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Color Vision

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A. Color Mixing Experiments. B. Spectral Sensitivity & Color Matching. C. ... B. Cerebral Achromatopsia. I. The Color Stimulus. 3 Dimensions of Color ... Achromatopsia ... – PowerPoint PPT presentation

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Title: Color Vision


1
PSY 3520 Sensation and Perception
Color Vision
2
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

3
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.

4
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
5
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
6
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.

7
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

8
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.

9
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

10
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

11
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.
12
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
13
Spectral Sensitivity Functions
  • Stiles identified 3 cone types

440-480 nm
540 nm
575-587 nm
LWS
MWS
SWS
Spectral Sensitivity
14
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

15
Color Matching
.90
.60
Relative Response
.13
600 nm ?
16
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
17
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.

18
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
19
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
20
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

21
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
22
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
23
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

24
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

25
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

26
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27
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28
Simultaneous Contrast
29
Mutually Exclusive Colors
  • There are no greenish reds or bluish yellows
  • There are greenish blues
  • and yellowish reds

30
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

31
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
32
Red-Green Deficiency
33
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
34
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35
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
36
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

37
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
38
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

39
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
40
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

41
Current Perspective
Opponent-process
Trichromatic
B/W ML
B/Y (ML)-S
R/G (LS)-M
42
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)

43
Cerebral Achromatopsia
  • Color blindness that results from a brain injury
    or stroke injury to the visual cortex
  • Other visual functions are intact
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