i'm working on a phd in psychology and this is one of my examples of receptor flood. as a very basic explanation, your eyes have receptors that detect shading, shape, and movement. this graphic shows how optical receptoirs are flooded by a repeating pattern and as their saturation level is reached, they purge the information to receive more. if you've ever seen an infant, you kind of see how this works. infants are only able to absorb so much stimuli before they deter their gaze from you onto the perephera. anyway, this purging gives the illusion of movement because you are purging the information that you are taking in simultaneously. in many ways what's going on here is why you perceive car wheels as going backwards on video media (although that's due to fps).
to see how saturation is related to focus, unfocus your stare. picture stops moving. focus and the perephera moves.