Chapter 9: Perceiving Color

Overview of Questions Why do we perceive blue dots when a yellow flash bulb goes off? What does someone who is “color-blind” see? What colors does a honeybee perceive?

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 9.2 Normally colored fruit and inappropriately colored fruit. (From Tanaka, J. W., Weiskopf, D., & Williams, P. (2001). The role of color in high-level vision, , Trends in Cognitive Sciences, 5,

How Can We Describe Color Experience? 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

What Is the Relationship Between Wavelength and Color Perception? 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

Figure 9.4 The visual spectrum.

Colors of Objects 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 9.5 Reflectance curves for surfaces that appear white, gray, and black, and for blue, green and yellow pigments. Adapted from Clulow, F. W. (1972). Color: Its principles and their applications. New York: Morgan and Morgan.

Table 9.1 Relationship between predominant wavelengths reflected and color perceived

Color of Objects - 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 Mixing 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 9.6 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.

Table 9.2 Mixing blue and yellow lights (Additive color mixture). Parts of the spectrum that are reflected from a white surface for blue and yellow spots of light projected onto the surface. Wavelengths that are not reflected are indicated by shaded squares.

Figure 9.7 Color mixing with paint. Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color mixture.

Table 9.3 Mixing blue and yellow paints (Subtractive color mixture). Parts of the spectrum that are absorbed and reflected by blue and yellow paint. The colors that are totally absorbed are indicated by shaded squares for each paint. Light that is usually seen as green is the only light that is reflected in common by both paints.

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.

Figure 9.8 In a color-matching experiment, the observer adjusts the amount of three wavelengths in one field (right) so it matches the color of the single wavelength in other field (left).

Color Matching Experiments 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.

Physiological Evidence for the Trichromatic Theory Researchers measured absorption spectra of visual pigments in receptors (1960s). –They found pigments that responded maximally to: Short wavelengths (419nm) Medium wavelengths (551nm) Long wavelengths (558nm) Later researchers found genetic differences for coding proteins for the three pigments (1980s).

Figure 9.9 Absorption spectra of the three cone pigments. From Dartnall, H. J. A., Bowmaker, J. K. and Mollon, J. D. (1983). Human visual pigments: Microspectrophotometric results from the eyes of seven persons. Proceedings of the Royal Society of London B, 220,

Response of Cones 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 9.10 Patterns of firing of the three types of cones to different colors. The size of the cone symbolizes the size of the receptor’s response.

Figure 9.11 Principle of 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. Because there is no difference in the responses of the two sets of receptors so that two fields are perceptually indistinguishable.

Are Three Receptor Mechanisms Necessary for Color Perception? 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.

Figure 9.12 (a) Absorption spectrum of Jay’s visual pigment. The fractions of 550-nm and 590-nm lights absorbed are indicated by the dashed lines. (b) The size of the cone indicates activation caused by the reflection of 1,000 photons of 550-nm light by Mary’s dress. (c) The activation caused by the reflection of 1,000 photons of 590-nm light by Barbara’s dress. (d) The activation caused by the reflection of 2,000 photons of 590-nm light from Barbara’s dress. Notice that the cone response is the same in (b) and (d).

Figure 9.13 The same as Figure 9.12 but with a second pigment added. (a) Absorption spectrum of pigment 2, with the fraction absorbed by 550-nm and 590-nm indicated by the dashed lines. (b) Response of the two types of cones when they absorb light from Mary’s dress. The response of cone 1 is on the right. (c) Response caused by light reflected from Barbara’s dress at the same intensity. (d) Response from Barbara’s dress at a higher intensity. Notice that the cone response is different in (b) and (d).

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 9.14 (a) Ishihara plate for testing for color deficiency. 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 from of red-green color deficiency.

Color Experience for Monochromats 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

Color Experience for Dichromats 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

Color Experience for Dichromats - 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

Color Experience for Dichromats - 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 9.16 How the visual spectrum appears to (a) protanopes; (b) deuteranopes; (c) tritanopes; and (d) trichromats. Spectra courtesy of Jay Neitz

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 9.18 Color matrix for afterimage and simultaneous contrast demonstrations.

Table 9.4 Results of afterimage and simultaneous contrast demonstration

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 9.19 The three opponent mechanisms proposed by Hering.

Physiology of Opponent-Process 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 9.20 Responses of B+ Y- and R+ G- opponent cells in the monkey’s lateral geniculate nucleus. From Devalois R. L., Jacobs, G. H. (1968). Primate color vision. Science, 162,

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

Figure 9.21 Our experience of color is shaped by physiological mechanisms, both in the receptors and in opponent neurons.

Figure 9.22 Neural circuit showing how the blue-yellow and red-green mechanisms can be created by excitatory and inhibitory inputs from the three types of cone receptors.

Figure 9.23 (a) Response curves for the M and L receptors. (b) Bar graph indicating the size of the responses generated in the receptors by wavelengths 1 (left pair of bars) and 2 (right pair). (c) Bar graph showing the opponent response of the R+ G- cells to wavelengths 1 and 2. The response to 1 is inhibitory and the response to 2 is excitatory.

Color Processing in the Cortex There is no single module for color perception –Cortical cells in V1, V2, 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

Perceiving Colors Under Changing Illumination 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 9.24 The wavelength distribution of sunlight and of light from a tungsten light bulb. From Judd, D. B., McAdam, D. L., & Wyszecki, G. (1964). Spectral distribution of typical daylight as a function of correlated color temperature. Journal of the Optical Society of America, 54,

Figure 9.25 Reflectance curve of a sweater and light reflected from the sweater when it is illuminated by tungsten light and by white light.

Possible Causes of Color Constancy 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

Chromatic Adaptation 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 9.27 The three conditions in Uchikawa et al.’s (1989) experiment. See text for details.

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

Possible Causes of 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

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 9.28 A black-and-white checkerboard illuminated by (a) tungsten light and (b) sunlight.

Possible Causes of Lightness Constancy 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.

Possible Causes of Lightness Constancy - continued 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

Lightness Perception Under Uneven Illumination 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.

Sources of Information About Illumination Orientation of surfaces provides information about illumination and reflectance edges. Perceptual organization of objects in a display affects perception of lightness.

Figure 9.32 Viewing a shaded corner. (a) Illuminate a folded card so one side is illuminated and the other is in shadow. (b) View the folded card through a small hole so the two sides of the corner are visible, as shown.

Figure 9.33 (a) Four dark discs partially covered by a white mist; (b) four light discs partially covered by a dark mist. The discs are identical in (a) and (b). Anderson, B. L., & Winawer, J. (2005). Image segmentation and lightness perception. Nature, 434,

Perceptual Experiences are Creations 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 at 350nm, which is outside human perception. –We cannot tell what color the bee actually “sees.”

Figure 9.34 Absorption spectra of honeybee visual pigments.