1. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.1 Color Perception The Physical and Psychological variables Grassman’s Laws of color mixing Additive vs. Subtractive color mixing Color facts and findings Physiological findings Balance Theory

2. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.2 3 Physical And Psychological Variables PHYSICAL –wavelength –intensity –purity PERCEPTUAL –hue –brightness –saturation

3. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.3 Double Cone Color Solid GRAPH of the 3 physical variables Some colors are not on the graph: extra-spectral purple non-spectral - gold, silver, bronze, brown Every point on the graph represents a color

4. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.4 Grassman’s Laws of Additive Color Mixture A normal person can match any hue with a combo of 3 non-complementary colors Complementaries –equal mix produces gray –unequal produces de-saturation of the dominant color Non - complementaries –de-saturated intermediate hue Metamers –appear the same, but composed of different wavelengths

5. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.5 Additive vs. Subtractive Color Mixing Additive color mixing is a mix of lights –stage lighting –pointillism (paintings) Subtractive color mixing is usually from reflected light –most of our color experience is reflected light

6. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.6 Additive vs. Subtractive Color Mixing Subtractive Color Mix The paint absorbs or subtracts out wavelengths and the color you see if the wavelengths that were reflected back to you (not absorbed) Additive mixture The wavelengths are added together so the final color you see is the sum of the wavelengths

7. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.7 Color Facts Grassman’s laws, esp. 3 colors can make any hue Lack of reddish-green or bluish-yellow 3 cone types 4 primary colors Complementary afterimages Simultaneous contrast Color map of the retina Color blindness

8. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.8 Color Blindness Normal color vision - 3D trichromats Dichromats - 2D vision (2% males) –Protanopes see blue and yellow (no red or green) blind to deep red light –Deuteranopes see blue and yellow but can see all wavelengths –Tritanope see red and green Monochromat - hue blind

9. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.9 Physiological Findings Wald - measures spectral sensitivity of cones –three cone types short, medium, and long wavelength detectors De Valois –opponent process cells in LGN –excitatory to one color and inhibitory to another R+G-; G+R-; B+Y-; Y+B-

10. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.10 Hurvich and Jameson’s Balance Theory Retina –three cone types (short, medium, and long wavelength detectors) LGN –opponent process cells Cortex –“balances” that produce all spectral colors by balancing inhibition and excitation

11. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.11 Hurvich and Jameson’s Balance Theory Note the pattern of excitatory and inhibitory connections between the cones in the retina and the opponent process cells in LGN

12. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.12 Balance Theory Color is determined by the pattern of responses in the three different cone types.

13. Sensation and Perception - color.ppt © 2001 Dr. Laura Snodgrass, Ph.D.13 Hurvich and Jameson’s Balance Theory Balances in cortex are needed to account for colors beyond four primaries Blue Yellow Green Red E.g. Yellow & Red = Orange Blue & Red = Purple SM + LMS