There are two classes of photoreceptor—rods and cones. Rods are involved in supporting vision under low-light conditions (technically called scotopic vision). Cones subserve color vision and other characteristic features of daylight vision (photopic vision), such as high spatial and temporal sensitivity. The conversion of energy from light into nerve signals is accomplished in the photoreceptors. The initial step in this transduction process involves the absorption of photon energy by photopigments. Molecules of cone pigment are densely packed in a series of parallel membranes making up one end of the photoreceptor, a physical arrangement that is particularly effective in trapping incoming light. The pigment molecule has two essential components—a protein called opsin and a covalently linked chromophore, the latter being a derivative of vitamin A. Absorption of energy from a photon of light causes the chromophore to undergo a conformational change, an isomerization. This change is virtually instantaneous, complete in no more than 200 fsec (fsec=10~15 sec). It serves as a first step in a cascade of molecular changes that produce a modulation in the flow of ionic current across the photoreceptor membrane. This induced electrical change spreads along the length of the photoreceptor and in turn alters the rate of release of neurotransmitter to second-order cells in the retina and thereby communicates a signal onward into the retinal network.
The efficiency with which photopigments absorb light varies continuously as a function of the wavelength of the light. Figure 4 illustrates the absorption spectra for the three classes of cone pigment found in the retinas of people with normal color vision. When properly scaled the shapes of absorption spectra for all photopigments are similar, and thus they can be economically specified by using one number, the wavelength to which they are maximally sensitivity (1max). The human cone pigments have lmax values at about 420, 530, and 560 nm. The shape and width of the absorption spectrum for the photopigment are dependent on features of the chromophore, whereas the spectral positioning of the pigment along the wavelength axis depends on structural features of the opsin. Note that there is a significant amount of overlap in the wavelengths of light absorbed by these pigments. This is important because it is the comparison of the amount of light absorbed by different pigments that constitutes the basis for the nerve signals that lead to color vision. Among vision scientists, the receptors containing these three pigments are usually termed S, M, and L cones, respectively (shorthand for short-, middle-, and long-wavelength sensitive). The three classes of cone are unequally represented in the retina, having an overall ratio of 1S:3M:6L, with some significant individual variations. As discussed later, this fact is important in understanding some features of human color vision.
A fundamental feature of the operation of pigments is that their response to light is proportional to the number of photons they absorb independent of the wavelength of these photons so that, once absorbed, each photon contributes equally to any generated signal. This means that the signal provided by a given type of receptor contains no information about the wavelength of the light that was absorbed. The blindness of a single type of photopigment to wavelength differences is formally called the principle of univariance. An important functional consequence of pigment univariance is that a retina having only a single type of photopigment could not support any color vision capacity. This is one reason why colors disappear under scotopic conditions when only a single (rod) pigment is operational.
Color matching behavior can be traced directly to the univariant property of the cone pigments. Thus, in a color match of the sort described previously, the subject is actually adjusting the amounts of the primary lights so that the three types of cone will absorb equal numbers of photons from the primary lights and from the test light. A consequence is that color matches are very sensitive to the relative spectral positions of the cone pigments. Indeed, other things being equal, the retinas of two individuals who set different color matches must contain photopigments that differ in their spectral positioning. This single fact constitutes one of the earliest and still most persuasive examples of a compulsive linkage between behavior and nervous system organization. Most humans have trichromatic color vision because their retinas contain three classes of cone pigment, each of which behaves univariantly. It is obvious from the absorption spectra
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