Sensation & PerceptionSensation & PerceptionCh. 9: Perceiving Colorg© Takashi Yamauchi (Dept. of Psychology, Texas A&M University)Main topicsMain topicsFunctions of colorTrichromatic theoryyColor deficiencyLightness constancych 9 1A Sunday afternoon on the island of :La Grande Jatte: GS tAlfalfa field, Saint-Denis: G. SeuratG. Seuratch 9 2Water Lilies: C. MonetSisleych 9 3Ai l l i b fB id VilllAristotle contemplating a bust of homer: Rembrandt (1653)Bridge at Villeneuve-la-Garenne: Sisleych 9 4Why are impressionists’Why are impressionists paintings so bright?• Subject matter–Landscape, not much mythological figuresLandscape, not much mythological figures•Scientists discovered the mechanism underlying the perception of colorppch 9 5Color• Physiologists:–Helmholtz (1821-1894)• http://en.wikipedia.org/wiki/Hermann_von_HelmholtzHering (1834-1918)–Hering (1834-1918)•Impressionist painters:Impressionist painters:– Monet (1840-1926)–Seurat (1859-1891)()– Pissarro (1830-1903)– Sisley (1839-1899)ch 9 6• Impressionists were influenced by the theory of color perceptionyppch 9 7ch 9 8ch 9 9ch 9 10ch 9 11ch 9 12© Takashi Yamauchich 9 13© Takashi YamauchiColors, what are they?• A brief review on visionch 9 14What we see is electromagnetic energych 9 15What does the eye do?y•Capturing electromagnetic energy•Capturing electromagnetic energy.– As the radio and TV doDi i diff l h–Discerning different wave lengthsch 9 16What does the eye do?What does the eye do? Transducing light energy into neural energych 9 17Two types of photo receptors• Cones and Rods•Cones are for color visionCones are for color visionch 9 18EyeEye• Photo receptors•Two types of photoTwo types of photo receptors – rod & conech 9 193 types of conesyp• There are three types cones that are selectively tuned to three different lengths of electromagnetic waves (Short, Medium, and Long)ch 9 20•Short:•Short: – These cones react primarily to the waves whose length are about 419 nm (nanometer)length are about 419 nm (nano-meter)• Medium:Th i il h i h–These cones react primarily to the waves with 531 nmL•Long– These cones react primarily to the waves with 558558 nmch 9 21Color perception is produced by combinations of these 3 types of cones:of these 3 types of cones:Demonstration:VL 9-4ch 9 22Different objects reflect light in a differentlight in a different manners.Some objects (tomato) j( )absorb short waves while reflect long waves create a red surfacecreate a red surface.Absorb short waves but reflect long wavesch 9 23Mixing paintsch 9 24Demonstration: Mi i li ht (VLMixing lights (VL 9-1)ch 9 25What the computer does is theWhat the computer does is the same thing•Pixel•BitBit• Illuminating the phosphor with three different electron guns.•Illuminate each pixel•Illuminate each pixel with red, green, and blue electron gunsch 9 26g3 types of electron guns: a red gun, a green gun, and a blue gunch 9 27Why do the impressionists’Why do the impressionists paintings so bright?They did the same thing.Computer scientists and impressionistsComputer scientists and impressionists developed their ideas based on the psychological theory of color perception.py g y p pch 9 28ch 9 29ch 9 30ch 9 31My paintingsch 9 32ch 9 33ch 9 34ch 9 35After Imagech 9 36ch 9 37ch 9 38ch 9 39ch 9 40Why do we get after-images?• After-images have something to do with adaptation.p• What is adaptation?Motion–Motion– Spatial frequencyLktthd tti f ti•Look at the demonstration of motion adaptationch 9 41ch 9 42ch 9 43Simultaneous Color Contrast• Why do the X’s look different in the two rectangles?• Some sort of ihibi h iinhibitory mechanism is going on?ch 9 44More Phenomenological ObservationsObservations• Visualize something red (apple, fire engine)N ddi hll–Now reddish-yellow– Now reddish-green•Visualize something blue (sky pool)Visualize something blue (sky, pool)– Now bluish-green– Now bluish-yellow• Which is hardest to visualize?• Note – very little overlap in color sensation of bl d ll d f d dblue and yellow, and of red and green• People who are color-deficient for red also lack green; same with blue and yellowch 9 45green; same with blue and yellowch 9 46ch 9 47TrichromaticOpponent-Trichromatic OpponentprocessTrichromatic stage:Trichromatic stage:3 kinds of cone receptors(S, M, L)Opponent processAt the bipolar or ganglion cells, the difference between S and (M+L) (Circuit 1) and the difference between M and L (Circuit 2) is assessed.The first circuit (Circuit 1) processes the blue-yellow difference.The second circuit (Circuit 2)ch 9 48The second circuit (Circuit 2) processes the red-green difference.Opponent-process theory (H i /H i h & J )(Hering/Hurvich & Jameson)• Color vision is caused by opposing responses generated by blue and yellow, pg y y,and by red and green.ch 9 49After images:Red GreenGreen  RedBlue  YellowYellow  BlueR G B Ych 9 50Color perceptionRGB YdttiGadaptationadaptationAfter imageAfter imagech 9 51After imageAfter imageTrichromaticOpponenct-Trichromatic OpponenctprocessTwo stages in color perceptionTwo stages in color perceptionTrichromatic stage:3 kinds of cone receptors(S,M,L)(S, M, L)Opponent processAt the bipolar or ganglian cells, pgg ,the difference between S and (M+L) (Circuit 1) and the difference between M and L (Circuit 2) is assessed.()The first circuit (Circuit 1) processes the blue-yellow difference.Th d i it (Ci it 2)ch 9 52The second circuit (Circuit 2) processes the red-gree difference.ch 9 53Fig. 7-20, p. 155Responses of opponent cells in the monkey’s LGNF S P l (1999)ch 9 54From S. Palmer (1999) Vision ScienceOpponent-process theorySM LSM LS, M, and L cones haveS M LRelativresponS M Lcones have inhibitory and excitatory tive nsesconnections at LGN.InhibitoryEich 9 55ExcitatoryOpponent-process theoryOpponentprocess theory•The opponent-process is more like a mechanism pppto detect the different responses in the three types of cones.•This allows theThis allows the visual system to record differencesbt thbetween the responses of cones, rather than each ch 9 56type of cone's individual response.Why is this process efficient?• Distinguishing the two wavelengths, 1 and 2 is very hard using justis very hard using just the M and L receptors.•Opponent process pp pcreate two categorical responses  + or –ch 9 57ch 9 58ch 9 59ch 9 60Huichol masks (Mexican Indian