Figure 7A. Extracting color from sets of one-dimensional signals. The operations in Figure 6B convert an incident spectrum to one number at each point in time (and space on the retina). In order to retain some information about the spectrum of incident light, animals use cones which have different absorption spectra. The top plot shows the spectra of the three cones found in humans. The bottom plot shows the absorption spectra of the outer segments of the four cone types in the retina of the European starling. The shapes of the curves vary slightly in systematic ways as a function of the wavelength where the absorption peaks. Therefore, the cones can be characterized by that peak wavelength. The absorption peaks for each of the seven pigments are inset in the two plots. See Figure 7B.