Color Vision, Opponent Theory
Opponency in human color vision refers to the idea that our perceptual color mechanisms are arranged in an opponent fashion. One mechanism, the red-green mechanism, signals colors ranging from red to green; the other one, the yellow-blue mechanism, signals colors ranging from yellow to blue. This opponency is often referred to as hue opponency, as opposed to cone opponency.
Behavioral Evidence for Color-Opponent Processing
Linearity and Constancy of the Color-Opponent Mechanisms
Figure 4 demonstrates the two major features of the color-opponent mechanisms: (1) Consistent with Krantz and colleagues, the red-green opponent mechanism was found to be approximately linear, that is, unique yellow and blue are colinear ; the yellow-blue opponent mechanism on the other hand is not a single linear mechanism , that is, unique red and green do not lie on a line through the neutral gray background. Therefore, one needs to either postulate a highly nonlinear yellow-blue mechanism or, which is more likely, two separate unipolar mechanisms, one signaling yellow and the other one signaling blue. (2) The red-green opponent mechanism is fairly color constant in comparison to the yellow-blue opponent mechanism; the unique yellow and blue settings are not affected by the changes in the ambient illumination (solid, dashed, and dotted lines are coincident), while large shifts in unique hue settings are observed for the yellow-blue equilibria. Unique green settings, in particular, shift toward yellow when viewed under typical office light (CWF).
Physiological Basis for Opponent Hue Processing: Hue Opponency Versus Cone Opponency
Firstly, the color-opponent mechanisms obtained using behavioral measures such as hue cancellation are not aligned with the cone-opponent mechanisms that have been identified in the retina and the lateral geniculate nucleus. It may therefore be more appropriate to refer to these mechanisms as hue-opponent and cone-opponent mechanisms, respectively. Secondly, the red-green opponent mechanism (yielding unique yellow and blue) is an approximately linear mechanism. In contrast, the yellow-blue opponent mechanism (yielding unique red and green) cannot be modeled as a single linear opponent mechanism since unique red and green do not lie on a line through the neutral gray origin. The most parsimonious explanation is to postulate two separate yellow-blue mechanisms; when at equilibrium, one of them signals red, the other one green. Thirdly, while the red-green opponent mechanism is almost completely color constant (unique yellow and blue settings are invariant under changes in ambient illumination), the equilibria point of the yellow-blue mechanism change under changes in ambient illumination: unique green in particular undergoes a major shift toward yellow when viewed under CWF in comparison to simulated daylight (D65). We speculate that this failure of constancy for unique green might have the same neural origin as the relatively large interobserver variability found in unique green settings [11, 12].
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