(A) A simple model by which a 2F chromatic response might be generated. Cone signals generated by the red and green stimulus waveforms in the top row show saturation. We assume these hypothetical signals, shown in the second row, are prior to the bipolar cells, so the cone response is hyperpolarizing. This model predicts a 2F chromatic response, but no major 2F distortions for silent substitution conditions. (B) Since such distortions were present, a more complex model was required to account for the data. The model is schematically indicated here and discussed in more detail in the Results section.