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FIGURE 15 Spectral sensitivity of cone photopigments and chromatic aberration in the eye. (A) Four types of photoreceptors in the human retina each contain different photopigments that are maximally about 560 nm and are the only photopigments sensitive to red light. Short wavelengths are refracted more strongly than long ones; therefore, a full-spectrum image will not be focused in the same plane and will appear blurred. There are two special adaptations to decrease this chromatic aberration. First, short-wavelength receptors (blue-sensitive cones and rods containing rhodopsin with peak absorption at 500 nm) are absent from the fovea. Second, an inert pigment, macular yellow, which strongly absorbs blue and violet (and hence looks yellow) is deposited in the macula and blocks short-wavelength light from affecting green and red foveal cones.

Color-Opponent Pathways

The three types of cones are coupled in mutually inhibitory, color-opponent fashion to produce color-specific receptive fields in neurons of the visual pathway. Two color-opponent pairs are established: red versus green and blue versus yellow. Figure 16 illustrates the general circuitry for these pathways and the types of color-opponent receptive fields that can result. Ganglion cells in the red/green pathway have single-opponent, ON and OFF, center-surround receptive fields created in part by horizontal cells providing inhibitory "surrounds" to cone/bipolar connections. The four types of ganglion cells in this pathway are classified according to whether they are stimulated or inhibited by red or green light in the center of their receptive fields. Opposite effects are caused by red or green light in the surround portion of the receptive field.

The perception of yellow is generated by combined (rather than opposing) input from red- and green-sensitive cones. Input from yellow (red plus green)-sensitive bipolar cells is compared with input from blue-sensitive cones to form blue versus yellow color-opponent, ON and OFF, ganglion cell types that lack center-surround receptive fields.

Color-coded information is transmitted along P ganglion cells via the lateral geniculate to blob regions of the primary visual cortex (VI). There, information is combined to form double-opponent cells that are sensitive to two wavelengths of light in both center and surround sensitive to different wavelengths. (B) Wavelength absorption for the four types of photopigments in the human retina. Different colors or hues are created by the nervous system depending on the ratio of light absorption by the three photopigments. (C) Rays of light containing a single wavelength of light, such as 600 nm (red), can be focused on the retina through the actions of the lens. If the light is a mixture of long and short wavelengths (for example, purple), the short wavelengths will be more refracted than long wavelengths, and the lens cannot focus both wavelengths simultaneously on the retina; the image will be blurred. (D) A simulation of the appearance of an object (left) when the object is red (middle) or purple (right).

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