did he have eye alignment issues?
i'm not going to pretend i came up with this on my own. when i was reading the description of margaret livingstone's book, vision and art: the biology of seeing, she mentions great artists who had strabismus, and how that may have contributed to their, well, greatness. she named a few - rembrandt and picasso included - and i may be imagining that she named van gogh as well. well, this was from my neuroscience textbook, and i'm not likely to open that in the next month at least....so i'm just going to have to buy livingstone's book soon, or else be forever curious :)
i found this interesting article by her, though: http://harvardmedicine.hms.harvard.edu/fascinoma/fivesenses/vision/viewmasters.php