400 450 500 550 600 650 Wavelength (nm)
Figure 4 Response properties of photopigments and nerve cells. (Top) The absorption curves for the three classes of cone found in the eyes of people with normal color vision. The peak sensitivities of the three cone types are at 430 nm (S), 530 nm (M), and 560 nm (L). The three graphs at the bottom illustrate the responses of nerves cells in the visual pathway that combine inputs from the three cone types. (Middle) Spectrally opponent cells that show additive and subtractive combinations of cone signals. The inputs are indicated. E, excitation; I, inhibition. (Bottom) Response property of a nonopponent cell that additively combines signals from M and L cones.
of Fig. 4 that S cones absorb very little light from the middle- to long-wavelength portion of the spectrum. A consequence of this is that only two primary lights are required to complete color matches in this part of the spectrum (i.e., over those wavelengths, three-variable, trichromatic color vision gives way to two-variable, dichromatic color vision).
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