Chapter 7: Color Vision

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Chapter 7 Color Vision How do we perceive color?
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Color Vision
Light Stimulus
EYE BRAIN
VISION

• Vision consists of the perception of
• Color
• Size
• Shape
• Depth
• Distance
• Speed

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• Features of Color Vision
• Trichromacy
• For complete specification of any color, at
  least three parameters are needed.
• HUE Dominant wavelength, ?D
• CHROMA Saturation or Purity, p
• VALUE Brightness, Y

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Features of Color Vision 2. Color Constancy
Ability of the eye to compensate for changes in
illumination and viewing conditions.
Object appears more or less the same since eye
becomes less sensitive to blue under sunlight!
Candlelight
Reflects less blue
Any object
Reflects more blue
Sunlight
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Color Constancy (Contd.)
Note Color Photography Does Not Have Color
Constancy.                                     
                                                  
                                                  
                                                 
                               These images
show how a standard daylight film responds to
different illuminants. The visual system is able
to compensate color appearances for illuminant
shifts.
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• Features of Color Vision
• 3. Contrast Effects
• Simultaneous B W Contrast
• White Brightest Grey
• Black Darkest Grey

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• Contrast Effects (Contd.)
• Simultaneous Color Contrast
• The appearance of a color changes based on the
  colors surrounding it .

http//webexhibits.org/colorart/contrast.html
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• Contrast Effects (Contd.)
• Successive Color Contrast
• The appearance of a color changes based on the
  colors seen previously.
• For example, green may appear bluish after
  viewing orange!

http//www.lifesci.ucsb.edu/mrowe/SuccessiveColor
Contrast.html
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Features of Color Vision (Contd.) 4.
Afterimages Latency, persistence, and positive
afterimages The photo-receptors do not respond
immediately to light. The electro-chemistry takes
about 0.05 seconds to respond (the latency
period) and persists for some time after the
stimulus (about 0.15 sec for cones, anywhere from
0.2 to more than one second for rods). These
delays and the persistence in later cell
communications produce a positive afterimage.
Such afterimages are visible after glancing at
the Sun or any other bright light.
Desensitization and negative
afterimages Continued exposure to light
desensitizes the photo-receptors over time scales
of 1 to 30 seconds and longer. This
desensitization produces negative afterimages
when the image field is changed.
http//staff.washington.edu/chudler/chvision.html
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Afterimages (Contd.)
Follow the movement of the rotating pink dot, you
will only see one color, pink.  If you stare at
the black in the centre, the moving dot turns
to green.  Now, concentrate on the black in the
centre of the picture.  After a short period of
time, all the pink dots will slowly disappear,
and you will only see a green dot rotating!
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Features of Color Vision (Contd.) 5. Lightness
Constancy Our judgment of lightness or darkness
remains unimpaired even under different
illumination.
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Color Vision Defects Genetic factors or
disease can eliminate one or more types of cones.
Monochromats (0.003) Have B W vision due to
rods only. Cones do not function.
Dichromats (4 males 0.4 females) Unable to
distinguish R from G. Also called R-G blindness.
Protanopes R-G blind and abnormal insensitivity
to long wavelengths.
Deuteranopes R-G blind and abnormal insensitivity
to middle wavelengths.
Tritanopes (very rare) Abnormal insensitivity to
short wavelengths.
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Color Vision Defects (Contd.)
Protanopia and deuteranopia are red-green
defects. Persons with red-green defects have
difficulty distinguishing between reds, greens
and yellows but can discriminate between blues
and yellows. Protanopes often can name red and
green correctly because green looks lighter to
them than red. Hereditary tritanopia is a
blue-yellow defect. Persons with blue-yellow
defects cannot see the difference between blues
and yellows but can distinguish between reds and
greens.
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Color Vision Defects (Contd.)
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Color Vision Test
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Theories of Color Vision Early Theories Newton
Particles of light excited vibrations of
different sizes on the retina. Young -
Postulated three types of photoreceptors,
each responding to some part of the
spectrum. For example, Y light excites R and G
receptors equally! Note Youngs
Trichromatic theory forms the basis for the
Component Theory.
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The Component Theory

• Adopted modified by Helmholtz.
• Color perceived is uniquely determined by
  relative stimulation of the three basic retinal
  receptors (Components).
• Explains positive afterimages and R-G color
  blindness.
• Drawbacks Color constancy, lightness constancy,
  and negative afterimages cannot be explained.

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Achromatic Response
Seems to indicate that there are four
psychologically pure primaries.
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The Opponent Theory

• Based on the Opponent Theory by Herring.
• Cone photoreceptors are linked together to form
  three opposing color pairs blue/yellow,
  red/green, and black/white.  Activation of one
  member of the pair inhibits activity in the
  other.
• Explains many psychological effects and color
  blindness.

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Trichromatic Theory or Opponent Theory? Both
theories are needed to explain what is known
about color vision. The trichromatic theory
explains color vision phenomena at the
photoreceptor level the opponent-process theory
explains color vision phenomena that result from
the way in which photoreceptors are
interconnected neurally.
Outputs compared in higher visual centers
L cone
Component Theory
M cone
S cone
Perceptions combined in higher visual centers
L cone
Opponent Theory
M cone
S cone