Opponent theory PSY 310 Greg Francis. Lecture 18. Trichromatic theory

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1 PSY 310 Greg Francis Lecture 18 Reach that last 1%. Trichromatic theory Different colors are represented as a pattern across the three basic colors Nicely predicted the existence of the three cone types and their properties Forms the basis for color perception Still more to explain What we see is not the pattern at the retina! 1

2 Trichromats: have all three cones Normal color vision Most of us Dichromats: are missing one cone Several different types Depending on which cone is missing Have abnormal color vision Do see some colors Monochromats: missing two (rare) or three (more common) cone types Do not see color at all Everything is shades of gray Ishihara plates are often used to test color blindness Subject is to report the number 2

3 Prof. Greg Francis This is how the image would look to a person with one type of color blindness has some real implications for life style Find the red crayons 3

4 This is how it looks to one type of dichromat Find the red crayons People who give talks or print documents should take color blindness into account Choices of colors can cause serious problems The default choices in Microsoft products tend to be rather bad 4

5 This is better because the letters are defined by luminance as well as chromatic color Ewald Hering ( ) noticed that the colors reported by color blind people are not haphazard People who cannot see red are also color blind to green People who cannot see blue are also color blind to yellow 5

6 Prof. Greg Francis Color context What color you perceive depends on surrounding colors Here you can tell that the six rectangles differ in color Color context They are physically the same here, but they look like different shades of gray 6

7 Color contrast There is only one shade of pink in this image What matters is whether the surrounding dots are green or white Color contrast The big and small rings are the same physically 7

8 Color context The blue inner circles are identical throughout Color properties There are interesting combinations of colors A reddish yellow is orange A reddish blue is purple A greenish yellow is light green A greenish blue is teal 8

9 Color properties So what about A reddish green? A yellowish blue??? There seem to be no such colors. Why not? Red and green, and blue and yellow Are intimately connected Color afterimage 9

10 competition between opposite colors red-green blue-yellow black-white habituating gate offset of one color leads to rebound in other competition between opposite colors red-green blue-yellow black-white Initial balance Neither color wins competition 10

11 competition between opposite colors red-green blue-yellow black-white Extra input to green Green wins competition competition between opposite colors red-green blue-yellow black-white Extra input to green Fading of green signal 11

12 competition between opposite colors red-green blue-yellow black-white Offset of green Rebound of red signal competition between opposite colors red-green blue-yellow black-white Recovery of green pathway Disappearance of rebound Return to initial state 12

13 You have similar systems for Black-white Blue-yellow Two theories Historically, trichromatic color theory and opponent color theory were seen as alternative theories But people just didn t think it through carefully Physiological studies demonstrate that they are both part of color perception Cones Opponent cells (Ganglion cells) 13

14 Two theories Rough schematic At the retina? We ve mostly discussed color perception at the retina Surely the brain does something? Huge areas of the brain are sensitive to variations in color But we don t really understand how it works Moreover, there is some evidence that many aspects of color perception can be explained with retinal mechanisms E.g., you don t have to see a stimulus to get an afterimage aidots.gif demonstration 14

15 Conclusions Color context and color contrast Color properties Color afterimages Next time Constancy Achromatic color The amazing range of visual perception 15