According to Wikipedia (I'm too tired to do my own research tonight) "A defining moment was the publication of Bildnerei der Geisteskranken (Artistry of the mentally ill) in 1922, by Dr Hans Prinzhorn." Here are some images I copied from the Prinzhorn Collection's website.
According to the website, "The core collection comprises approximately 5000 pieces of art created by approx. 450 patients of psychiatric institutions. These pieces comprise mostly drawings, water colors, writings, like letters, notes, drafts of books and exercise books, which were often self-manufactured, as well as oil paintings, material manual work, collages and 70 wooden sculptures." What a wealth.
