PSC129: Chapter 5 The Perception of Color

I) Basic Principles of color Perception
-Color is the result of the interaction of physical stimulus with a particular nervous system.

1. The Problem of Univariance
-Different wavelengths of light give rise to different experiences of color, and the varying responses of this photoreceptor to different wavelengths could provide a basis for color vision.
-Problem: two different wavelengths can produce the same response –
-Output of a single photoreceptor is ambiguous à problem of univariance
II) Trichromacy
1. Rods and Cones
-Rods: sensitive to low light (scotopic); contain photopigment, rhodopsin; have same sensitivity to wavelength à can’t tell color
-Cones: three types of photoreceptors with different photopigment which give each type of cone a distinctive wavelength sensitivity
a. S-cones: short-wavelength cones; 440nm; blue cone
b. M-cones: middle-wavelength cones; peak at 535nm; green cone
c. L-cones: long-wavelength cones; peak at 565nm; red cone
-Different outputs from the three cones; any wavelength will produce a unique set of three responses
-If increase intensity of light, the response sizes wil change but the proportions will not
-Trichromacy: the theory that the color of any light is defined in our visual system by the relationships between a set of three numbers, the outputs of three receptor types now known to be the three cones.

2. Metamers
-Usually see a mixture of wavelengths
-A single wavelength can excite the cones equally
-The rest of the nervous system knows only what the cones tell it.
-Metamers: different mixtures of wavelengths that look identical; if mixture of red plus green produces the same cone output as the single wavelength of yellow, then the mixture and the single wavelength must look identical and are called metamers.
a. Mixing wavelengths does not change the physical wavelengths; If mix 500 and 600, the stimulus contains 500 and 600 nm wavelengths and not 550.
b. For the mixture of red and green light to look perfectly yellow, we would have to have just the right red and just the right green.

3. Lights and Fingerpaints
-Additive color mixture: a mixture of lights; taking one wavelength and adding it to another
-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; pigment absorbs some wavelengths, subtracting them from white light and reflecting the rest.
-Hue: the chromatic aspect of color (red, blue, green, etc.)
-Saturation: the amount of hue present in a light; white has zero saturation and red is fully saturated
-Brightness: physical intensity of light; the distance from black (zero brightness) in color space

III) Opponent Processes
1. Opponent Cells in the Lateral Geniculate Nucleus
-Some cells are excited by the L-cone onset in their center and inhibited by M-cone onsets in their surrounds --> color-opponent cell
-Color-opponent cell: a neuron whose output is based on a difference between sets of cones; different sources of chromatic information are pitted against each other
-Opponent color theory: the theory that perception of color is based on the output of three mechanisms, each of them on an opponency between two colors: red-green, blue-yellow, and black-white.

2. Psychophysical Roots of Opponent Color Theory
-Red and green are opposed to each other
-Unique hues: red, yellow, blue and green; can only be described with one color

3. Afterimages
-Negative Afterimages: used to see opponent colors in action
a. If look at one color, a subsequent achromatic region will appear to take on a color opposite to the original color
b. The first colored stimulus is called the adapting stimulus
c. The illusory color is the negative afterimage
-Neutral point: the point at which an opponent color mechanism is outputting no signal

4. Color in the Visual Cortex
-In opponent-process, found three opponency: red-green, blue-yellow, and black-white
-Mismatch between color perception and responses of LGN cells: LGN is not the end of color processing
-Axons go to visual cortex from LGN; LGN contains cells whose opponency is based on adding and subtracting the outputs of cones
-Achromatopsia: loss of color vision

IV) Does Everyone See Colors the Same Way?
1. Does Everyone See Colors the Same Way? - Yes
-Standard measures of color vision is same as others

2. No
-Color blindness: genes don't encode one or more of the three cone photopigments
a. Genes coding M and L-cone photopigments are in the X chromosome
b. Can still see color but 2D
-Deuteranope: has no M-cones; classify 560 and 610 lights as the same
-Protanope: no L-cones
-Tritanope: no S-cones
-Color anomalous: have three cone photopigments, but two is very similar
-Cone monochromat: one type of cone in retina; world is a shade of gray
-Agnosia: can see but fail to recognize
-Anomia: can recognize but fail to name

3. Maybe
-Cultural relativism: determined in part by the cultural environment.
-Color perception is not much influenced by culture and language

V) From the Color of Lights to a World of Color
-Unrelated colors: a color that can be experienced in isolation
-Related colors: a color, such as brown or gray, that is seen only in relation to other colors.

1. Color Constancy
-Illuminant: the light that illuminates a surface; not all illuminants are the same
-Color constancy: the tendency for colors to appear relatively unchanged in spite of substantial changes in the illuminant.

2. Problem witht he Illuminant
-Reflectance: percentage of light that is reflected
-What we perceive is the combination of illuminant and reflectance
-Mondrians: stimuli that are strongly influenced by the environment; the presence of other colors allowed the colors of the test patches to remain constant over a change of illumination

3. Physical Constraints Make Constancy Possible
-Assumptions of physics of the world influence color perception

Summary
1. Probably the most important fact to know about color vision is that lights and surfaces look colored because a particular distribution of wavelengths of light is being analyzed by a particular visual system. Color is a psychophysical phenomenon, not a physical phenomenon. Many animal species have some form of color vision. It seems to be important for identifying possible mates, possible rivals, and good things to eat. Color vision has evolved several time in several different ways in the animal kingdom.
2. Rod photoreceptors are sensitive to low (scotopic) light levels. There is only one type of rod photoreceptor. It yields one "number" for each location in the visual field. Rods can support only a one0dimensional representation of color from dark to light. Thus, scotopic vision is achromatic vision.
3. There are three types of cone photoreceptors with different sensitivities to the wavelength of light. Cones operate at brighter light levels, producing three "numbers" at each location; the pattern of activity over the different cone types defines the color.
4. If two regions of an image produce the same response in the three cones types, they will look identical; that is, they will be metamers. And they will look identical even if the physical wavelengths coming from the two regions are different.
5. In additive color mixture, two or more lights are mixed. Adding a light that looks blue to a light that looks yellow will produce a light that looks white. In subtractive color mixture, paints or other pigments that absorb some wavelengths and reflect others are mixed. Mixing a typical blue paint an a typical yellow paint will subtract most long and short wavelengths from the light reflected by the mixture, and the result will look green.
6. Information from the three cones is combined to form three opponent processes. Cones sensitive to long wavelengths (L-cones) are pitted against medium-wavelength (M) cones to create an L-M process that is roughly sensitive to the redness or greenness of a region. L+M cones are pitted against short-wavelength (S) cones to create a process roughly sensitive to the blueness or yellowness of a region. The third process is sensitive to the overall brightness of a region.
7. Color blindness is typically caused by the congenital absence or abnormality of one cone type - usually the L or M cones, usually in males. Most color-blind individuals are not blind to differences in wavelength. Rather their color perception is based on the outputs of two cone types instead of the normal three.
8. The goal of color vision is to describe the properties of surfaces in the world and to ignore the color of the light shining on the surface. Mechanisms of color constancy use implicit knowledge about the world to correct for the influence of different illuminants and to keep the apple looking red under a wider range of conditions.