as Mace said,...
http://en.wikipedia.org/wiki/Peripheral_drift_illusionThe peripheral drift illusion (PDI) refers to a motion illusion generated by the presentation of a sawtooth luminance grating in the visual periphery. This illusion was first described by Faubert and Herbert (1999), although a similar effect called the "escalator illusion" was reported by Fraser and Wilcox (1979). A variant of the PDI was created by Kitaoka and Ashida (2003) who took the continuous sawtooth luminance change, and reversed the intermediate greys. Kitaoka has created numerous variants of the PDI, and one called "rotating snakes"
thats the picture BlkMgk posted has become very popular. The latter demonstration has kindled great interest in the PDI.
The illusion is easily seen when fixating off to the side, and then blinking as fast as possible. Most observers see the illusion easily when reading text with the illusion figure in the periphery. Motion is consistently perceived in a dark-to-light direction, so the two circles to the right should spin in opposite directions.
Two recent papers have been published examining the neural mechanisms involved in seeing the PDI (Backus & Oruç, 2005; Conway et al., 2005). Faubert and Herbert (1999) suggested the illusion was based on temporal differences in luminance processing producing a signal that tricks the motion system. Both of the recent articles are broadly consistent with those ideas, although contrast appears to be an important factor (Backus & Oruç, 2005).
