Nonlinear codes for the yellow/blue mechanism.

The opponent yellow/blue mechanism is studied by means of an iso-cancellation technique. A qualitative and quantitative analysis of different photopigment models for the yellow/blue code is presented. On the basis of this analysis we provide a new model which ascribes the nonlinear character of the code to a power transformation of the short wave cone activity. This new model leads to predictions concerning the wavelength shift of the short wave component of a unique red. Tests of these predictions support the new model.