Chapter 7: Color perception

1. 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.

2. 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.

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

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

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

9. 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.

13. Non-spectral colors

14. 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.

15. 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.

17. 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.

18. 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.

20. 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.

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

22. 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.

23. Color Adaptation • Tinted glasses • Color afterimages (Plate 9)

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

25. 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.

34. 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

35. 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.

40. 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.

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

43. 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.

44. 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

45. Color opponency involves: • one achromatic system • two chromatic systems

47. The channels account for brightness of some colors versus others

48. 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.