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Chapter 9: Perceiving Color

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1 Chapter 9: Perceiving Color

2 Figure 9. 1 (a) Red berries in green foliage
Figure 9.1 (a) Red berries in green foliage. (b) These berries become more difficult to detect without color vision. Figure 9-1 p200

3 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

4 Figure 9.6 Reflectance curves for surfaces that appear white, gray, and black, and for blue, green, and yellow pigments and a red tomato. Figure 9-6 p202

5 Table 9-1 p202

6 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

7 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

8 Figure 9. 7 Color mixing with light
Figure 9.7 Color mixing with light. Superimposing a blue light and a yellow light creates the perception of white in the area of overlap. This is additive color mixing. Figure 9-7 p202

9 Figure 9. 8 Color mixing with paint
Figure 9.8 Color mixing with paint. Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color mixture. Figure 9-8 p203

10 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.

11 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.

12 Figure 9.9 In a color-matching experiment, the observer adjusts the amount of three wavelengths in one field (right) so that it matches the color of the single wavelength in the other field (left). Figure 9-9 p204

13 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).

14 Figure 9.10 Absorption spectra of the three cone pigments.
Figure 9-10 p205

15 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.

16 Figure 9. 12 Principle behind metamerism
Figure 9.12 Principle behind metamerism. The proportions of 530- and 620-nm lights in the field on the left have been adjusted so that the mixture appears identical to the 580-nm light in the field on the right. The numbers indicate the responses of the short-, medium-, and long-wavelength receptors. There is no difference in the responses of the two sets of receptors, so the two fields are perceptually indistinguishable. Figure 9-12 p206

17 Figure 9. 16 Ishihara plate for testing color deficiency
Figure 9.16 Ishihara plate for testing color deficiency. (a) A person with normal color vision sees a “74” when the plate is viewed under standardized illumination. (b) Ishihara plate as perceived by a person with a form of red–green color deficiency. Figure 9-16 p208

18 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.

19 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.

20 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

21 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

22 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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