Art-science blog Bioemphemera has an excellent piece on how Renaissance artists depicted madness as involving a stone in the head. Numerous paintings from the 16th and 17th century show operations to remove the stone and presumably cure the insane of their ‘folly’.
Despite the widespread depiction of this procedure, many examples of which are wonderfully illustrated in the Bioemphemera post, it’s not clear whether these paintings were documenting widespread practices of medical fakery, or whether they were entirely metaphorical.
Perhaps owing to this element of mystery, and to the striking artworks, the topic is often featured in science and medical journals.
A 1999 article in Trends in Neurosciences is probably the most comprehensive treatment, and makes an excellent complement to the Bioephemera piece.
Link to Bioephemera post ‘The Stone of Madness’.
Link to TINS article ‘Psychosurgery in Renaissance art’.
Link to PubMed entry for article.
Mind Hacks 
I’m assuming this is were the phrase “He has rocks in his head” came from?