Anthony J Greene1 COLOR VISION I The Spectrum II Trichromatic Vision –Cones 1.Additive Mixing 2.Subtractive Mixing III Color Opponency –Complimentary Colors –Color Blindness –Color Constancy IV Color Vision –Memory & Imagery –Form & Motion

Anthony J Greene2 Light and Frequency The Rays, to speak properly, are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour... So Colours in the Object are nothing but a Disposition to reflect this or that sort of Rays more copiously than the rest. -Isaac Newton

Anthony J Greene3 Light and Frequency Color is how we discriminate frequencies Light does not have color, light has frequency Objects do not have color, objects have pigments which absorb all frequencies except those which we see

The Spectrum Color Properties Brightness - amount of light Saturation - richness of color: desaturated colors are are grayish or whitish Hue - frequency : position on the spectrum Violet - Indigo - Blue - Green - Yellow - Orange - Red Short Medium Long 400 nm nm nm

Anthony J Greene5 Color Solid Hue Brightness Saturation

Anthony J Greene6 Trichromatic ColorVision Each of the three primary colors can be considered as a dimension on a 3-D graph - Combinations of these three colors, or positions on the graph specify all visible colors Combinations of cone activity likewise specify all percievable colors

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8 Color Mixing Additive mixing => mixing light - when two colored lights are mixed wavelengths which were present in either original source are now present in the mixture

Anthony J Greene9 Color Mixing Subtractive mixing => mixing pigments - because pigments absorb all colors except for that which you see reflected, when two colored pigments are mixed the result is that only wavelengths which were reflected in both original pigments are reflected in the mixture

Anthony J Greene10 Subtractive Color Mixing

Anthony J Greene11 Color Opponency Three types of cones connect to two types of ganglion cells

Anthony J Greene12 Color Opponency Starting with an Y-B pathway & the evolution of an R-G pathway Implications for human R-G color blindness and animal vision

Anthony J Greene13 Color Opponency Red-Green cells will increase their firing rate in response to green and decrease their firing rate in response to red (or vice-versa) At-home-experiment: Take a half circle of yellow paper, and a half circle of blue paper, glue them to a cardboard disk, push a pencil through the center of the disk, then spin the disk rapidly: What is the hypothesis?

Anthony J Greene14 Color Opponency Color opponency determines complementary colors When complementary colors are additively combined (i.e., simultaneous activation of red and green or of blue and yellow) opponent processes cancel and a shade of gray is percieved

Anthony J Greene15 Color Opponency Color opponency determines complementary colors

Anthony J Greene16 Color Opponency Color After-images

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Anthony J Greene18 Color Opponency Color After- images

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Anthony J Greene20 Color Opponency Color After- images

Anthony J Greene21 Color Opponency Color Blindness

Anthony J Greene22 Color Constancy Discounting the illuminant - adaptation The color of the ambient lighting quickly fatigues photorecptors to that color -- There is no eye position that allows the photoreceptors to recover Once fatigued to the ambient color, that color is subtracted, or discounted from the visual scene and colors appear close to the way they would in white light

Anthony J Greene23 Color Constancy There is a cognitive component as well.

Anthony J Greene24 Color Constancy There is a cognitive component as well.

Anthony J Greene25 Color constancy

Anthony J Greene26 Color Vision 1.Memory & Imagery –Achromatopsia 2.Form & Motion –Interactions of color system with other visual components

Anthony J Greene27 Case of Achromatopsia Damage to V4 can cause the complete loss of color vision (as opposed to red-green color blindness): V4 is more sensitive to oxygen deprivation In addition, color imagery and color memory are also lost What are the implications for perception, imagery and memory?

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Anthony J Greene31 Color, Form & Motion Although V4 interacts with other areas (V3 & V5 are monochromatic), its interactions are limited Equiluminant color conditions makes form and motion perception difficult -- but not impossible

Anthony J Greene32 Equiluminant Colors

Anthony J Greene33 Equiluminant Colors

Anthony J Greene34 Equiluminant Colors

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Dad: Wow Honey, your missing a beautiful sunset out here. Mom: I’ll count to 10 and then… POW Calvin: Dad, how come old photographs are always black and white. Didn’t they have color film back then Dad: Sure they did. In fact those old photographs are in color. Its just the world was black and white then. Calvin: Really Dad: Yep. The world didn’t turn color until sometime in the 1930s, and it was pretty grainy color for a while too. Calvin: That’s really weird. Dad: Well, truth is stranger than fiction Calvin: But then why are old paintings in color? If the world was black and white, wouldn’t artists have painted it that way? Dad: Not necessarily, a lot of great artists were insane. Calvin: But… but how could they have painted in color anyway? Wouldn’t their paints have been shades of gray back then? Dad: Of course, but they turned colors like everything else did in the 30s. Calvin: so why didn’t old black and white photos turn color too? Dad: Because they were color pictures of black and white, remember? Calvin: The world is a complicated place Hobbes. Hobbes: Whenever it seems that way, I take a nap in a tree and wait for dinner.