Chapter 7: Color perceptionChapter 7: Color perception

Color is an important source of Color is an important source of information independent of information independent of luminance (which we discussed luminance (which we discussed extensively in Chapter 5). extensively in Chapter 5).

Facilitates detection, identification, Facilitates detection, identification, and discrimination.and discrimination.

Color termsColor terms

Different cultures have different color Different cultures have different color terms.terms.

In our culture, there are a few common In our culture, there are a few common color terms upon which there is color terms upon which there is widespread agreement, but beyond this, widespread agreement, but beyond this, there are divergences.there are divergences.

Having terms for colors facilitates their Having terms for colors facilitates their recall.recall.

Innateness of color Innateness of color perceptionperception

Habituation paradigmHabituation paradigmHabituate to 480 nm (blue) light.Habituate to 480 nm (blue) light.Show 450 nm (blue) light or 510 nm Show 450 nm (blue) light or 510 nm

(green) light.(green) light.

Color discriminationColor discrimination

Simultaneous comparison: ~2.3 Simultaneous comparison: ~2.3 million.million.

Serial comparisons: < 12.Serial comparisons: < 12.

Color terminologyColor terminology

Hue: the quality that distinguishes Hue: the quality that distinguishes red from yellow from blue.red from yellow from blue.

Brightness: related to illumination.Brightness: related to illumination.Saturation: vividness or paleness.Saturation: vividness or paleness.

White light is compositeWhite light is composite

White light can be produced by different White light can be produced by different combinations of complementary “pure combinations of complementary “pure colors”.colors”.

Yellow + blue Yellow + blue lightlight looks white to humans. looks white to humans.

Orange + greenish-blue Orange + greenish-blue light light looks white to looks white to humans.humans.

Colorimeters, however, are not fooled.Colorimeters, however, are not fooled.

Non-spectral colorsNon-spectral colors

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

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

Updating Newton’s wheelUpdating Newton’s wheel

Delete the sharp boundaries between Delete the sharp boundaries between the colors.the colors.

Colors must be spaced around the Colors must be spaced around the wheel so that complementary colors wheel so that complementary colors are separated by 180 degrees.are separated by 180 degrees.

The improved wheel is not The improved wheel is not perfect:perfect:

1. Some colors appear brighter than 1. Some colors appear brighter than others. This has to do with the others. This has to do with the greater sensitivity of the eye to greater sensitivity of the eye to certain colors.certain colors.

The improved wheel is not The improved wheel is not perfect:perfect:

Bright colors tend to appear Bright colors tend to appear desaturated (Cf. Plate 7.)desaturated (Cf. Plate 7.)

The amount of light needed to The amount of light needed to desaturate a given color is shown in desaturate a given color is shown in Figure 7.8.Figure 7.8.

The colors of objectsThe colors of objects

Most environments provide a broad Most environments provide a broad spectrum of light.spectrum of light.

Thus, most objects have the colors Thus, most objects have the colors they do in virtue of absorbing some they do in virtue of absorbing some frequencies of light and reflecting the frequencies of light and reflecting the others.others.

The color reflecting property of an The color reflecting property of an object is its object is its spectral reflectancespectral reflectance..

The color of objectsThe color of objects

The exact spectrum of light, The exact spectrum of light, however, varies under certain however, varies under certain conditions.conditions.

SunsetSunset Incandescent lightIncandescent lightFlorescent lightFlorescent light

Color constancyColor constancy

Despite the variations in illuminating Despite the variations in illuminating light, objects retain the general color.light, objects retain the general color.

This requires making compensations This requires making compensations for illuminating light in computing for illuminating light in computing spectral reflectance.spectral reflectance.

This appears to work by color This appears to work by color adaptation, akin to light adaptation.adaptation, akin to light adaptation.

Color AdaptationColor Adaptation

Tinted glassesTinted glasses

Color afterimages (Plate 9)Color afterimages (Plate 9)

Color adaptation is not the whole Color adaptation is not the whole story on color constancystory on color constancy

Color contrast in adjacent regions is Color contrast in adjacent regions is an important element maintaining an important element maintaining color constancy.color constancy.

Color constancy needs broadband Color constancy needs broadband illumination.illumination.

Highlights provide cues.Highlights provide cues.

TrichromacyTrichromacy

What would it be like to have just one type What would it be like to have just one type of color sensitive cell?of color sensitive cell?

You have a massive ambiguity problem.You have a massive ambiguity problem.

A single cell type just fires with a A single cell type just fires with a frequency that indicates the intensity of frequency that indicates the intensity of the incident light, i.e. firing rate is the incident light, i.e. firing rate is proportional to number of photons hitting proportional to number of photons hitting the cell.the cell.

A single type of cone is another A single type of cone is another instance of the ambiguity problem instance of the ambiguity problem discussed in Chapters 4 and 5.discussed in Chapters 4 and 5.

In other words, many combinations In other words, many combinations of incident light frequencies will be of incident light frequencies will be metameric.metameric.

Individuals with only one cell type Individuals with only one cell type are monochromatsare monochromats

TrichromacyTrichromacy

What would it be like to have just two What would it be like to have just two types of color sensitive cells?types of color sensitive cells?

Although no single cell provides color Although no single cell provides color information, in tandem the two types do.information, in tandem the two types do.

This would provide at least some help in This would provide at least some help in solving the ambiguity problem: it would solving the ambiguity problem: it would make ensemble coding of color possible.make ensemble coding of color possible.

Four eye pigments?Four eye pigments?

This would reduce the number of color This would reduce the number of color metamers.metamers.

Evolutionarily unlikely:Evolutionarily unlikely: Would require rare mutationsWould require rare mutations There would have to be more central There would have to be more central

information processing structures to use the information processing structures to use the four types of receptors.four types of receptors.

The additional discriminatory capacity must The additional discriminatory capacity must increase fitness.increase fitness.

The three cone typesThe 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:Cone facts:

The number of cones per person is The number of cones per person is highly variable.highly variable.

Spatial distribution of cones is highly Spatial distribution of cones is highly variable.variable.

Cone pigment types have a common Cone pigment types have a common evolutionary ancestor.evolutionary ancestor.

Color opponencyColor opponency

Brightness contrast exaggerates Brightness contrast exaggerates differences between adjacent areas.differences between adjacent areas.

Color contrast exaggerates Color contrast exaggerates differences between adjacent areas.differences between adjacent areas.Red-greenRed-greenBlue-yellowBlue-yellowCan’t have reddish-green lightCan’t have reddish-green lightCan’t have bluish-yellow lightCan’t have bluish-yellow light

Color opponency involves:Color opponency involves:

one one achromaticachromatic system systemtwo two chromaticchromatic systems systems

The channels account for The channels account for brightness of some colors versus brightness of some colors versus

othersothers

Physiological evidence for color Physiological evidence for color opponency channels in parvo opponency channels in parvo

LGNLGNSome ON cells respond to all Some ON cells respond to all

wavelengths of light (non-opponent wavelengths of light (non-opponent achromatic channel).achromatic channel).

Some cells increase activity in Some cells increase activity in response to long wavelengths, but response to long wavelengths, but decrease activity in response to short decrease activity in response to short wavelengths. (chromatic opponent wavelengths. (chromatic opponent channels)channels)

The opponent channels vary in their The opponent channels vary in their wavelength sensitivities.wavelength sensitivities.