Why does this still image appear to move?

I think we've proven, through various optical illusions, that your eyes know precisely nothing about anything. And yet it's still creepy when a still image is crawling across your screen. Check out a couple of reasons why people think this happens.

There's not much to be said, here. The image is moving. I see it. You see it. We all see it. Why is it happening? The answer, as is so often the case, is "no one knows." If scientists could claw open our eyeballs and dig back to our brains to figure it out they would, bless them, but that doesn't work. There are a couple theories.

One explanation is that the color white turns the receptors on our retinas on, while the color black shuts them off. Having white and black in close proximity, and in certain arrangements, makes them flicker between on and off. The changes from on to off and on again are interpreted by the brain as motion.

Another theory holds that our retina sections aren't flicking off and on, they're perceiving color changes. It's been noticed that, when black and white are striped, and those stripes move in different directions - for example the stripes are oriented differently in each of four sections of a grid - the eyes see illusory colors. The curves that we see around the dots might serve as different angled black and white lines, and we might see color changes that we interpret as motion.

All I know is that when I stare intently at a section it stills, but the sections I see in my peripheral vision move around fast. It also moves faster as I move my eyes. So whatever's happening is always happening just at the edges of my vision as I sweep back and forth. Computer ghosts?

Image: Paul Nasca

Via Akiyoshi's Illusion Pages.