Opponentprocess theory

Ewald Hering (1878) put forward an opponent-process theory that handles some findings that cannot be explained by the Young-Helmholtz theory. Hering's key assumption was that there are three types of opponent processes in the visual system. One type of process produces perception of green when it responds in one way and of red when it responds in the opposite way. A second type of process produces perception of blue or yellow in the same fashion. The third type of process produces the perception of white at one extreme and of black at the other.

Evidence consistent with opponent-process theory was reported by Abramov and Gordon (1994). They presented observers with single wavelengths, and asked them to indicate the percentage of blue, green, yellow, and red they perceived. According to Hering's theory, it is not possible to see blue and yellow together, or to see red and green together, but the other colour combinations can occur. That is what Abramov and Gordon (1994) found.

Opponent-process theory helps to explain colour deficiency and negative afterimages. Red-green deficiency occurs when the high- or medium-wavelength cones are damaged or missing, and so the red-green channel cannot be used. In similar fashion, individuals lacking the short-wavelength cones cannot make effective use of the yellow-blue channel, and so their perception of these colours is disrupted. Negative afterimages can be explained by assuming that prolonged viewing of a given colour (e.g., red) produces one extreme of activity in the relevant opponent process. When attention is then directed to a white surface, the opponent process moves to its other extreme, and this produces the negative afterimage. Thus, the operation of opponent processes can account for negative afterimages.

DeValois and DeValois (1975) obtained physiological evidence in monkeys that was broadly consistent with Hering's theory. They discovered what they called opponent cells. These are cells located in the lateral geniculate nucleus that show increased activity to some wavelengths of light but decreased activity to others. For some cells, the transition point between increased and decreased activity occurred between the green and the red parts of the spectrum. As a result, they were called red-green cells. Other cells had a transition point between the yellow and blue parts of the sprectrum, and so they were called blue-yellow cells.

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