Divariant Color Vision

Figure 10 A comparison of the divariant color vision of most mammals and about 2% of human males with that of trivariant color vision.

the center and antagonistic signals from the opponent cone mechanism in the surrounding receptive field. It would be more appropriate to receive the antagonism from the same area of visual space (i.e., its receptive field center rather than from surrounding retina). Nevertheless, this drawback has been disregarded in most models of color vision, which routinely employ this variety of retinal cell. It has been intuitively assumed that visual cortex can correct this deficiency by assembling groups of these cells that subserve coextensive areas of visual space.

A second complexity involved in using the midget system to transmit information about color is that it is also transmitting information about achromatic contrast. Therefore, the brain needs two different detectors to extract the achromatic from the chromatic information from the same neural channel. A scheme suggesting how chromatic and achromatic information, "multiplexed," in a single neural channel is demultiplexed by the brain is shown in Fig. 11. In this scheme synergistic signals from L on and M off channels become logical ways to facilitate the perception of red; similar ones from M on and L off channels are logical to facilitate the perception of green. On the other hand, neurons receiving synergistic inputs from either L on or M on would lose their advantage for chromatic contrast detection but could function to detect achromatic lightness. Similarly, L off and M off channels could function to detect achromatic darkness. By disregarding the source of the cone input achromatic contrast can exploit the fine pixel grain of the fovea. This model does not solve how midget cells with concentric cone opponent fields compare cone responses over coextensive areas of visual space.

A different model has been proposed by Rodieck, who assumes that the midget system does not mediate chromatic contrast. Instead, he postulates a much smaller group of retinal ganglion cells that receive coextensive inputs from L and M cones. These cells would be similar to the S cone retinal channel and perhaps also involve bistratified retinal ganglion cells. There is no anatomical evidence that such retinal cells exist; however, there is physiological evidence that such cells do exist. Further research is required to distinguish which of these two models is correct.

3. Simultaneous Color Contrast

Simultaneous chromatic contrast is not as strong as simultaneous brightness contrast, probably because the comparison of cones in the same area of visual space provides a unique local signal not present in achromatic contrast. Cells sensitive to simultaneous chromatic contrast are first found in visual cortex. Such cells respond best to a red area surrounded by green or the converse. The logical way to establish such cells is to have similar color comparison in one area of space inhibit a similar comparison in neighboring areas or have dissimilar comparisons excite each other (Fig. 12). Cells organized in this manner are first encountered in striate cortex but have also been detected in higher visual areas.

Figure 11 A scheme showing how the same signals from the midget cell system are processed by different sets of parallel circuits, which extract a chromatic signal on the left and an achromatic signal on the right. The chromatic detector mixes synergistic signals from L-on and M-off cells. The achromatic detectors mix only on or only off signals but from either L or M cones.

Figure 11 A scheme showing how the same signals from the midget cell system are processed by different sets of parallel circuits, which extract a chromatic signal on the left and an achromatic signal on the right. The chromatic detector mixes synergistic signals from L-on and M-off cells. The achromatic detectors mix only on or only off signals but from either L or M cones.

Figure 12 For simultaneous color contrast, the logical arrangement is to have cells responsive to a particular color (in this case ''red'' or ''green'' in one area of space) inhibit the same type of neuron in a neighboring area of space. In addition, a cell detecting one color should excite cells detecting the opponent color in neighboring areas of space. In this way, red/green or yellow/blue borders can be enhanced.

Figure 12 For simultaneous color contrast, the logical arrangement is to have cells responsive to a particular color (in this case ''red'' or ''green'' in one area of space) inhibit the same type of neuron in a neighboring area of space. In addition, a cell detecting one color should excite cells detecting the opponent color in neighboring areas of space. In this way, red/green or yellow/blue borders can be enhanced.

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