(Animated #gif below may take a while to load, so please be patient until it full loads.) If you observe the rotation of the central text label below, do you see it turning smoothly all the time? Does it seem to “freeze” from time to time? If you manage to cover or isolate the surround (swaying dots), you can easily see that the text is really turning continuously!
One possible explanation of this phenomena states that “assuming a stable surround, our visual system probably uses the surround as a reference to measure motion of the included objects”.