openly about what he jokingly referred to as his 'phantasmagoria'.

He would see horses and bicycles racing, and whole landscapes whizzing by as if he were on a train. On one occasion, he found himself trapped in an apparently endless tunnel.

Hallucinations tend to have common themes: simple geometric patterns, disembodied faces with jumbled features, landscapes, groups of people, musical notes, vehicles and miniature figures in Victorian or Edwardian costume. They can be in black and white or colour, moving or still, but they are always silent.

The condition was named after Charles Bonnet, an 18th-century Swiss natural philosopher whose grandfather had seen people, patterns and vehicles that were not really there. Bonnet was the first person to identify that you could have visual hallucinations and still be mentally sound.

The condition can affect anybody at any age with diminishing eyesight. Even people with normal vision can develop it if they blindfold themselves for long enough.

On a related note, this is why artificial intelligence, insofar as it means replicating human cognition, will work only if flaws are designed in. The brain is naturally buggy.

[via BoingBoing's David Pescovitz, who also links to an interview with the perpetually fascinating Dr. Sacks.]