If you watch the small center dot, the colors of the large dots appear to stop changing when the circle rotates. But if you track the motion of the circle with your eyes, you'll see they continue to change. The same thing happens with a grey circle, changing the luminance or even the size of the larger dots.
I can't help but think that something analogous happens in our social perceptions as well. The "motion" of our busy, media-drenched lives suppresses our awareness, and major changes can happen without notice, You must look carefully and deliberately to see the truth rather than the illusion.