At Bioephemera, Jessica has a fascinating post about depictions of madness in 15th-17th century art, during which time mental illness was popularly attributed to the presence of a "stone of madness" (or "stone of folly") in the head.
One of the earliest depictions of this is found in the above painting, Hieronymous Bosch's The Cure of Folly (The Extraction of the Stone of Madness), and similar scenes were subsequently depicted by other Renaissance artists.
As Jessica explains, historians of art and medicine are in dispute about the underlying meaning of the paintings. Were they depictions of real procedures, or allegorical statements about the human condition? Did medieval quacks claim to cure madness by removing stones from the head, or were they aware of the placebo effect?
Find out more at Bioephemera.
The book 'Madness: A Brief History' by Roy Porter is a very interesting read in this regard. What I found interesting was the way much of delusional or hallucinatory content were traced to religious modes of thinking (even pre-Christian religions). It was a useful source to flick through when I was writing coursework essays on certain phenomenology associated with schizophrenia.