The French artist Isia Léviant created this optical illusion in 1984. Typically one will see motion in the rings. The artwork was called Enigma because scientists initially did not understand what was happening in the human visual system. The controls only change the background.
The concentric circles appear to be flowing because the eyes constantly make tiny movements .
 X. Troncoso, S. Macknik, J. Otero-Millan, and S. Martinez-Conde, "Microsaccades Drive Illusory Motion in the Enigma Illusion," Proceedings of the National Academy of Sciences of the United States of America, 105(41), 2008 16033–16038. www.pnas.org/content/105/41/16033.full.
 A. Seckel, Optical Illusions: The Science of Visual Perception, Buffalo, NY: Firefly Books, 2006.