What to see
At top right there is a yellowish ring, we need that later. At top left and bottom right there are two grey disks in whose center there is a darker disk. Compare the two large disks, they are quite alike, no?
What to do
You can grab the yellow ring with the mouse and move it around. Place it on the center of the large top left disk – the darker center is visible in the hole. Now place it over the bottom right disk. Unexpectedly, the center shows the same lightness as the surround!
Using the slider at bottom left you can increase the contrast. With high levels of contrast, it becomes immediately obvious that the bottom right central disk has no constant grey level.
The retinal ganglion cells encode the incoming luminance profile via their centre-surround luminance profile in a sort of “delta code”. In the cortex this is integrated to perceive the veridical square luminance profile.
The profile of the Craik-O’Brien-Cornsweet disk on the right is (almost) a fixpoint for this encoding, thus sending a nearly identical spatial code up the cortex. The cortex thinks: “Oh, I know, that’s the ganglion cells doing their usual spiel”, integrates it and arrives at the same perceptual result (yes, I know this is a simplification).
O’Brien V (1959) Contrast by contour-enhancement. Am J Psychol 72:299–300
Craik KJW (1966) The nature of psychology (Sherwood SL, ed). Cambridge, UK: Cambridge UP
Cornsweet TN (1970) Visual perception. Academic, New York
Demo from Ted Adelson’s site