How are these circles not moving?

Question

See the below post. The circles are stationary…

https://www.reddit.com/r/gifs/comments/k0tiue/the_circles_are_stationary/?utm_source=share&utm_medium=ios_app&utm_name=iossmf

In: Other

Answer 1

If you look carefully at the edges of the segments, they’re “broken” on the sides that are in the direction of the illusory movement. For instance, the blue segment will have a one-pixel yellow border on the inside edge of one half, and a one-pixel yellow border on the outside of the other half. It’s easy to see if you pause the video at different stages of the animation. As the direction of illusory movement changes, which sides of the circle have “broken” segments also changes.

I don’t know the specifics of how exactly that leads to the perception of motion, but I suspect that it tricks the brain in a similar way as the [café wall illusion](https://en.wikipedia.org/wiki/Caf%C3%A9_wall_illusion).