If you look at the left (black/white) pattern you will probably notice that the vertical pattern in the center appears to move relative to the horizontal pattern in the surround.

 

explanation

You will have noticed that the pattern in the center is similar to that in the surround but rotated 90 degrees. At first one might think that the important difference is the horizontal spatial frequency. That is to say the width and repetition of the elements in the center and the surround.two gratings of identical but rotated spatial frequencies also produced relative movement. They concluded from their experiments that "The illusion arises because there is a failure to integrate two motion signals into a single motion vector which characterizes rigid motion.

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