Chapter 9: Perceiving Color

What Are Some Functions of Color Vision?

• Color signals help us classify and identify objects.

• Color facilitates perceptual organization of elements into objects.

• Color vision may provide an evolutionary advantage in foraging for food.

Figure 9-1 p200

Figure 9-2 p201

What Colors Do We Perceive?

• Basic colors are red, yellow, green, and blue

• Color circle shows perceptual relationship among colors

• Colors can be changed by:

– Intensity which changes perceived brightness

– Saturation - adding white to a color results in less saturated color

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Figure 9-4 p201

Color and Wavelength

• Color perception is related to the wavelength of light:

– 400 to 450nm appears violet

– 450 to 490nm appears blue

– 500 to 575nm appears green

– 575 to 590nm appears yellow

– 590 to 620nm appears orange

– 620 to 700nm appears red

Color and Wavelength - continued

• Colors of objects are determined by the wavelengths that are reflected

• Reflectance curves - plots of percentage of light reflected for specific wavelengths

• Chromatic colors or hues - objects that preferentially reflect some wavelengths

– Called selective reflectance

• Achromatic colors - contain no hues

– White, black, and gray tones

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Table 9-1 p202

Color and Wavelength - continued

• Selective transmission:

– Transparent objects, such as liquids, selectively allow wavelengths to pass through

• Simultaneous color contrast - background of object can affect color perception

Color and Wavelength - continued

• Additive color mixture:

– Mixing lights of different wavelengths

– All wavelengths are available for the observer to see

– Superimposing blue and yellow lights leads to white

• Subtractive color mixture:

– Mixing paints with different pigments

– Additional pigments reflect fewer wavelengths

– Mixing blue and yellow leads to green

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Table 9-2 p203

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Trichromatic Theory of Color Vision

• Proposed by Young and Helmholtz (1800s)

– Three different receptor mechanisms are responsible for color vision.

• Behavioral evidence:

– Color-matching experiments

• Observers adjusted amounts of three wavelengths in a comparison field to match a test field of one wavelength.

Behavior Evidence of the Theory

• Results showed that:

– It is possible to perform the matching task

– Observers with normal color vision need at least three wavelengths to make the matches.

– Observers with color deficiencies can match colors by using only two wavelengths.

Figure 9-9 p204

Physiological Evidence for the Theory

• Researchers measured absorption spectra of visual pigments in receptors (1960s).

– They found pigments that responded maximally to:

• Short wavelengths (419nm)

• Medium wavelengths (531nm)

• Long wavelengths (558nm)

• Later researchers found genetic differences for coding proteins for the three pigments (1980s).

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Cone Responding and Color Perception

• Color perception is based on the response of the three different types of cones.

– Responses vary depending on the wavelengths available.

– Combinations of the responses across all three cone types lead to perception of all colors.

– Color matching experiments show that colors that are perceptually similar (metamers) can be caused by different physical wavelengths.

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Figure 9-12 p206

Are Three Receptor Mechanisms Necessary for Color Vision?

• One receptor type cannot lead to color vision because:

– absorption of a photon causes the same effect, no matter what the wavelength is.

– any two wavelengths can cause the same response by changing the intensity.

• Two receptor types (dichromats) solve this problem but three types (trichromats) allow for perception of more colors.

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Figure 9-15 p207

Color Deficiency

• Monochromat - person who needs only one wavelength to match any color

• Dichromat - person who needs only two wavelengths to match any color

• Anomalous trichromat - needs three wavelengths in different proportions than normal trichromat

• Unilateral dichromat - trichromatic vision in one eye and dichromatic in other

Figure 9-16 p208

Monochromatism

• Monochromats have:

– A very rare hereditary condition

– Only rods and no functioning cones

– Ability to perceive only in white, gray, and black tones

– True color-blindness

– Poor visual acuity

– Very sensitive eyes to bright light

Dichromatism

• There are three types of dichromatism:

– Protanopia affects 1% of males and .02% of females

• Individuals see short-wavelengths as blue

• Neutral point occurs at 492nm

• Above neutral point, they see yellow

• They are missing the long-wavelength pigment

Dichromatism - continued

• Deuteranopia affects 1% of males and .01% of females

– Individuals see short-wavelengths as blue

– Neutral point occurs at 498nm

– Above neutral point, they see yellow

– They are missing the medium wavelength pigment

Dichromatism - continued

• Tritanopia affects .002% of males and .001% of females

– Individuals see short wavelengths as blue

– Neutral point occurs at 570nm

– Above neutral point, they see red

– They are most probably missing the short wavelength pigment

Figure 9-17 p209

Figure 9-18 p210

Opponent-Process Theory of Color Vision

• Proposed by Hering (1800s)

– Color vision is caused by opposing responses generated by blue and yellow, and by green and red.

• Behavioral evidence:

– Color afterimages and simultaneous color contrast show the opposing pairings

– Types of color blindness are red/green and blue/yellow.

Figure 9-19 p210

Figure 9-20 p211

Table 9-4 p211

Opponent-Process Theory of Color Vision - continued

• Opponent-process mechanism proposed by Hering

– Three mechanisms - red/green, blue/yellow, and white/black

– The pairs respond in an opposing fashion, such as positively to red and negatively to green

– These responses were believed to be the result of chemical reactions in the retina.

Figure 9-21 p211

Physiology Evidence for the Theory

• Researchers performing single-cell recordings found opponent neurons (1950s)

– Opponent neurons:

• Are located in the retina and LGN

• Respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other

Figure 9-22 p212

Trichromatic and Opponent-Process Theories Combined

• Each theory describes physiological mechanisms in the visual system

– Trichromatic theory explains the responses of the cones in the retina

– Opponent-process theory explains neural response for cells connected to the cones further in the brain

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Figure 9-24 p212

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Color in the Cortex

• There is no single module for color perception

– Cortical cells in V1, and V4 respond to some wavelengths or have opponent responses

– These cells usually also respond to forms and orientations

– Cortical cells that respond to color may also respond to white

Types of Opponent Neurons in the Cortex

• Single-opponent neurons

• Double-opponent neurons

Figure 9-26 p214

Color Constancy

• Color constancy - perception of colors as relatively constant in spite of changing light sources

– Sunlight has approximately equal amounts of energy at all visible wavelengths

– Tungsten lighting has more energy in the long-wavelengths

– Objects reflect different wavelengths from these two sources

Figure 9-27 p215

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Color Constancy - continued

• Chromatic adaptation - prolonged exposure to chromatic color leads to receptors:

– “Adapting” when the stimulus color selectively bleaches a specific cone pigment

– Decreasing in sensitivity to the color

• Adaptation occurs to light sources leading to color constancy

Figure 9-29 p216

Color Constancy - continued

• Experiment by Uchikawa et al.

– Observers shown sheets of colored paper in three conditions:

• Baseline - paper and observer in white light

• Observer not adapted - paper illuminated by red light; observer by white

• Observer adapted - paper and observer in red light

Figure 9-30 p216

Color Constancy - continued

• Experiment by Uchikawa et al. results showed that:

– Baseline - green paper is seen as green

– Observer not adapted - perception of green paper is shifted toward red

– Observer adapted - perception of green paper is slightly shifted toward red

• Partial color constancy was shown in this condition

Color Constancy - continued

• Effect of surroundings

– Color constancy works best when an object is surrounded by many colors

• Memory and color

– Past knowledge of an object’s color can have an impact on color perception

Figure 9-31 p217

Experiment by Hansen et al.

• Experiment by Hansen et al.

• Observers saw photographs of fruits with characteristic colors against a gray background.

– They adjusted the color of the fruit and a spot of light.

– When the spot was adjusted to physically match the background, the spot appeared gray.

– But when this done for the fruits, they were still perceived as being slightly colored.

Lightness Constancy

• Achromatic colors are perceived as remaining relatively constant.

– Perception of lightness:

• Is not related to the amount of light reflected by an object

• Is related to the percentage of light reflected by an object

Figure 9-32 p218

Lightness Constancy - continued

• The ratio principle - two areas that reflect different amounts of light look the same if the ratios of their intensities are the same

• This works when objects are evenly illuminated.

Lightness Perception Under Uneven Illumination

• Lightness perception under uneven illumination

– Perceptual system must distinguish between:

• Reflectance edges - edges where the amount of light reflected changes between two surfaces

• Illumination edges - edges where lighting of two surfaces changes

Figure 9-33 p219

Lightness Perception Under Uneven Illumination - continued

• Sources of information about illumination:

– Information in shadows - system must determine that edge of a shadow is an illumination edge

• System takes into account the meaningfulness of objects.

• Penumbra of shadows signals an illumination edge.

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Color Is a Construction of the Nervous System

• Physical energy in the environment does not have perceptual qualities.

– Light waves are not “colored.”• Different nervous systems experience

different perceptions.

• Honeybees perceive color which is outside human perception.

– We cannot tell what color the bee actually “sees.”

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Infant Color Vision

• It is a complex problem to know what an infant really “sees”– Chromatic color

– Brightness

• Bornstein et al (1976)– Habituation

– Young infants have color vision

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