When a gradient stimulus, whose luminance contrast ranges gradually from white on one side to black on the other, is made to disappear all at once so that only the uniform white background remains visible, illusory motion is perceived. This motion lasts ~700ms, as if the stimulus moves from the low to the high luminance contrast side. This gradient-offset induced motion does not occur for equiluminant color-defined gradient offsets, suggesting that it relies mainly on the magnocellular pathway. We hypothesize that this illusion is caused by the difference of decay rates within the gradient afterimage.
See different versions of the illusion
Illusory motion induced by the offset of stationary luminance-defined gradients Po-Jang Hsieh, Gideon P. Caplovitz & Peter U. Tse Vision Research. 2006. 46:970-8Gradient-Offset Induced Motion,