Figure 4.

The lower sequence of images shows the stimulus presented first, and the upper sequence of images shows the subsequent stimulus. The lower sequence of images shows the enhanced Cornsweet effect, in which the upper gray surface is commonly perceived as darker than the lower one, although the brightness of both surfaces is exactly the same. Novice viewers do not see this illusion until it is demonstrated that the brightness of both surfaces is exactly the same. Thus, by covering the gradient region containing the border between the two gray surfaces (upper sequence of images), observers are supposed to see that the brightness of both surfaces was the same.