Perception of Color
Color vision processing in the primate visual system is initiated by absorption of light by three different spectral classes of cones. Consequently, color vision is described as being trivariant or trichromatic, and initial psychophysical studies demonstrated that colors could be matched by the use of three different primaries. In 1802, Thomas Young proposed a model that perception of color can be coded by three principal color receptors rather than thousands of color receptors coding for individual colors.
Spectral sensitivity of cones can be determined through several methods. Two of these methods include isolating receptoral responses (Baylor et al, 1984) using calculation from color matching function of normals and dichromats (Smith and Pokorny, 1975; a dichromat is a subject whose retina has one cone photopigment missing), microspectrometry (Bowmaker and Dartnall, 1980) or reflection densitometry (Rushton, 1963, 1966). The microspectrometer technique involves isolating a single cone and passing light through it. The change in transmission of different wavelengths can be used to calculated the spectral absorption of the cone or determine the change in electrical response. Reflection densitometry involves directing light in the retina and determining the change in absorption as a function of wavelength. These results are subsequently used to calculate spectral absorption.
Three classes of cones in the human retina have been isolated from the above techniques. These three classes of cones are the short-wavelength sensitive (S-cones), middle-wavelength sensitive (M-cones) and long-wavelength sensitive (L-cones), and all have different but overlapping spectral sensitivities. The spectral sensitivity of S-cones peak at approximately 440 nm, M-cones peak at 545 nm and L-cones peak at 565 nm after corrected for pre-retinal light loss, although the various measuring techniques result in slightly different maximum sensitivity values (figure 4).
Figure 4. Spectral sensitivity of the S-cone, M-cone and L-cone. Combined results from various authors using different methods including retinal densitometry from Rushton, microspectrometry from Brown and Wald and increment threshold producing artificial monochromasy from Brinley (D and s ) and increment threshold measurements from Wald (5 ) (From Moses, R. A. and Hart, W. M. (Ed) Adler's Physiology of the Eye, Clinical Application. St. Louis: The C. V. Mosby Company, 1987)
The trichromatic theory was first proposed by Thomas Young in 1802 and was explored further by Helmholtz in 1866. This theory is primarily based on color mixing experiment and suggests that a combination of three channels explain color discrimination functions.
Evidence for the trichromatic theory includes:
However, the trichomatic theory fails to account for the four unique colors: red, green, yellow and blue, and also fails to explain why dichromats can perceive white and yellow. It also fails to fully explain color discrimination functions and opponent color percepts.
The opponent color theory was first proposed by Hering in 1872. At the time, this theory rivalled the well accepted trichromatic theory which explains the trichromasy of vision and predicts color matches. Hering's opponent color theory suggests that there are three channels: red-green, blue-yellow and black-white, with each responding in an antagonist way. That is, either red or green is perceived and never greenish-red. Hering, however, never challenged the initial stages of processing expressed by the trichromatic theory. He simply argued that any color vision theory should explain our perception, that is, color opponency as revealed by colored after images.
Hurvich and Jameson (1957) provided quantitative data for color opponency. Using hue cancellation paradigms, the psychophysical color opponent channels were isolated. The V; function was used to brightness discrimination to describe the perception of blackness and whiteness. Therefore, by adjusting the amount of blue or yellow and red or green, any sample wavelength can be matched (figure 5). Complementary wavelengths can be used to cancel each other for all wavelengths except the four unique hues (blue, green, yellow and red).
Figure 5. Hurvich and Jameson experiment using blue or yellow and red or green to match all wavelengths of the visible spectrum (Hurvich and Jameson's data (1957) from Benjamin, W. J. (Ed), Borish's Clinical Refraction. Philadelphia: W. B. Saunders Company, 1998)
Other evidence supporting the opponent color theory include:
1. Electrical recordings of horizontal cells from fish retina show blue-yellow opponent process and red-green opponent (Svaetichin, 1956).
Stage Theory: This has led to the modern model of normal color vision which incorporates both the trichromatic theory and the opponent color theory into two stages (figure 6). The first stage can be considered as the receptor stage which consists of the three photopigments (blue, green and red cones). The second is the neural processing stage where the color opponency occurs. The second stage is at a post-receptoral level, and occurs as early as the horizontal cell level.
Figure 6. Model for normal human colour vision
Figure 7. The visual pathways from retina to visual cortex of the human brain
Inhabiting a world in which an organism can only distinguish light and dark without color is a handicap. Therefore early in the evolution of vision, color must have appeared. For this, two different types of cones were necessary, one responding best to one part and a second responding best to another part of the visible spectrum, i.e. sunlight. By this means the brain can compare two signals to distinguish color. Again the brain senses relative rather than absolute differences, in this case of wavelengths rather than energies of light.
Figure. 8. The wavelength sensitivities of the different photoreceptor types in the vertebrate retina.(Blue-violet and yellow-green wavelengths maximally stimulate the two cone types in the divariant mammalian retina)
A second cone system evolved that was most sensitive to the short wavelength region of the visible spectrum, the region we call bluish-violet (Fig. 8). The output of these cones could be compared with the earlier long wave cones that evolved to detect light and dark, and are most sensitive to the yellow-green part of the spectrum (Fig. 8). The spectral sensitivity of a cone is determined by the absorption spectrum of its opsin. The further apart these absorption spectra are, the greater is the potential color contrast. are comparatively small as in mice, an ultra-violet cone becomes more tractable.
With two different spectral images of an object and/or its background, the brain can derive differences that are impossible to detect with only one spectral image. Now the brain can distinguish objects, which are not just lighter or darker than their background but have as new attribute, color. An object is uniquely white, gray or black if the two different cone mechanisms are affected equally by the reflected energy of the object and its background. If an inequality exists between the two cone mechanisms, detecting the light reflected by either the object or its background, the brain will see color, making the object or its background appear different than white, gray or black. Objects can have the same brightness as their backgrounds but stand out because of this inequality. This is a powerful way to see objects in a world where most things reflect about the same light as their background.
Nature uses these two cone mechanisms differently. The long wave cones are the sole determinant of light and dark. The short wave cones are only used for color contrast. This strategy minimizes chromatic aberration. For this reason there are about ten times as many long than short wave sensitive cones in most retinas. In the human central fovea there are very few short wave cones and borders are detected by brightness alone.