Chapter 7: 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

Familiarity and recognition

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 which adds white to a color resulting in less saturated color

Color Wheel – Known for centuries by artists

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

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

Table 7.1 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

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 to match a comparison field to a test field

Color Matching Experiments

• Results showed that:

– It is possible to perform the matching task without three colors in the S, M and L wavelengths

– Observers with normal color vision need at least 3 wavelengths to make the matches

– Observers with color deficiencies can match colors by using only 2 wavelengths

• They think it looks OK!!

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 7.8 Absorption spectra of the three cone pigments. (From Dartnall, Bowmaker, and Mollon, 1983.)

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

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

Figure 7.12 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 that two fields are perceptually indistinguishable.

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 7.11 Mixing blue paint and yellow paint creates a paint that appears green. This is subtractive color mixture.

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 - called the principle of univariance

– Any two wavelengths can cause the same response by changing the intensity

• Two receptor types (dichromats) solves this problem but 3 types (trichromats) allows for perception of more colors

• Try This!• http://www.cs.brown.edu/exploratories/freeSoftware/catalogs/color_theory.html

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 7.15 Ishihara plate for testing for 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 3 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

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 7.17 Color matrix for afterimage and simultaneous contrast demonstrations.

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 positive to red and negatively to green

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

Figure 7.19 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

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

Figure 7.22 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.

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 7.24 The reflectance curve of a sweater (green curve) and the wavelengths reflected from the sweater when it is illuminated by daylight (white) and by tungsten light (yellow).

Possible Causes of Color Constancy

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

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

– Sensitivity to the color decreases

• Adaptation occurs to light sources leading to color constancy

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

– Memory for color is not exact, so we don’t notice slight changes caused by illumination changes

Lightness Constancy

• Achromatic colors are perceived as remaining relatively constant

– Perception of lightness:

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

• Is related to the percentage of light reflected by object

Figure 7.27 A black-and-white checkerboard illuminated by tungsten light and by 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

– Shadows cause problems• Reflectance edges - edge where the

reflectance of two surfaces changes

• Illumination edges - edge where illumination of two surfaces changes

Figure 7.28 This unevenly illuminated wall contains both reflectance edges and illumination edges. The perceptual system must distinguish between these two types of edges to accurately perceive the actual properties of the wall and other parts of the scene, as well.

Figure 7.29 The pattern created by shadows on a surface is usually interpreted as a change in the pattern of illumination, not as a change in the material making up the surface. The fact that we see all of the bricks on this wall as made of the same material, despite the illumination changes, is an example of lightness constancy.

Figure 7.30 (a) A cup and its shadow. (b) The same cup and shadow with the penumbra covered by a black border.

Creating Color Experience

• Light waves are not “colored”

• Color is a creation of our physiology

– Animals with different sensory apparatus, such as honey bees, experience something we cannot

• All of our sensory experiences are created by our nervous system