The Neural Control of Vision
G. The Function of the On and Off Channels in Vision

That our perception depends a lot on assessing local differences in illumination is demonstrated in Figure 19. In the upper section we see two identical bright squares on a gray background. In the lower section the same two squares are shown, but the background is ramped from left to right from bright to dark. As a result we perceive the two squares as different. The one of the left is a dark square and the one on the right is a bright square.