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COLOR PERCEPTION

COLOR PERCEPTION. Physical and Psychological Properties Theories Trichromatic Theory Opponent Process Theory Color Deficiencies Color and Lightness Constancies. Properties of Color. Trichromatic Theory.

francis-vaughan
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COLOR PERCEPTION

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  1. COLOR PERCEPTION • Physical and Psychological Properties • Theories • Trichromatic Theory • Opponent Process Theory • Color Deficiencies • Color and Lightness Constancies

  2. Properties of Color

  3. Trichromatic Theory • All colors are perceived as combinations of three primaries: red, green, and blue • This refers to additive mixing, in which color combining happens in the visual system (subtractive mixing is when pigments or dyes are mixed)

  4. Trichromatic Theory • People with normal color vision need three primaries to match any color • Color TV is based on this principle

  5. Trichromatic Theory • Three types of cone cells with three types of photopigments: • Short (419 nm) • Medium (531 nm) • Long (558 nm)

  6. Opponent Process Theory • All colors are perceived as a combination of opponent pairs of colors: • red-green • blue-yellow

  7. Opponent Process Theory • We don’t see colors that combine opponents (e.g., reddish green) • Negative color afterimages • Simultaneous color contrast - change in appearance of a color caused by surrounding color

  8. Opponent Process Theory • Opponent neurons in LGN: • B+Y- • Y+B- • R+G- • G+R-

  9. Which Theory is Correct ? • Both! • Trichromatic Theory describes processing of cones • Opponent Process Theory describes processing at higher levels in the visual system

  10. S L M cones bipolars R+G- B+Y- ganglions

  11. Blue light S L M R+G- B+Y- “blue”

  12. Yellow light S M L R+G- B+Y- “yellow”

  13. Red light S M L R+G- B+Y- “red”

  14. Green light S M L R+G- B+Y- “green”

  15. Color Vision Deficiencies • cerebral achromatopsia - no color vision; due to damage in secondary visual cortex • monochromat - no color vision; due to abnormal or missing photopigments

  16. Color Vision Deficiencies • dichromat - can match all colors using only two primaries • protanopia - no L pigment (red-green loss) • deuteranopia - no M pigment (red-green loss) • tritanopia - no S pigment (blue-yellow loss)

  17. Color Vision Deficiencies • anomalous trichromat – three cone systems, but two of them are too close together • most common color vision problem

  18. Color Constancy • We perceive the hue of an object as constant even when the wavelength illuminating it changes • May be due to comparing reflectance patterns from various objects

  19. Lightness and Brightness • lightness refers to perception of the intensity of an achromatic color • brightness refers to perception of the intensity of chromatic colors

  20. Lightness Constancy • Perception of an object with an achromatic color is the same even when the amount of light reflected changes • Ratio principle: compare amount of light reflected from other objects to perceive lightness of an object

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