Prof. Greg Francis Trichromatic theory PSY 310 Greg Francis 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! Lecture 18 Reach that last 1%. Trichromats: have all three cones Normal color vision Most of us Dichromats: are missing one cone Several different types Ishihara plates are often used to test color blindness Subject is to report the number 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 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 1
Prof. Greg Francis 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 This is better because the letters are defined by luminance as well as chromatic color What color you perceive depends on surrounding colors Here you can tell that the six rectangles differ in color People who cannot see red are also color blind to green People who cannot see blue are also color blind to yellow Ewald Hering (18341918) noticed that the colors reported by color blind people are not haphazard They are physically the same here, but they look like different shades of gray 2
Color contrast Color contrast There is only one shade of pink in this image What matters is whether the surrounding dots are green or white The big and small rings are the same physically The blue inner circles are identical throughout 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 So what about A reddish green? Color afterimage A yellowish blue??? There seem to be no such colors. Why not? Red and green, and blue and yellow Are intimately connected 3
red-green blue-yellow black-white habituating gate offset of one color leads to rebound in other red-green blue-yellow black-white Initial balance Neither color wins competition red-green blue-yellow black-white Extra input to green Green wins competition red-green blue-yellow black-white Extra input to green Fading of green signal red-green blue-yellow black-white Offset of green Rebound of red signal red-green blue-yellow black-white Recovery of green pathway Disappearance of rebound Return to initial state 4
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) Rough schematic Two theories 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 Conclusions and color contrast Color afterimages Next time Constancy Achromatic color The amazing range of visual perception 5