Prof. Greg Francis Opponent theory PSY 310 Greg Francis Lecture 18 Reach that last 1%. Purdue University 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! Purdue University PSY 310: Sensory and Perceptual Processes 1 Prof. Greg Francis Color blindness 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 Purdue University Color blindness Ishihara plates are often used to test color blindness Subject is to report the number Purdue University PSY 310: Sensory and Perceptual Processes 2 Prof. Greg Francis Color blindness This is how the image would look to a person with one type of color blindness Purdue University Color blindness Color blindness has some real implications for life style Find the red crayons Purdue University PSY 310: Sensory and Perceptual Processes 3 Prof. Greg Francis Color blindness This is how it looks to one type of dichromat Find the red crayons Purdue University Color blindness 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 Purdue University PSY 310: Sensory and Perceptual Processes 4 Prof. Greg Francis Color blindness This is better because the letters are defined by luminance as well as chromatic color Purdue University Color blindness Ewald Hering (1834- 1918) 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 Purdue University PSY 310: Sensory and Perceptual Processes 5 Prof. Greg Francis Color context What color you perceive depends on surrounding colors Here you can tell that the six rectangles differ in color Purdue University Color context They are physically the same here, but they look like different shades of gray Purdue University PSY 310: Sensory and Perceptual Processes 6 Prof. Greg Francis Color contrast There is only one shade of pink in this image What matters is whether the surrounding dots are green or white Purdue University Color contrast The big and small rings are the same physically Purdue University PSY 310: Sensory and Perceptual Processes 7 Prof. Greg Francis Color context The blue inner circles are identical throughout Purdue University 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 Purdue University PSY 310: Sensory and Perceptual Processes 8 Prof. Greg Francis 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 Purdue University Color afterimage Purdue University PSY 310: Sensory and Perceptual Processes 9 Prof. Greg Francis Opponent theory competition between opposite colors red-green blue-yellow black-white habituating gate offset of one color leads to rebound in other Purdue University Opponent theory competition between opposite colors red-green blue-yellow black-white Initial balance Neither color wins competition Purdue University PSY 310: Sensory and Perceptual Processes 10 Prof. Greg Francis Opponent theory competition between opposite colors red-green blue-yellow black-white Extra input to green Green wins competition Purdue University Opponent theory competition between opposite colors red-green blue-yellow black-white Extra input to green Fading of green signal Purdue University PSY 310: Sensory and Perceptual Processes 11 Prof. Greg Francis Opponent theory competition between opposite colors red-green blue-yellow black-white Offset of green Rebound of red signal Purdue University Opponent theory competition between opposite colors red-green blue-yellow black-white Recovery of green pathway Disappearance of rebound Return to initial state Purdue University PSY 310: Sensory and Perceptual Processes 12 Prof. Greg Francis Opponent theory You have similar systems for Black-white Blue-yellow Purdue University 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) Purdue University PSY 310: Sensory and Perceptual Processes 13 Prof. Greg Francis Two theories Rough schematic Purdue University 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 Purdue University PSY 310: Sensory and Perceptual Processes 14 Prof. Greg Francis Conclusions Color blindness Color context and color contrast Color properties Color afterimages Opponent theory Purdue University Next time Constancy Achromatic color The amazing range of visual perception Purdue University PSY 310: Sensory and Perceptual Processes 15.