Presentation on theme: "Chapter 9: Perceiving Color. Figure 9-1 p200 Color and Wavelength - continued Colors of objects are determined by the wavelengths that are reflected."— Presentation transcript:
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. ”