Color Perception

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Chapter 7

• Color Perception

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Why is it important to see colors?
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• Color provides unique source of information for
  picking out object from its background
  (detection).
• Color helps us recognize (identification) and
  distinguish (discrimination).

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A Multitude of Color Names

• In most cultures, everyday language uses about a
  dozen basic terms for color.
• Different cultures categorize colors differently.
• Habituation occurs when infants are repeatedly
  presented with the same color.

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The Insights of Isaac Newton

• Color is psychological and subjective.
• Hue is the quality that distinguishes colors.

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The Insights of Isaac Newton
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The Insights of Isaac Newton

• Brightness is related to the intensity of light.
• Saturation distinguishes pale from vivid.

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The Insights of Isaac Newton
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The Insights of Isaac Newton

• Newton used a prism to separate sunlight into a
  rainbow of spectral colors.
• By passing light through a series of prisms,
  Newton separated pure light from composite light.

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Newtons Prism and a Rainbow of Spectral Colors
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Metamers and Color

• Physically different combinations of energy can
  produce identical color experiences.
• Metamers suggest that the visual system can
  generate equivalent neural responses to
  physically different stimuli.

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Newtons Color Circle

• Newtons color circle can be used to predict the
  color produced by mixing other colors.
• Newtons color circle was based on the erroneous
  assumption that there should be seven discrete
  pure colors.

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Newtons Color Circle

• To make accurate predictions, Newtons color
  circle must be revised.
• A revised color circle must consider that colors
  are continuous, not discrete.
• A revised color circle should place complementary
  colors opposite each other.

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Revised Color Circles
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Constancy of Color

• Color constancy is the propensity for an objects
  color to remain constant despite changes in the
  spectrum of light falling on that object.
• Color induction is where color appearance is
  influenced by preceding color stimulation or by
  presence of other colors elsewhere in the visual
  field.

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How Many Cone Pigments?

• Each photopigment responds to a preferred
  wavelength, but the neural response of the
  photoreceptor does not specify wavelength.
• Univariance principle a photoreceptors response
  is summarized by one variable that specifies the
  amount of light absorbed.

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How Many Cone Pigments?

• A monochromat is a person or animal that cannot
  differentiate colors because it has only one cone
  pigment.
• A dichromat, with two cone pigments, is
  susceptible to color confusions.

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How Many Cone Pigments?

• Human eye has three photopigments, so it is
  trichromatic (three-colored).
• Any wavelength will produce three responses, a
  trio that is not likely to be confused with any
  other single wavelength.

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How Many Cone Pigments?

• A goldfish has an eye with four pigments and is
  sensitive to wavelengths in the ultraviolet
  range.
• Tetrachromatic (four-colored) vision allows for
  even better color discrimination.

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What are the three cone types?

• Microspectrophotometry is the technique of
  directly measuring the light absorption of
  photopigments.
• Each pigment type is most sensitive to light of a
  particular wavelength.

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What are the three cone types?

• S cones are short-wavelength sensitive.
• M cones are medium-wavelength sensitive.
• L cones are long-wavelength sensitive.

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Geography of Cones

• A small proportion of S cones appear among larger
  numbers of M and L cones.
• Parts of the retina may have one cone type.
• Color vision varies in different parts of the
  retina.

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Evolution of Cone Photopigments

• The genes that encode the cone pigments and
  rhodopsin share structural features that suggest
  a common ancestor.
• Trichromatic vision can evolve only in a species
  that possesses fine visual acuity.

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Abnormalities of Color Perception

• Color deficiency can be attributed to many
  factors
• Heredity
• Eye disease
• Brain damage
• Abnormal early visual experience
• Protanopia - The protanope has only M and S
  cones--no L (red).
• Deteranopia - The deteranope has only L and S
  cones--no M (green).
• Tritanopia - The tritanope has only L and M
  cones--no S (blue).