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Bouncing Brains by Thorsten Hansen, Kai Hamburger, & Karl R. Gegenfurtner
University of Giessen, Germany
http://illusioncontest.neuralcorrelate.com/2007/bouncing-brains/

"The perceived location of the separation between light and dark regions changes as the background is modulated, causing each brain to jiggle and bounce. In some regions these illusory motions of neighboring brains are coherent and are grouped together to give rise to an even stronger illusion."