Stare at the centre of the figure for a while. Some scintillating activity will build up in the violet and blue annuli. Some observers also report a circular rotation within these regions; things will begin to “run around in circles”.
This image “The Enigma” is by Isia Leviant (1984).
Mechanism currently unknown.
Zeki S, Watson JD, Frackowiak RS (1993). Going beyond the information given: the relation of illusory visual motion to brain activity. Proc R Soc Lond B Biol Sci 252: 215–22
Barch D, Kumar T, & Glaser DA (2003) Modeling the illusory motion of Enigma with an excitable neuronal array [Abstract]. Journal of Vision, 3(12), 72a
Zanker J (2004) Naturwissenschaften 91:149–156