Chapter 7: Color Vision How do we perceive color?
Color Vision Light Stimulus EYE + BRAIN VISION • Vision consists of the perception of: • Color • Size • Shape • Depth • Distance • Speed
Intensity lD l (nm) • 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
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 Sunlight Reflects more blue
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.
Features of Color Vision • 3. Contrast Effects: • Simultaneous B & W Contrast: • White = Brightest Grey • Black = Darkest Grey
Contrast Effects (Contd.) • Simultaneous Color Contrast: • The appearance of a color changes based on the colors surrounding it . http://webexhibits.org/colorart/contrast.html
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/SuccessiveColorContrast.html
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
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!
Features of Color Vision (Contd.) 5. Lightness Constancy: Our judgment of lightness or darkness remains unimpaired even under different illumination.
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.
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.
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: Young’s “Trichromatic” theory forms the basis for the “Component Theory”.
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.
Achromatic Response Seems to indicate that there are four “psychologically pure” primaries.
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 inhibitsactivity in the other. • Explains many psychological effects and color blindness.
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