The bar in the center of the image is actually one solid gray bar, not a gradient. This is hard to believe. You can just place a piece of paper above the bar on the screen. Still not convinced? To see the proof, click here, and scroll down to the animated version – and picture credit.
This shows me how much our perception is subjective. I would swear in a court that I saw a bar that was lighter on the left and darker on the right.