Trivariance

In the 1850s, Maxwell in England demonstrated that all human color perception depends on three variables, which he assumed reflected three different types of cones. In Germany, Helmholtz was arriving at a similar conclusion, led by the intuitive insights of Grassmann. The ability to define color by measured amounts of three physically defined variables led to colorimetry. Any color can now be defined by three international standards. The spectral properties of the

1. Blue-Yellow Opponency

A scheme that combines physiology and Hering's color opponency is shown in Fig. 9. The operations occurring at the retinal and geniculate level are separated from those thought to be occurring in visual cortex. For blue-yellow color vision, the brain combines excitatory signals from S cone on bipolars with excitatory signals from long-wave cone off bipolars at the retinal level. This on-off-S cone channel excites a cell in the cortex that signals blue; this cell also receives antagonistic (inhibitory) signal from L and M

Figure 9 Circuitry that logically organizes the retinogeniculate inputs from the parvocellular system to establish cells responsive to opponent colors in local areas of visual space. (Right) The circuitry of blue-yellow opponent colors in which two different cortical cells receive inputs from the retinal S cone channel, on the one hand, and the long-wavelength tonic system, on the otherhand. The latter excites and the former inhibits the cell detecting ''yellow.'' The converse arrangement affects the cell detecting ''blue.'' If neither cell is excited, achromatic vision determines the color as white, gray, or black. (Left) The circuitry of red-green opponent colors. L cone on and M cone off signals excite and L cone off and M cone on signals inhibit a cell detecting ''red''; the converse arrangement detects ''green.''

Figure 9 Circuitry that logically organizes the retinogeniculate inputs from the parvocellular system to establish cells responsive to opponent colors in local areas of visual space. (Right) The circuitry of blue-yellow opponent colors in which two different cortical cells receive inputs from the retinal S cone channel, on the one hand, and the long-wavelength tonic system, on the otherhand. The latter excites and the former inhibits the cell detecting ''yellow.'' The converse arrangement affects the cell detecting ''blue.'' If neither cell is excited, achromatic vision determines the color as white, gray, or black. (Left) The circuitry of red-green opponent colors. L cone on and M cone off signals excite and L cone off and M cone on signals inhibit a cell detecting ''red''; the converse arrangement detects ''green.''

cones. This on-off-S cone channel also antagonizes (inhibits) a cell in cortex that signals yellow; this same cell receives an excitatory signal from the L and M cone on-system. These two cells form a blue-yellow opponent system.

If the S cone on-system and the L-M cone on system are excited, the blue and yellow cells are both silent (inhibited) and the system defaults to black-and-white (achromatic) vision. If the S cone on system is excited and the L-M cone on system is not excited (its off system is excited), the color is blue (and dark). If the S cone on system is not excited and the L-M cone on system is, the color is yellow. If both the S cone and the L-M cone on systems are not excited, the color is black (and dark). Whether white and yellow are dark (i.e., gray or brown) is determined by achromatic simultaneous brightness contrast. Humans share this prototypical blue-yellow color vision system, with many other mammals.

The scheme as it stands, however, is deficient in not addressing simultaneous color contrast and in disregarding a fundamental principle in the Land model of color vision, which requires that each cone's system's response be normalized over the visual scene before a comparsion is made for color.

2. Red-Green Opponency

In primates, a second comparison arose with the evolution of two different long-wavelength-sensitive opsins, which split the bright and yellow part of the visible spectrum in two. This provided a new dimension of chromatic contrast (i.e., red-green) (Figs. 9 and 10). The M on and L off channels are not brought together in the same retinal neuron as is the case for the S cone system, presumably because the L and M systems mediate achromatic as well as chromatic contrast. At the cortical level, where many more neurons are available, these systems are brought together in an opponent manner to form cells that respond uniquely to color (i.e., red or green). Again, this scheme disregards simultaneous chromatic contrast as well as the requirement required in the Land model of color vision for normalization of the responses of each cone mechanism over space before color is determined.

This model has another weakness in disregarding a role for the S cone system in determining redness. There is evidence for trivariant interactions in the perception of red-green opponent colors, and this has not been incorporated into this scheme.

There is another controversy in neural modeling of the red-green opponent system. There are two competing models that have been proposed to mediate red-green opponent responses as depicted in Fig. 9. One theory employs all the midget cells of the fovea as well as hypothetical midget-like cells in the parafovea. This is attractive because these cells are very numerous and they possess an essential requirement for transmitting signals for color to the brain. They isolate in one neural channel the signals of one spectral type of cone. However, they have an inappropriate retinal receptive field for a cell mediating color vision. These cells receive excitatory signals from one cone mechanism in

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