The receptive fields are represented as a disk (+) and annulus (-). The center disk is an excitatory area and the annulus an inhibitory area. The receptive fields in the uniformly white and uniformly black areas receive about the same stimulation in their excitatory centers and inhibitory surrounds. Therefore the center excitations are in balance with the surround inhibitions.
The receptive field over the bright Mach Band gives a stronger response in the center because part of the surround is in the darker area. Therefore it receives less inhibition from the surround than did the center at the extreme left and right ends. The receptive field over the dark band receives more surround inhibition because part of the surround is in the brighter area. Therefore, the excitatory response is less and this results in our seeing that the area as darker.