Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor perceptionPSY 310Greg FrancisLecture 17Which cracker do you want to eat?Purdue UniversityImportance of color For most people color is an integral part of living It is useful for identifying properties of objects e.g., ripe fruitProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityImportance of color For most people color is an integral part of living It is useful for identifying properties of objects e.g., raw meatPurdue UniversityImportance of color Some judgments impossible without color It is useful for identifying properties of objects e.g., raw meatProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityImportance of color Some stimuli are defined almost entirely by color Isoluminant 'Impression Sunrise', Claude Monet, 1873Purdue UniversityImportance of color Remove the color, and the sun almost disappears 'Impression Sunrise', Claude Monet, 1873Prof. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityImportance of color People find colors beautifulPurdue UniversityImportance of color People find colors beautifulProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityImportance of color We expect things to be certain colors When they are not, they look very oddPurdue UniversityWhat is color? People used to think (up until the 19th century) That color was a basic property of objects It’s part of our language The wall is red. Her eyes are blue. But color is actually a percept It depends on Properties of the object Properties of the illuminating light Properties of background stimuli The physiology of the viewerProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversitySome terms Wavelength: the frequency of light This is a physical measurementPurdue UniversitySome terms Chromatic color: reds, greens, blues, and so on Sometimes called hue or just color Achromatic color: white, gray, blackProf. Greg Francis7PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCones There are threekinds of conesthat respond todifferentwavelengths oflight Short (blue) Medium (green) Long (red) This is the start ofcolor visionPurdue UniversityTrichromatic theory Thomas Young (1773-1829) Hermann von Helmholtz (1821-1894) Developed before knowledge about the photoreceptors of theeye Predicted by psychophysical experiments Color matching The color on the left could be any combination of wavelengthsof lightProf. Greg Francis8PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor matching Subject changes the intensity of the three colors to make theleft and right rectangles look the samePurdue UniversityColor matching Subject changes the intensity of the three colors to make theleft and right rectangles look the sameYou can alwaysget a match!Prof. Greg Francis9PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor matching Significantly, if I give you only two colors to work with, youcannot match some colorsThis is the bestI could getwithout red.Purdue UniversityColor matching You can always get a match with three colors because colorperception is based on the responses of the three cone types The two stimuli (though physically different) can activate the cones inthe same wayProf. Greg Francis10PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor matching The same neurophysiological response must lead to the samepercept Metamer colors This is the basis for all color computer monitors and printersPurdue UniversityColor matching By measuring the wavelengths of color metamers, Helmholtzderived how the photoreceptors must respond to differentwavelengths of light Not bad!Prof. Greg Francis11PSY 310: Sensory and Perceptual ProcessesPurdue UniversityColor coding One can think of the representation of colors as apattern of activity across the cone typesPurdue UniversityDichromats Some animals (and people) have only two cones They can see color, but the same as normals A picture like thisProf. Greg Francis12PSY 310: Sensory and Perceptual ProcessesPurdue UniversityDichromats Some animals (and people) have only two cones They can see color, but the same as normals Looks like this (to a horse)Purdue UniversityConclusions Color Importance Trichromatic theory Color matching Note, we didn’t discuss the Garner interferenceCogLab assignment I think there is a problem with the lab, so the data are nogoodProf. Greg Francis13PSY 310: Sensory and Perceptual ProcessesPurdue UniversityNext time Opponent-process theory of color vision Afterimages Color contrast Neural
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