B. Opponent processing in the red - green system

L cone is red sensitive

M cone is green sensitive

red-ON GC red-OFF GC green-ON GC green-OFF GC

L cone is red sensitive

M cone is green sensitive blue-ON GC active in short wavelengths blue-OFF GC active in long wavelengths red-ON GC red-OFF GC green-ON GC green-OFF GC

Blue- ON / yellow-OFF V1 blob cell blue-OFF / yellow-ON V1 blob cell

Red- ON / green-OFF V1 blob cell red-OFF / green-ON V1 blob cell

FIGURE 16 Color-opponent receptive fields of ganglion cells and neurons in the blob regions of the V1 cortex. (A) Three types of cones contribute to blue/yellow-opponent receptive fields in ganglion cells (GCs) and cells within blobs of cortical area V1. Excitatory input from blue-sensitive (S) cones and inhibitory input from redsensitive (L) and green-sensitive (M) cones cause one class of ganglion cells, blue-ON, to be excited in blue light (B + ) and to be inhibited in yellow light, as detected by the combined responses of both red- and green-sensitive cones (R-, G-). Another class of ganglion cells, blue-OFF, has the opposite response and is inhibited by blue light and excited by yellow light. Both ganglion cell types send information (via the lateral geniculate nucleus; not shown) to two major types of cortical cells that are clustered in blue/yellow-sensitive V1 blobs and exhibit doubleopponent, center-surround receptive fields. (B) Two types of cones contribute to red/green-opponent receptive fields. There are four types of single-opponent ganglion cells with center-surround receptive fields. They send information to two types of red/green-sensitive V1 blob cells that display double-opponent, center-surround receptive fields.

portions of their receptive fields. Understanding this circuitry can be helpful in appreciating certain aspects of color perception. For example, it explains why differently colored backgrounds can affect the apparent color of an object (e.g., why a red vase looks redder when placed on a green background or why it is impossible to perceive the color of an object as red-green [in humans, there is no such color as reddish green]). The latter occurs because red and green input either cancel each other out in the red versus green pathway or become additive to produce the perception of yellow in the yellow versus blue pathway.

Most nonhuman mammals have only two cone pigments. One is sensitive to shorter wavelengths (S, blue sensitive); the other is sensitive to longer wavelengths of light (L, covering the green to red portions of the spectrum). This more primitive S versus L system has been supplemented in humans through duplication and mutation of the gene for the L pigment to produce two slightly different photopigments, allowing discrimination of multiple hues in the middle (green) to long (red) wavelengths. Genes for red- and green-sensitive pigments are very similar and located on the X chromosome near the 5' end. Thus, recombination can result in the placement of three genes on one chromosome, leaving only a single L or M gene on the other. A male who inherits a maternal X chromosome containing only the L or M pigment gene will be red or green color blind, respectively.

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