Gilbert's very interesting talk last night reminded me of a book I read a few years ago on the rules that our brain uses to create what we see. It was a fascinating read. http://www.cogsci.uci.edu/%7Eddhoff/vi.html He has created a number of Java applets to demonstrate how we create the motions that we see. http://www.cogsci.uci.edu/%7Eddhoff/vi6.html -- Bill