Chapter 9:
Color Vision
Overview of Questions
• How do we perceive 200 different colors with
only three cones?
• What does someone who is “color-blind” see?
• How does our memory of color affect our
perception of color?
Fruit of a different color just doesn’t look right.
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.
How Can We Describe Color Experience?
• Colors can be changed by:
– Intensity which changes perceived
brightness
– Saturation - adding white to a color results
in less saturated color
Chromatic Colors
• Basic colors are red,
yellow, green, and blue
• Color circle shows
perceptual relationship
among colors
• Chromatic colors or
hues - objects that
preferentially reflect
some wavelengths
Color wheel with exceptions
•
•
•
•
•
•
•
Some colors not in the color spectrum.
White
Gray
Black
Brow
Achromatic colors –
contain no hues
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.
Table 9.1 Relationship between predominant wavelengths reflected and color perceived
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
– Called selective reflectance
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.
Selective transmission
• Selective transmission:
– Transparent objects, such as liquids,
selectively allow wavelengths to pass
through
– Cranberry juice
– Transmits long wavelengths
– Looks red
Additive: Mixing lights of different wavelengths
All wavelengths are available for the observer to see
Superimposing blue and yellow lights leads to white
Color Mixing
• Subtractive color mixture:
– Mixing paints with different pigments
– Additional pigments reflect fewer
wavelengths
– Mixing blue and yellow leads to green
Figure 9.7 Color mixing with paint. Mixing blue paint and
yellow paint creates a paint that appears green. This is
subtractive color mixture.
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)
Figure 9.9 Absorption spectra of the three cone pigments.
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 9.14 (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 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
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
Dichromacy
• moderately severe color vision defect in
which one of the three basic color
mechanisms is absent or not functioning. It
is hereditary and, in the case of Protanopia
or Deuteranopia, sex-linked, affecting
predominantly males. Dichromacy occurs
when one of the cone pigments is missing
and color is reduced to two dimensions.
Protanopia (no red cone)
• Protanopia is a severe type of color vision
deficiency caused by the complete absence
of red retinal photoreceptors. It is a form of
dichromatism in which red appears dark. It is
hereditary, sex-linked, and present in 1% of
males and .02% of females
Deuteranopia (no green cone)
• a color vision deficiency in which the
green retinal photoreceptors are absent,
moderately affecting red–green hue
discrimination. It is likewise hereditary
and sex-linked. Deuteranopia affects
1% of males and .01% of females
Dichromats
Protanopia
Deuteranopia
Red-green deficiency
Tritanopia (no blue cone)
• Tritanopia is a very rare color vision
disturbance in which there are only two cone
pigments present and a total absence of blue
retinal receptors. Not sex-linked. Tritanopia
affects .002% of males and .001% of females
Blue-yellow deficiency
Scenes with color deficiency
Afterimages
Stare a box for 30 seconds
a
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.
Color matrix for afterimage and simultaneous contrast demonstrations.
Results of afterimage and simultaneous contrast demonstration
Three pairs of connections
• Opponent-process mechanism
– 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
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 9.9 Absorption spectra of the three cone pigments.
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