Color Vision

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PSY 3520 Sensation and Perception
Color Vision
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Color Vision Lecture Outline

• I. Color Stimulus
• A. Dimensions of Color
• B. Color Mixing
• 1. Additive
• 2. Subtractive
• II. Trichromatic Theory
• A. Color Mixing Experiments
• B. Spectral Sensitivity Color Matching
• C. Physiological Evidence
• 1. Absorption Spectra
• 2. Genetics
• 3. Relative Numbers and Retinal Distribution
• D. Color Vision Deficiencies
• III. Opponent-Process Theory
• A. Phenomenological Observations
• B. Quantitative (Psychophysical) Evidence
• C. Physiological Evidence
• IV. Two Theories of Color Vision?
• V. Higher Level Processing of Color Information

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I. The Color Stimulus

• 3 Dimensions of Color
• Physical Psychological
• Dominant l Hue/Color
• Purity Saturation
• Intensity Brightness
• Bezold-Brücke Effect
• All three of these dimensions can affect our
  perception of color.

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Color Mixing

• Additive related to light
• Occurs when two or more lights are added together
• Wavelengths are added together to form a
  different hue perception

• Primaries
• Red
• Green
• Blue

Red Green Yellow Red Green Blue White
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Color Mixing

• Subtractive associated with mixing paints
• Reflection of light from a surface
• Some wavelengths are selectively removed

• Primaries
• Red
• Blue
• Yellow

Blue Yellow Green Red Yellow Blue
Black/Brown
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2 Major Theories of Color Vision

• Both theories proposed in the 1800s.
• Both theories based on phenomenological and
  psychophysical evidence.
• Both theories were not supported by physiological
  evidence until over 70 years after they were
  proposed.

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II. Trichromatic Theory

• First proposed by Thomas Young in 1802
• Later supported by Hermann von Helmholtz in 1852
• Sometimes called the Young-Helmholtz theory of
  color vision
• Proposed that there are 3 receptor mechanisms,
  each having different spectral sensitivities

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Trichromatic Theory

• These 3 mechanisms are 3 cone types
• Each mechanism varies in terms of the range of
  wavelengths to which they are sensitive
• For any given wavelength there is a ratio of
  activity in the 3 receptor mechanisms this
  results in the perception of a certain color.

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Color Mixing Experiments

• First studies to support this theory were color
  mixing studies
• Experiments completed by James Clerk Maxwell
• Maxwell proposed that if you mixed the correct
  three primaries (460 nm blue, 530 nm green, 650
  nm red) you only need 3 wavelengths to match
  any wavelength in the visible spectrum

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Color Matching Studies

• The color mixing research expanded into color
  matching studies
• Set the foundation for the studies
• Spectral Sensitivity Functions psychophysical
  research completed to identify the 3 mechanisms
  identified in the trichromatic theory

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Spectral Sensitivity Functions

• W.S. Stiles (1953) two-color threshold method

Test Light
Adapting Field
The adapting field is used to selectively bleach
the photopigment of 2 different cone types. Test
light used to measure the threshold of the
remaining cone type.
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Two-color Threshold Technique

• Examples

Adapting Field - Long l Short l Test Field
Middle l
Adapting Field - Middle l Short l Test Field
Long l
Adapting Field - Long l Middle l Test Field
Short l
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Spectral Sensitivity Functions

• Stiles identified 3 cone types

440-480 nm
540 nm
575-587 nm
LWS
MWS
SWS
Spectral Sensitivity
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Color Matching

• Once spectral sensitivity of cone photoreceptors
  was identified the information could be used to
  describe color matching
• To a wavelength of light there will be a ratio of
  activity in the 3 cone mechanisms, which results
  in a perception of color

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Color Matching
.90
.60
Relative Response
.13
600 nm ?
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Color Matching Experiments

• Experiments were based on the knowledge that the
  ratio of receptor activity is related to hue
  perception

420 nm 560 nm 640 nm
Comparison Light
500 nm
Test Light
S
M
L
S
M
L
.90
.60
.13
.90
.60
.13
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Color Matching Experiments

• Although two physically different stimuli are
  being presented they are perceptually identical
• This is called a metamer
• Basic principle of metamers 2 lights with
  physically different wavelength distributions
  appear the same if they stimulate the S, M, L
  receptors in the same ratios.

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Physiological Evidence

• Absorption Spectra Brown Wald (1964) used
  microspectrophotometry (MSP) to measure the
  absorption spectra of human cone photopigments

Light
Photoreceptor isolated in a micropipette and
light is shown through the photopigment while it
is still inside of the receptor Measure the
amount of light absorbed greater absorption
more sensitive Principle of Univariance
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Absorption Spectra

• Brown Wald confirmed that there are 3 cone
  photoreceptors that can be differentiated by
  their absorption spectra

Peak wavelength or l max
Brown Wald (1964) SWS 419 nm MWS 531 nm LWS
558 nm
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Genetics

• Genetics research has uncovered individual
  differences in the absorption spectra of the 3
  cone types in particular M l cones
• Nathans, Thomas Hogness (1986) isolated the
  genes responsible for coding the opsin (protein)
  part of the photopigment molecules of the 3 cone
  types
• Mollon (1993) suggested the individual
  differences are reflected in the sequences of
  amino acids that make up the opsin molecule
• Different amino acid sequences cause shifts in
  the absorption spectra of the cone photopigments
• Can cause a cumulative shift of up to 12 nm

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Relative Numbers and Retinal Distribution of
Photoreceptors

• Cicerone Nerger (1989) identified a 21 ratio
  of LM cones using psychophysical data
• S-cones thought to be a small percentage of the
  total cones

L M S 32 16 1
Relative s
Cone Mosaic
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Color Vision Deficiencies

• Rushton 1964 used Retinal Densitometry to measure
  visual pigments in observers with color vision
  deficiencies
• He found they were missing one type of cone
  photopigment

P
D
T
Protanope missing LWS Deuteranope missing
MWS Tritanope missing SWS
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Color Vision Deficiencies

• Color deficient observers are called dichromats
  because they only have 2 cone photopigments
• For color matching dichromats only need 2
  wavelengths to match any wavelength in the
  visible spectrum (2 wavelengths needed to make a
  metameric match)
• Cicerone Nerger (1989) found that dichromats
  have the same of cones as trichromats

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Color Vision Deficiencies

• Color vision deficiencies are X-linked so more
  males have color vision deficiencies than females
• There is also a truly color-blind individual with
  no functional cones (rod monochromat)
• Truly color blind
• Rod vision only
• Black and white
• Reduced acuity
• 10 out of 1 million people

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Opponent-process Theory

• Ewald Hering (1878) proposed the opponent-process
  theory of color vision because of the phenomena
  that he observed that could not be explained by
  the trichromatic theory of color vision
• Afterimages
• Simultaneous contrast
• Mutually exclusive colors
• Color vision deficiencies

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Simultaneous Contrast
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Mutually Exclusive Colors

• There are no greenish reds or bluish yellows
• There are greenish blues
• and yellowish reds

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Opponent-process Theory

• Because of these observations Hering proposed
  that we have 3 opponent neural mechanisms
• Red/Green Blue/Yellow Black/White
• These mechanisms have antagonistic responses
  excitatory response to one end of the visible
  spectrum and inhibitory response to the other end
  of the visible spectrum

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Color Vision Deficiencies

• Protanopes and deuteranopes have difficulty
  discriminating between reds and greens
• Red-green deficiency

Blue
Yellow
492
400
700
Neutral Point/Gray
Blue
Yellow
498
400
700
Neutral Point/Gray
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Red-Green Deficiency
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Color Vision Deficiencies

• Tritanopes have difficulty discriminating between
  blues and yellows
• Blue-yellow deficiency
• Trichromatic theory can not explain the inability
  to discriminate between colors of opponent pairs

Red
Green
570
400
700
Neutral Point/Gray
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Opponent-process Theory

• Herings theory was controversial because there
  was no experimental evidence to support his
  theory
• Finally in 1955 Hurvich Jameson provided
  psychophysical evidence through hue cancellation
  studies

Dorothea Jameson
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Hue Cancellation Functions

• Used psychophysics to determine the strengths of
  the blue, yellow, green, and red components of
  the Blue/Yellow and Red/Green mechanisms across
  the visible spectrum
• Measure the strength of the blue channel by
  measuring the amount of yellow light needed to
  cancel out the blue perception in a stimulus

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Hue Cancellation Functions
Unique Blue 475 nm Unique Green 500
nm Unique Yellow 580 nm
Unique Green
Unique Blue
650 nm stimulus Add green to measure strength
of the red mechanism
Unique Yellow
Only have the perception of yellow because the
red is cancelled by green
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Physiological Evidence

• Svaetchin (1956) identified s-potentials
• He recorded electrical responses in the
  horizontal cells of the goldfish retina
• Found an excitatory response to light on one end
  of the spectrum and an inhibitory response to
  light on the other end of the spectrum

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Physiological Evidence

• DeValois (1960) found cells in monkey LGN that
  had opponent properties
• He identified 4 types
• B Y-
• G R-
• Y B-
• R G-

Russell DeValois
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Current Perspective

• Instead of two theories in opposition to one
  another the two theories describe two sites of
  color vision processing
• 1st Site trichromatic theory
• 3 cone photoreceptors that provide information to
  the second site
• 2nd Site opponent-process theory
• Cone photoreceptor inputs are combined to form
  opponent channels

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Current Perspective
Opponent-process
Trichromatic
B/W ML
B/Y (ML)-S
R/G (LS)-M
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Color Coding in the Striate Cortex

• Most cells in the striate cortex have opponent
  properties like those in the LGN
• One type of cell that is particular to the
  striate cortex is the double color-opponent cell
• Livingstone Hubel (1984)

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Cerebral Achromatopsia

• Color blindness that results from a brain injury
  or stroke injury to the visual cortex
• Other visual functions are intact