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

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. Having terms for colors facilitates their recall.

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

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

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

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

Non-spectral colors

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.

Updating Newton’s wheel Delete the sharp boundaries between the colors. 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.) 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. 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 of objects The exact spectrum of light, however, varies under certain conditions. Sunset Incandescent light Florescent light

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

Color Adaptation Tinted glasses 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 constancy based on local and global factors. Color constancy needs broadband illumination. Highlights provide cues.

Trichromacy What would it be like to have just one type of color sensitive cell? 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 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. Individuals with only one cell type are monochromats

Trichromacy 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. 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. Evolutionarily unlikely: Would require rare mutations 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 three cone types S: Peak sensitivity at 430 nm. M: Peak sensitivity at 530 nm. L: Peak sensitivity at 570 nm.

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

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

Color opponency involves: one achromatic system 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 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.

R = M-L opponent cells at long wavelengths G = M-L opponent cells at short wavelengths Y = S – (M+L) opponent cells at long wavelengths B = S – (M+L) opponent cells at short wavelengths

Normal Protanopia (No L) Deuteranopia (No M) Tritanopia (No S)

http://webexhibits.org/causesofcolor/2.html#vissamp

Gross features of dichromacy Protanopic and deuteranopic vision are relatively similar, since the L-cones and M-cones have relatively similar absorption spectra. Protanopic and deuteranopic vision lose red, since the signals for red are reduced. Tritanopic vision is relatively “bright” since the darkening blue signal is removed.

A Puzzle Why do protanopes and deuteranopes see violet abnormally? Don’t both of these colors essentially not stimulate the M- and L-cones? Maybe the textbook curves are mere “cartoons”.

Acquired color abnormalities Glaucoma & diabetes can disturb S cones. Alcoholism leads to a B12 deficiency which causes optic nerve damage causing reduced sensitivity to red wavelengths. Yellowing of the lens. Brain damage (to v4). Can lose color constancy, while preserving color discrimination and color naming.

Macaca fuscata Reared w/ 12 hours/day of monochromatic light 465, blue 517, green 592, yellow 641, red

Macaca fuscata results after one year Slow to learn color matching Developed distinct color categories No apparent color constancy These problems appear to involve the post-receptoral processing.

Neitz, et al., (2002) Further evidence for post-receptoral processing of color: Wearing tinted lens over one eye, leads to color compensation in the other eye. Suggests that the brain assumes that ambient light is roughly white, hence tries to compensate.

Color synesthesia Letter-color pairings are idiosyncratic, but reliably constant in each individual. They are probably “real”: They are perceived effortlessly They induce “popout” They are vivid and saturated They are limited to the letter They are not confused with actual colors.