Chapter 7: Color perception Color is an important source of information independent of luminance (which we discussed extensively in Chapter 5). Color is an important source of information independent of luminance (which we discussed extensively in Chapter 5). Facilitates detection, identification, and discrimination. Facilitates detection, identification, and discrimination.

Color terms Different cultures have different color terms. Different cultures have different color terms. In our culture, there are a few common color terms upon which there is widespread agreement, but beyond this, there are divergences. In our culture, there are a few common color terms upon which there is widespread agreement, but beyond this, there are divergences. Having terms for colors facilitates their recall. Having terms for colors facilitates their recall.

Innateness of color perception Habituation paradigm Habituation paradigm Habituate to 480 nm (blue) light. Habituate to 480 nm (blue) light. Show 450 nm (blue) light or 510 nm (green) light. Show 450 nm (blue) light or 510 nm (green) light.

Color discrimination Simultaneous comparison: ~2.3 million. Simultaneous comparison: ~2.3 million. Serial comparisons: < 12. Serial comparisons: < 12.

Color terminology Hue: the quality that distinguishes red from yellow from blue. Hue: the quality that distinguishes red from yellow from blue. Brightness: related to illumination. Brightness: related to illumination. Saturation: vividness or paleness. Saturation: vividness or paleness.

White light is composite White light can be produced by different combinations of complementary “pure colors”. White light can be produced by different combinations of complementary “pure colors”. Yellow + blue light looks white to humans. Yellow + blue light looks white to humans. Orange + greenish-blue light looks white to humans. Orange + greenish-blue light looks white to humans. Colorimeters, however, are not fooled. Colorimeters, however, are not fooled.

Non-spectral colors

Newton’s color wheel gives the hue of a mixture of light. Newton’s color wheel gives the hue of a mixture of light. Newton’s color wheel also gives the saturation of a mixture of light. Newton’s color wheel also gives the saturation of a mixture of light.

Updating Newton’s wheel Delete the sharp boundaries between the colors. Delete the sharp boundaries between the colors. Colors must be spaced around the wheel so that complementary colors are separated by 180 degrees. Colors must be spaced around the wheel so that complementary colors are separated by 180 degrees.

The improved wheel is not perfect: 1. Some colors appear brighter than others. This has to do with the greater sensitivity of the eye to certain colors.

The improved wheel is not perfect: Bright colors tend to appear desaturated (Cf. Plate 7.) Bright colors tend to appear desaturated (Cf. Plate 7.) The amount of light needed to desaturate a given color is shown in Figure 7.8. The amount of light needed to desaturate a given color is shown in Figure 7.8.

The colors of objects Most environments provide a broad spectrum of light. Most environments provide a broad spectrum of light. Thus, most objects have the colors they do in virtue of absorbing some frequencies of light and reflecting the others. Thus, most objects have the colors they do in virtue of absorbing some frequencies of light and reflecting the others. The color reflecting property of an object is its spectral reflectance. The color reflecting property of an object is its spectral reflectance.

The color of objects The exact spectrum of light, however, varies under certain conditions. The exact spectrum of light, however, varies under certain conditions. Sunset Sunset Incandescent light Incandescent light Florescent light Florescent light

Color constancy Despite the variations in illuminating light, objects retain the general color. Despite the variations in illuminating light, objects retain the general color. This requires making compensations for illuminating light in computing spectral reflectance. This requires making compensations for illuminating light in computing spectral reflectance. This appears to work by color adaptation, akin to light adaptation. This appears to work by color adaptation, akin to light adaptation.

Color Adaptation Tinted glasses Tinted glasses Color afterimages (Plate 9) Color afterimages (Plate 9)

Color adaptation is not the whole story on color constancy Color contrast in adjacent regions is an important element maintaining color constancy. Color contrast in adjacent regions is an important element maintaining color constancy. Color constancy needs broadband illumination. Color constancy needs broadband illumination. Highlights provide cues. Highlights provide cues.

Trichromacy What would it be like to have just one type of color sensitive cell? What would it be like to have just one type of color sensitive cell? You have a massive ambiguity problem. You have a massive ambiguity problem. A single cell type just fires with a frequency that indicates the intensity of the incident light, i.e. firing rate is proportional to number of photons hitting the cell. A single cell type just fires with a frequency that indicates the intensity of the incident light, i.e. firing rate is proportional to number of photons hitting the cell.

A single type of cone is another instance of the ambiguity problem discussed in Chapters 4 and 5. A single type of cone is another instance of the ambiguity problem discussed in Chapters 4 and 5. In other words, many combinations of incident light frequencies will be metameric. In other words, many combinations of incident light frequencies will be metameric. Individuals with only one cell type are monochromats Individuals with only one cell type are monochromats

Trichromacy What would it be like to have just two types of color sensitive cells? What would it be like to have just two types of color sensitive cells? Although no single cell provides color information, in tandem the two types do. Although no single cell provides color information, in tandem the two types do. This would provide at least some help in solving the ambiguity problem: it would make ensemble coding of color possible. This would provide at least some help in solving the ambiguity problem: it would make ensemble coding of color possible.

Four eye pigments? This would reduce the number of color metamers. This would reduce the number of color metamers. Evolutionarily unlikely: Evolutionarily unlikely: Would require rare mutations Would require rare mutations There would have to be more central information processing structures to use the four types of receptors. There would have to be more central information processing structures to use the four types of receptors. The additional discriminatory capacity must increase fitness. The additional discriminatory capacity must increase fitness.

The three cone types S: Peak sensitivity at 430 nm. S: Peak sensitivity at 430 nm. M: Peak sensitivity at 530 nm. M: Peak sensitivity at 530 nm. L: Peak sensitivity at 570 nm. L: Peak sensitivity at 570 nm.

Cone facts: The number of cones per person is highly variable. The number of cones per person is highly variable. Spatial distribution of cones is highly variable. Spatial distribution of cones is highly variable. Cone pigment types have a common evolutionary ancestor. Cone pigment types have a common evolutionary ancestor.

Color opponency Brightness contrast exaggerates differences between adjacent areas. Brightness contrast exaggerates differences between adjacent areas. Color contrast exaggerates differences between adjacent areas. Color contrast exaggerates differences between adjacent areas. Red-green Red-green Blue-yellow Blue-yellow Can’t have reddish-green light Can’t have reddish-green light Can’t have bluish-yellow light Can’t have bluish-yellow light

Color opponency involves: one achromatic system one achromatic system two chromatic systems two chromatic systems

The channels account for brightness of some colors versus others

Physiological evidence for color opponency channels in parvo LGN Some ON cells respond to all wavelengths of light (non-opponent achromatic channel). Some ON cells respond to all wavelengths of light (non-opponent achromatic channel). Some cells increase activity in response to long wavelengths, but decrease activity in response to short wavelengths. (chromatic opponent channels) Some cells increase activity in response to long wavelengths, but decrease activity in response to short wavelengths. (chromatic opponent channels) The opponent channels vary in their wavelength sensitivities. The opponent channels vary in their wavelength sensitivities.