I thought I had posted a link to this site before but couldn't find it using the search feature. Anyhow it was down for awhile as he just moved to a new host, so maybe you haven't seen it.
http://www.drjohnruss.com
I came across his site about a year ago I think; found a paper he wrote very interesting and useful as I explored drawing realistic textures. Here is a link
http://www.drjohnruss.com/downloads/seeing.pdf
Lot's of interesting stuff regarding how human vision works, how we percieve depth/distance, shapes and so on.
Vision - How it Works
Moderators: Víctor Paredes, Belgarath, slowtiger