You can
look at an animation of 'the rotating face mask' here.
With grateful thanks to Prof. Heinrich H. Bülthoff of the
Max
Planck Institute for Biological Cybernetics in Tübingen
(D).
(The animation might work best in the Google Chrome browser).
Even when we
understand what is happening, we still can't see it!
We cannot see the head turning only in one direction.
