Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOpponent theoryPSY 310Greg FrancisLecture 18Reach that last 1%.Purdue UniversityTrichromatic theory Different colors are represented as a patternacross the three basic colors Nicely predicted the existence of the three conetypes and their properties Forms the basis for color perception Still more to explain What we see is not the pattern at the retina!Purdue UniversityColor 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) conetypes Do not see color at all Everything is shades of grayPurdue UniversityColor blindness Ishihara plates areoften used to testcolor blindness Subject is to reportthe numberPurdue UniversityColor blindness This is how theimage would look toa person with onetype of colorblindnessPurdue UniversityColor blindness Color blindness has some real implications for life styleFind theredcrayonsProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor blindness This is how it looks to one type of dichromatFind theredcrayonsPurdue UniversityColor blindness People who give talksor print documentsshould take colorblindness into account Choices of colors cancause serious problems The default choices inMicrosoft products tendto be rather badPurdue UniversityColor blindness This is better because the letters are defined by luminance as well aschromatic colorPurdue UniversityColor blindness Ewald Hering (1834-1918) noticed that thecolors reported bycolor blind people arenot haphazard People who cannotsee red are also colorblind to green People who cannotsee blue are alsocolor blind to yellowPurdue UniversityColor context What color you perceive depends on surrounding colors Here you can tell that the six rectangles differ in colorPurdue UniversityColor context They are physically the same here, but they look like differentshades of grayProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor contrast There is only one shade of pink in this image What matters is whether the surrounding dots are green or whitePurdue UniversityColor contrast The big and small rings are the same physicallyPurdue UniversityColor context The blue inner circles are identical throughoutPurdue UniversityColor 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 tealPurdue UniversityColor 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 UniversityColor afterimageProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white habituating gate offset of one color leads torebound in otherPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white Initial balance Neither color winscompetitionPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white Extra input to green Green wins competitionPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white Extra input to green Fading of green signalPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white Offset of green Rebound of red signalPurdue UniversityOpponent theory competition betweenopposite colors red-green blue-yellow black-white Recovery of green pathway Disappearance of rebound Return to initial stateProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityOpponent theory You have similar systemsfor Black-white Blue-yellowPurdue UniversityTwo theories Historically, trichromatic color theory and opponent colortheory were seen as alternative theories But people just didn’t think it through carefully Physiological studies demonstrate that they are both part ofcolor perceptionConesOpponent cells(Ganglion cells)Purdue UniversityTwo theories Rough schematicPurdue UniversityAt the retina? We’ve mostly discussed color perception at theretina Surely the brain does something? Huge areas of the brain are sensitive to variationsin color But we don’t really understand how it works Moreover, there is some evidence that manyaspects of color perception can be explained withretinal mechanisms E.g., you don’t have to see a stimulus to get an afterimage aidots.gif demonstrationPurdue UniversityConclusions Color blindness Color context and color contrast Color properties Color afterimages Opponent theoryPurdue UniversityNext time Constancy Achromatic color The amazing range of visual
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