Photopigments and pseudo-pigments

The spectral properties of photopigments are often inferred from physiological measures of spectral sensitivity. From these measures pigments have recently been inferred with properties not corresponding to those of any pigment measured by spectrophotometry. It is proposed that in all these cases the properties of single pigments are not being measured; rather, they are pseudo pigments resulting from a specific form of neural interaction involving several real, known, photopigments. These pseudo-pigments were accepted as being related to single, but unusual, pigments largely because they obeyed the commonly accepted but incomplete criteria for identifying single pigments. Starting from the essential features of a pseudo-pigment, an analysis is developed; the conclusion is that pseudo-pigments can be resolved into standard photopigments, as identified by spectrophotometry, and that these interact in a specified form. A method is described for identifying the number andλ_max's of the photopigments involved in any physiological spectral sensitivity function, as well as certain details of the physiological interactions involved.