Opponent-process theory of color vision refers to a theory originally proposed by Hering, which claimed that human perception of color is determined by the activity of two ("9 opponent mechanisms: a blue–yellow mechanism and a red–green mechanism. The responses to the two (2) colors in each mechanism oppose each other, one being an excitatory response and the other an inhibitory response. In addition, this theory also includes a black–white mechanism, which is concerned with the perception of brightness. Please see also Opponent neuron.