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.
Sources:
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
