Here's an explanation that I read:
http://blogs.discovermagazine.com/badast.../illusion/
More details here:
http://web.mit.edu/persci/people/adelson...ption.html
Quote:This classic illusion is due to the way you interpret color (or shading): it’s not done independently; your brain is always comparing things. In one case, the square is surrounded by lighter squares which make it look darker, and in the other case it’s surrounded by darker squares, making it look lighter. The shadow going across the squares messes with your perception as well, amplifying the effect.
http://blogs.discovermagazine.com/badast.../illusion/
More details here:
http://web.mit.edu/persci/people/adelson...ption.html