In the neural firing of our eyes, there is a duality rather a trichromy in the interpretation of color. Ewald Hering described the opponent-process theory to explain this. Neurons that fire in response to a red object will, in their sudden absence, create the illusion of having seen green; and vice versa. Neurons that fire in response to blue will see yellow in the sudden absence of blue. Neurons that fire in response to black stimulus, in their sudden absence, perceive white. Opponent-process is also an emotions theory. Emotions come in pairs.