Chapter 9:
Perceiving Color
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 9-1 p200
Figure 9-2 p201
What Colors Do We Perceive?
• 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
Figure 9-3 p201
Figure 9-4 p201
Color and Wavelength
• 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
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 9-5 p202
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
Table 9-2 p203
Table 9-3 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 9-11 p205
Figure 9-12 p206
Are Three Receptor Mechanisms
Necessary for Color Vision?
• 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 9-13 p206
Figure 9-14 p207
Figure 9-15 p207
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 9-16 p208
Monochromatism
• 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
Dichromatism
• 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
Dichromatism - 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
Dichromatism - 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 9-17 p209
Figure 9-18 p210
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 9-19 p210
Figure 9-20 p211
Table 9-4 p211
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 9-21 p211
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
Figure 9-22 p212
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 9-23 p212
Figure 9-24 p212
Figure 9-25 p213
Color in the Cortex
• There is no single module for color perception
– Cortical cells in V1, 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
Types of Opponent Neurons in the Cortex
• Single-opponent neurons
• Double-opponent neurons
Figure 9-26 p214
Color Constancy
• 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 9-27 p215
Figure 9-28 p215
Color Constancy - continued
• 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
Figure 9-29 p216
Color Constancy - continued
• 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 9-30 p216
Color Constancy - continued
• 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
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
Figure 9-31 p217
Experiment by Hansen et al.
• 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 9-32 p218
Lightness Constancy - continued
• 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.
Lightness Perception Under Uneven
Illumination
• 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
Figure 9-33 p219
Lightness Perception Under Uneven
Illumination - continued
• 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.
Figure 9-34 p219
Figure 9-35 p220
Figure 9-36 p220
Figure 9-37 p220
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.”
Figure 9-38 p221
Figure 9-39 p222
Figure 9-40 p222
Infant Color Vision
• It is a complex problem to know what an
infant really “sees”
– Chromatic color
– Brightness
• Bornstein et al (1976)
– Habituation
– Young infants have color vision
Figure 9-41 p223
Figure 9-42 p223
Figure 9-43 p223