Click here for a fascinating collection of Vincent Van Gogh's Letters. There are some interesting nuggets in here, including this one on hullucinations:
... Again - speaking of my condition - I am so grateful for yet another thing. I've noticed that others, too, hear sounds and strange voices during their attacks, as I did, and that things seemed to change before their very...I stumbled across this gem at the ever-so-indespensible Museum of Online Museums.
Letter from Vincent van Gogh to Theo van Gogh, 22 May 1889