COLOR VISION I The Spectrum II Trichromatic Vision вЂ“ 1. 2. Cones Additive Mixing Subtractive Mixing III Color Opponency вЂ“ вЂ“ вЂ“ Complimentary Colors Color Blindness Color Constancy IV Color Vision вЂ“ вЂ“ Memory & Imagery Form & Motion Anthony J Greene 1 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 Greene 2 Light and Frequency вЂў Color is how we discriminate frequencies вЂў Light does not have color, light has frequency вЂў Objects do not have color, objects have pigments which absorb all frequencies except those which we see Anthony J Greene 3 The Spectrum Color Properties вЂў Brightness amount of light вЂў Saturation richness of color: desaturated colors are are grayish or whitish вЂў Hue - frequency : position on the spectrum Violet - Indigo - Blue - Green - Yellow - Orange - Red Short -------------------Medium------------------------ Long 400 nm ----------------550 nm-----------------------700 nm Color Solid Hue Brightness Saturation Anthony J Greene 5 Trichromatic ColorVision вЂў 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 вЂў Combinations of cone activity likewise specify all percievable colors Anthony J Greene Red Green Blue 6 Anthony J Greene 7 Color Mixing вЂў 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 Greene 8 Color Mixing вЂў 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 Greene 9 Subtractive Color Mixing Anthony J Greene 10 Color Opponency вЂў Three types of cones connect to two types of ganglion cells Anthony J Greene 11 Color Opponency вЂў Starting with an Y-B pathway & the evolution of an R-G pathway вЂў Implications for human R-G color blindness and animal vision Anthony J Greene 12 Color Opponency вЂў Red-Green cells will increase their firing rate in response to green and decrease their firing rate in response to red (or vice-versa) вЂў 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 Greene 13 Color Opponency вЂў Color opponency determines complementary colors вЂў 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 Greene 14 Color Opponency Color opponency determines complementary colors Anthony J Greene 15 Color Opponency вЂў Color After-images Anthony J Greene 16 Anthony J Greene 17 Color Opponency вЂў Color Afterimages Anthony J Greene 18 Anthony J Greene 19 Color Opponency вЂў Color Afterimages Anthony J Greene 20 Color Opponency вЂў Color Blindness Anthony J Greene 21 Color Constancy вЂў Discounting the illuminant - adaptation вЂў The color of the ambient lighting quickly fatigues photorecptors to that color -- There is no eye position that allows the photoreceptors to recover вЂў 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 Greene 22 Color Constancy There is a cognitive component as well. Anthony J Greene 23 Color Constancy There is a cognitive component as well. Anthony J Greene 24 Color constancy Anthony J Greene 25 Color Vision 1. Memory & Imagery вЂ“ Achromatopsia 2. Form & Motion вЂ“ Interactions of color system with other visual components Anthony J Greene 26 Case of Achromatopsia вЂў 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 вЂў In addition, color imagery and color memory are also lost вЂў What are the implications for perception, imagery and memory? Anthony J Greene 27 Anthony J Greene 28 Anthony J Greene 29 Anthony J Greene 30 Color, Form & Motion вЂў Although V4 interacts with other areas (V3 & V5 are monochromatic), its interactions are limited вЂў Equiluminant color conditions makes form and motion perception difficult -- but not impossible Anthony J Greene 31 Equiluminant Colors Anthony J Greene 32 Equiluminant Colors Anthony J Greene 33 Equiluminant Colors Anthony J Greene 34 This is Difficult to Read This is Easier to Read This is Easiest to Read Anthony J Greene 35 Dad: Wow Honey, your missing a beautiful sunset out here. Mom: IвЂ™ll count to 10 and thenвЂ¦ POW Calvin: Dad, how come old photographs are always black and white. DidnвЂ™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 didnвЂ™t turn color until sometime in the 1930s, and it was pretty grainy color for a while too. Calvin: ThatвЂ™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, wouldnвЂ™t artists have painted it that way? Dad: Not necessarily, a lot of great artists were insane. Calvin: ButвЂ¦ but how could they have painted in color anyway? WouldnвЂ™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 didnвЂ™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.