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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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Purdue PSY 31000 - Color perception

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