From: The World In Your Head by Steven Lehar

Fig. 5.1 (A) The pattern of brightness of an object is a combination of its lightness, (i.e., perceived reflectance) and its illuminance. The objective of brightness perception is to factor these two components from the brightness image. (B) The situation can be modeled dynamically by providing three nodes (black circles) at every point in the percept (white circles) representing the three variables, with a mutually inhibitory relation between the brightness and lightness nodes, such that the more brightness that is attributed to lightness, the less is attributed to illuminance at each point.