what I found fascinating is that the effect is maintained even when you're not focused (like lens focus, not attention focus) on the circles. If you let your eyes relax their focus to a point beyond the screen, the blurry circles maintain the illusion.

And bonus points if you find the focal point where a third circle is in the middle which is a composite of both the left and right circles, and... it doesn't move. I don't know how to better explain the effects of letting your eyes wander out of focus and the artifacts it creates in your vision, but I'm sure someone will get it.