This image is not animated. Don’t believe me? Try staring at just one part of the image and you’ll see it will stop moving. This is an example of a “peripheral drift illusion(Opens in a new window).” It is thought that this illusion occurs because of the slight differences in time it takes to process different luminances (how intense the light is from a particular area). This slight lag in mental processing tricks the brain into perceiving movement that isn’t really there.

Illusion by Paul Nasca