Hallucinations

Great collection of the various medical conditions and altered states that play tricks on our mind and make us see/hear/feel things that aren't really there. Sensory deprivation, psychedelics, visual migraines, narcolepsy, etc.

Some fascinating conditions: Polyopia - perceiving multiple copies of the same visual image. Or the temporal variant of it: Palinopsia - images persist to some extent even after their corresponding stimulus has left. Or the Doppelgänger syndrome: perceiving a neutral mirror image of oneself.

My favorite Oliver Sacks anecdote is him taking a mix of cannabis, amphetamines and LCD and setting out to experience the color indigo, which he'd never managed before. He succeeds and has a very indigo trip. Afterwards he continues to look for indigo, yet without success, until years later he visits a classical concert at the Metropolitan Museum of Art. During the break - all high on music - he visits the Egyptian jewellery section of the museum and he manages to see indigo again. But only for the short duration for his music-caused ecstatic state.