The spectral distribution of primate cones and of the macular pigment: Matched to properties of the world?

We have asked quantitatively whether the spectral positions of primate phot opigments are optimized for discovering fruit signals against a background of foliage. In primary tropical rain forest, we have measured the spectral reflectances of fruits observed to be taken by a particular species of monk ey (Alouatta seniculus) and have also measured the foliage background again st which the signal must be discriminated. In the case of trichromatic monk eys, the actual positions of the long-wavelength and middle-wavelength phot opigments (which lie in the range 530 to 560 nm) can be shown to maximize t he signal-to-noise ratio of fruits to foliage in the newer subsystem of col or vision. The latter subsystem compares the quantum catches in the long-an d middle-wavelength cones and enjoys much better spatial resolution than th e older subsystem (which compares the short-wavelength signal with the othe r two). The advantage, however, may extend to a wider range of signals than fruits: modelling shows that the spectral positions of the long-and middle -wavelength photopigments are such as to minimize the variance of vegetable greens in the newer subsystem of color vision. The degree to which this va riance is minimized depends also on the optical density of the pre-receptor al, macular pigment of the retina; and the optimal positions of the photopi gments vary according to the density of the macular pigment. (C) 1999 The O ptical Society of America. [S1070-9762(99)00110-4].