Sensation & Perception
Ch. 7: Perceiving Color
© Takashi Yamauchi (Dept. of Psychology, Texas A&M
University)
Main topics
Trichromatic theory
Opponent process theory
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Color
• Physiologists:
– Helmholtz (1821-1894)
• http://en.wikipedia.org/wiki/Hermann_von_Helmhol
tz
– Hering (1834-1918)
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Trichromatic  Opponenctprocess
Two stages in color perception
Trichromatic stage:
3 kinds of cone receptors
(S, M, L)
Opponent process
At the bipolar or ganglion cells,
the difference between S and
(M+L) (Circuit 1) and the
difference between M and L
(Circuit 2) is assessed.
The first circuit (Circuit 1)
processes the blue-yellow
difference.
The second circuit (Circuit 2)
processes the red-green
difference.
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What we see is electromagnetic
energy
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Two types of photo receptors
• Cones and Rods
• Cones are for color vision
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Eye
• Photo receptors
• Two types of photo
receptors – rod &
cone
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3 types of cones
• There are three types cones that are
selectively tuned to three different lengths
of electromagnetic waves (Short, Medium,
and Long)
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Color perception is produced by combinations
of these 3 types of cones:
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Different objects reflect
light in a different
manners.
Some objects (tomato)
absorb short waves while
reflect long waves 
create a red surface.
Absorb short waves
but reflect long waves
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What the computer does is the
same thing
• Pixel
• Bit
• Illuminating the
phosphor with three
different electron
guns.
• Illuminate each pixel
with red, green, and
blue electron guns
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3 types of electron guns:
a red gun, a green gun,
and a blue gun
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Problems with the trichromatic
theory
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After Image
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More Phenomenological
Observations
• Visualize something red (apple, fire engine)
– Now reddish-yellow
– Now reddish-green
• People who are color-deficient for red also
lack green; same with blue and yellow
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Trichromatic  Opponentprocess
Trichromatic stage:
3 kinds of cone receptors
(S, M, L)
Opponent process
At the bipolar or ganglion
cells, the difference
between S and (M+L)
(Circuit 1) and the
difference between M and L
(Circuit 2) is assessed.
The first circuit (Circuit 1)
processes the blue-yellow
difference.
The second circuit (Circuit 2)
processes the red-green
difference.
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Opponent-process theory
(Hering/Hurvich & Jameson)
• Color vision is caused by opposing
responses generated by blue and yellow, and
by red and green.
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After images:
Red  Green
Green  Red
Blue  Yellow
Yellow  Blue
R
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B
Y
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Color perception
R
B
G
adaptation
After image
Y
adaptation
After image
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Responses of opponent cells in the monkey’s LGN
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Fig. 7-20, p. 155
Opponent-process theory
Relative
responses
S
M
L
S
M
L
S, M, and L
cones have
inhibitory
and
excitatory
connections
at LGN.
Inhibitory
Excitatory
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Trichromatic  Opponenctprocess
Two stages in color perception
Trichromatic stage:
3 kinds of cone receptors
(S, M, L)
Opponent process
At the bipolar or ganglian cells,
the difference between S and
(M+L) (Circuit 1) and the
difference between M and L
(Circuit 2) is assessed.
The first circuit (Circuit 1)
processes the blue-yellow
difference.
The second circuit (Circuit 2)
processes the red-gree
difference.
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Why is this process efficient?
• Distinguishing the two
wavelengths, 1 and 2
is very hard using just
the M and L receptors.
• Opponent process
create two categorical
responses  + or –
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Huichol masks (Mexican Indian tribe)
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Color blindness
• Can be caused by the disorder in cone
receptors, optic nerves, and/or other higher
brain areas responsible for color perception
(e.g., V4).
• Most problems in color vision is associated
with problems with the cone receptors in the
retina.
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• Monochromat
– Cannot distinguish any color from gray. (all or
two of the cone systems are compromised)
• Dichromat
– Only one cone system is compromised.
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Constancy
• Color constancy
– We perceive the colors
of objects as being
relatively constant
even under changing
illumination.
– A green sweater is green
no matter where you see
it.
– How come?
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• Background knowledge and your memory
contribute to color constancy.
• You compare the color of your sweater with
other surrounding things.
• Adaptation
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Lightness constancy
• Both top-down and bottom-up processes are going
on.
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Different amounts of
light are reflected from
the same surface in (a)
and (b).
But our perception of
lightness is pretty much
the same (lightness
constancy)
How come?
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Lightness constancy
• Constancy is so prevalent that we even
don’t notice that.
• My skin reflects much less light in this
room than in outside.
• But people don’t think that my skin color
changes every time I enter a building.
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Ratio principle
• Perceive lightness
of an object with
respect to the
surrounding area.
• E.g.,
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– When I enter a
building, everything
loses illumination,
so perceived
lightness of my face
remains the same.45
How does the
visual system
distinguish
reflected light from
illuminated light?
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• The LGN is the first
stop in the visual
pathway.
• The LGN receives
more input back from
the cortex than from
the retina.
LGN (Lateral Geniculate
Nucleus)
V1 (Striate Cortex)
Lateral Geniculate Nucleus
(LGN)
ch 7 Image courtesy of Dr. Paul Wellman
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