anthony j greene1 color vision i the spectrum ii trichromatic vision –cones 1.additive mixing...

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COLOR VISION

I The SpectrumII Trichromatic Vision

– Cones1. Additive Mixing2. Subtractive Mixing

III Color Opponency– Complimentary Colors– Color Blindness– Color Constancy

IV Color Vision– Memory & Imagery– Form & Motion

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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

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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 - RedShort -------------------Medium------------------------ Long400 nm ----------------550 nm-----------------------700 nm

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Color Solid

Hue

Brightness

Saturation

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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

Red

Green

Blue

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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

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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

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Subtractive Color Mixing

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Color Opponency• Three types of cones connect to two types

of ganglion cells

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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

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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?

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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

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Color Opponency

Color opponency determines complementary colors

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Color Opponency

• Color After-images

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Color Opponency• Color After-

images

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Color Opponency• Color After-

images

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Color Opponency

• Color Blindness

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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

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Color Constancy

There is a cognitive component as well.

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Color Constancy

There is a cognitive component as well.

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Color constancy

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Color Vision

1. Memory & Imagery– Achromatopsia

2. Form & Motion– Interactions of color system with other visual

components

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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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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

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Equiluminant Colors

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Equiluminant Colors

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Equiluminant Colors

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This is Difficult to Read

This is Easiest to Read

This is Easier to Read

Dad: Wow Honey, your missing a beautiful sunset out here.Mom: I’ll count to 10 and then… POWCalvin: Dad, how come old photographs are always black and white. Didn’t they have color film back thenDad: Sure they did. In fact those old photographs are in color. Its just the world was black and white then.Calvin: ReallyDad: 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 fictionCalvin: 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.