If you glance around at this optical illusion by Akiyoshi Kitaoka, the snakelike coils around the outer edge will start to move.
But if you let your eyes come to a rest, the snakes come to a stop. Try looking toward one spot and 'spacing out.' Then start looking around again, and the snakes start moving.
Apparently, scientists have concluded, the illusion is somehow tied to eye movements.
Science News: Snakes Illusion