PSYB51H3 Study Guide - Opponent Process, Subtractive Color, Lateral Geniculate Nucleus

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29 Mar 2014
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PSYB51 CHAPTER 5
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Basic Principles of Color Perception
- Some wavelengths are absorbed by the surfaces they hit, the more light that is absorbed, that
darker the surface will appear
- Color of a surface depends on the mix of wavelengths that reach the eye from the surface
- Color is the result of the interaction of physical stimulus with a particular nervous system
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Three Steps to Color Perception
- Detection: wavelengths must be detected. Light is differentially absorbed by three photo pig-
ments in the cones
- Discrimination: must be able to tell the different between one wavelength and another – differ-
ences are taken between cone types, creating cone-opponent mechanisms, important for wave-
length discriminations
- Appearance: want to assign perceived colors to lights and surfaces in the world. Want perceived
colors to go with the object and not to change dramatically as the viewing conditions change.
Further recombination of the signals creates color-opponent processes that support the color-op-
ponent nature of color appearance
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Step 1: Color Detection
- S-cone: a cone that is preferentially sensitive to short wavelengths; known as “blue cone”
- M-cone: a cone that is preferentially sensitive to short wavelengths, known as “green cone”
- L-cone: a cone that is preferentially sensitive to long wavelengths, known as “red cone”
- Cones work at daylight (phototopic) light levels
- Rods work in dimmer (scotopic) light levels
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Step 2: Color Discrimination
The Problem of Univariance
- Problem of univariance: the fact that an infinite set of different wavelength-intensity combi-
nations can elicit exactly the same response from a single type of photoreceptor. One photorecep-
tor type cannot make color discriminations based on wavelength
- Univariance explains the lack of color in dimly lit scenes
- Dim light stimulates only the rods, and the output of that single variety of photoreceptor does
not permit color vision
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The Trichromatic Solution
- Trichromatic theory of color vision: the theory that the color of any light is defined in our
visual system by the relationships of three numbers the outputs of three receptor types now
known to be the three cones. Also known as the Young-Hemholtz theory.
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Metamers
- Metamers: different mixtures of wavelengths that look identical. More generally, any pair of
stimuli that are perceived as identical in spite of physical differences
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There are two warnings
1. Mixing wavelengths does not change the physical wavelengths; color mixture is a mental
event, not a change in the physics of light
2. For mixture of a “red” and “green” light to look perfectly yellow, we would have to have
just the right red and just the right green; other mixes might look a bit more reddish or a
bit more greenish
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The History of Trichromatic Theory
- Schnapf et al., recorded the activity of single photoreceptors
- Nathans et al., found the genes that code for the different photopigments
- Isaac Newton – a prism would break up sunlight into the spectrum of hues, and a second prism
would put the spectrum back together in white
- Maxwells color matching experiment a color is presented on the left, and on the right, the
observer adjusts a mixture of the three lights to match the color on the left
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A Brief Digression into Lights, Filters, and Finger Paints
- Additive color mixture: a mixture of lights. If light A and light B are both reflected from a sur-
face to the eye, in the perception of color the effects of those two lights add together
- Finger paint looks a particular color because it absorbs some wavelengths, subtracting them
from the white light falling on a surface covered with the pigment
- Subtractive color mixture: a mixture of pigments. If pigments A and B mix, some of the light
shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to
the perception of color.
- Georges Seurat and other Postimpressionist artists of the late 10th century experimented with
Pointillism, a style of painting that involved creating many hues by placing small spots of just a
few colors in different textures
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From Retina to Brain: Repackaging the Information
- Cones in the retina are the neural substrate for detection of lights
- To tell difference between different lights, the nervous system will look at differences in the
activities of the three cone types
- Nervous system computes two differences: (L – M) and ([L + M] – S)
- Combining L and M signals is a pretty good measure of the intensity of light
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Cone-Opponent Cells in the Retina and LGN
- Cone signals exist in the lateral geniculate nucleus
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Document Summary

Some wavelengths are absorbed by the surfaces they hit, the more light that is absorbed, that darker the surface will appear. Color of a surface depends on the mix of wavelengths that reach the eye from the surface. Color is the result of the interaction of physical stimulus with a particular nervous system. Light is differentially absorbed by three photo pig- ments in the cones. Discrimination: must be able to tell the different between one wavelength and another differ- ences are taken between cone types, creating cone-opponent mechanisms, important for wave- length discriminations. Appearance: want to assign perceived colors to lights and surfaces in the world. Want perceived colors to go with the object and not to change dramatically as the viewing conditions change. Further recombination of the signals creates color-opponent processes that support the color-op- ponent nature of color appearance. S-cone: a cone that is preferentially sensitive to short wavelengths; known as blue cone .

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