Perception and Color

• Color and Wavelength

  color perception is related to the wavelength of light:

  • 400 to 450 nm appears violet
  • 450 to 490 nm appears blue
  • 500 to 575 nm appears green
  • 575 to 590 nm appears yellow
  • 590 to 620 nm appears orange
  • 620 to 700 nm appears red
• Color and Wavelength

  colors of objects are determined by the wavelengths that are reflected

  • selective reflection occurs when objects preferentially reflect some wavelengths
  • selective transmission: transparent objects, such as liquids selectively allow wavelengths to pass
• Trichromatic Theory of Color Vision

  • Three different receptor mechanisms are responsible for color vision.
  • color-matching experiments showed observers adjusted amounts of three wavelengths in a comparison field to match a test field of one wavelength.
• Behavior Evidence of the Trichromatic Theory

  • it is possible to perform the matching task
  • observers with normal color vision need at least three wavelengths to make the matches
  • observers with color deficiencies can match colors by using only two wavelengths
• Physiological Evidence for the Theory

  • researchers measured absorption spectra of visual pigments in receptors
  • they found pigments respond maximally to: short (419 nm), medium (531 nm), and long wavelengths (558 nm)
  • researchers found genetic differences for coding proteins for the three pigments
• Cone Responding and Color Perception

  Color perception is based on the response of the three different types of cones.

  • responses vary depending on the wavelengths available
  • combinations of the responses across all three cone types lead to perception of all colors
  • color matching experiments show that colors that are perceptually similar can be caused by different physical wavelengths
• Are Three Receptor Mechanisms Necessary for Color Vision?

  one receptor type cannot lead to color vision because:

  • absorption of a photon causes the same effect, no matter what the wavelength is
  • any two wavelengths can cause the same response by changing the intensity

  two receptor types (dichromats) solve this problem but three types (trichromats) allow for perception of more colors
• Color Deficiency

  • monochromat: person who needs only one wavelength to match any color
  • dichromat: person who needs only two wavelengths to match any color
  • anamalous trichromat: needs three wavelengths in different proportions than normal trichromat
  • unilateral dichromat: trichromatic vision in one eye and dichromatic in other
• Monochromatism

  • rare hereditary condition
  • only rods and no functioning cones
  • ability to perceive only in white, gray, and black tones
  • true color-blindness
  • poor visual acuity
  • sensitive eyes to bright light
• Dichromatism: Protanopia

  • individuals see short-wavelengths as blue
  • neutral point occurs at 492 nm
  • above neutral point they see yellow
  • missing the long-wavelength pigment
• Dichromatism: Deuteranopia

  • individuals see short-wavelengths as blue
  • neutral point occurs at 498 nm
  • above neutral point they see yellow
  • missing the medium wavelength pigment
• Dichromatism: Tritanopia

  • individuals see short wavelengths as blue
  • neutral point occurs at 570 nm
  • above neutral point they see red
  • missing the short wavelength pigment
• Opponent-Process Theory of Color Vision

  • color vision is caused by opposing responses generated by blue and yellow, and by green and red
  • color afterimages and simultaneous color contrast show the opposing pairings
  • types of color blindness are red/green and blue/yellow
• Opponent-Process Theory of Color Vision

  • three mechanisms: red/green, blue/yellow, and white/black
  • pairs respond in an opposing fashion, such as positively to red and negatively to green
  • responses believed to be the result of chemical reactions in the retina
• Physiology Evidence for the Theory

  researchers performing single-cell recordings found opponent neurons

  • located in the retina and LGN
  • respond in an excitatory manner to one end of the spectrum and an inhibitory manner to the other
• Trichromatic and Opponent-Process Theories Combined

  Each theory describes physiological mechanisms in the visual system

  • trichromatic theory explains the responses of the cones in the retina
  • opponent-process theory explains neural response for cells connected to the cones further in the brain
• light → trichromatic (receptors → color matching) → opponent-process (opponent cells → afterimages, simultaneous contrast) → to brain
• Color in the Cortex

  there is no single module for color perception

  • cortical cells in V1 and V4 respond to some wavelengths or have opponent responses
  • these cells also respond to forms and orientations
  • cortical cells that respond to color may also respond to white
• Types of Opponent Neurons in the Cortex

  • single-opponent neurons: circular receptive field
  • double-opponent neurons: square receptive field
• Color Constancy

  color constancy: perception of colors as relatively constant in spite of changing light sources

  • sunlight has approximately equal amounts of energy at all visible wavelengths
  • Tungsten lighting has more energy in the long-wavelengths
  • objects reflect different wavelengths from these two sources
• Color Constancy

  chromatic adaptation: prolonged exposure to chromatic color leads to receptors:

  • "adapting" when the stimulus color selectively bleaches a specific cone pigment
  • decreasing in sensitivity to the color

  adaptation occurs to light sources leading to color constancy
• Color Constancy

  Experiment in which observers are shown sheets of colored paper in three conditions

  • baseline: paper and observer in white light
  • observer not adapted: paper illuminated by red light; observer by white
  • observer adapted: paper and observer in red light
• Color Constancy

  Experiment results show:

  • baseline: green paper is seen as green
  • observer not adapted: perception of green paper is shifted toward red
  • observer adapted: perception of green paper is slightly shifted toward red (partial color constancy was shown in this condition)
• Color Constancy

  effect of surroundings

  • color constancy works best when an object is surrounded by many colors

  memory and color

  • past knowledge of an object's color can have an impact on color perception