Chapter 9: Perceiving Color

Figure 9-1 p200

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

Figure 9-6 p202

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

Figure 9-7 p202

Figure 9-8 p203

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).

Figure 9-10 p205

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.

Figure 9-12 p206

Figure 9-16 p208

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.

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.

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

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 farther in the brain

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.”