Color Vision I Flashcards
The ability to discriminate colors
Color vision
Evolution of color vision
May have evoked in primates by conferring a competitive advantage to monkeys in foraging for edible tropica fruits and leaves
What is chromatic contrast good for
Allows us to see objects that would otherwise be indistinguishable from the background in which they exist
-greatly adds to our visual capabilities
Trichromatic theory
- Thomas young
- hypothesized that color information is encoded by a limited number of cones, perhaps 3
- the relative activities of these differnt cones encode color
- referred to as the trichromatic theory
- it’s initial support came from psychophysical color matching experiments
- more plausible theory than thousands of cones
- does basis of modern color vision science
A person with only one photopigment
Monochromacy
-no color discrimination
Principle of univariance
Photopigment molecule absorbs a light quantum, it does not encode the wavelength of that quantum, all information regarding its wavelength is lost.
In monochromacity, what is the probability of Ya being absorbed
0.25
In monochromaicity, what is the probability of Yb being absorbed
0.50
How does a monchromat tell difference in color
Ya and Yb are perceived as being differnt brightnesses, and they probably interpret this as a difference in color
How would a monchromat not be able to distinguish Ya and Yb?
Double the intensity of the patch with Ya such that it now emits 200 quanta, but keep Yb at the same intensity
- now they both result in 50 quintal absorption’s for each wavelength
- both produce the same effect on the visual system
- indistinguishable
People with monochromatic Vision are unable to make distinctions based on
Wavelength
How are monochromats able to correctly label colors under certain conditions?
We show three patches of light to a person who only has rods (a person with rod monchromacy)
- 505, 570, and 620nm, each of which emits 5000 quanta
- tell the subject that one patch is green, one is yellow, and one red
- rhodopsin’s peak absorption is 507nm
- patient reports the 505 is bright, 570 is less bright, and the 620 patch is dim
- ask the subject to assign green, yellow, and red color labels to the patches of light, he or she might be able to do so correctly
How does someone with monochromatic vision label colors
Use brightness to label colors
- the person has noticed that other people have labeled dim objects red, and bright objects green, San has learned to label colors on the basis of brightness
- when confronted with natural objects, such as apples and bananas, the tanks of color labeling may be easier by the presence of other cues, such as the shape of the fruit
Does someone with rod monochromacy perceive colors?
Not easy tp answer
-we know that people with monochromatic vision can be fooled if we adjust the intensities of stimuli, they do not have the ability to distinguish stimuli on the basis of the wavelength alone
Dichromacy curves
- the person has two photopigment, M and L
- each peak at different wavelengths
- have overlapping absorption spectra throughout much of the spectrum
Number of quanta absorbed for M at Ya (dichromat)
60%
Number of quanta absorbed for L in Ya (dichromat)
20%
Number of quanta absorbed in Yb for M (dichromat)
40
Number of quanta absorbed in Yb for L
60
In dichromacy, if the individual is shown two patches of light, one consisting of Ya and other Yb, the subject is asked to adjust the intensity of Yb so that the two patches appear identical
If capable of matching these two wavelengths, the person does not have the ability to make wavelength-based discriminations
-if the person is unable to match these two patches of light, he or she possess the ability to make discrimination based purely on wavelength
It is possible for a dichromat to match Ya and Yb intensities?
No
Why can a dichromat not match intensities of Ya and Yb?
- there is no intensity at which the number of quanta absorbed by M and L or Yb equals the number absorbed by these same photopigment for Ya
- no matter how the intensities of these two patches of light adjusted, they never have identical effects on the visual system
- they are distinguishable because they are different wavelengths
Can a dichromat adjust the intensities of the Ya/Yc, and Yb such that each patch of light results in the same number of quanta absorption’s by photopigment M and L?
Yes
- each patch produces 200 quanta absorption’s by photopigment M and 300 by L
- from the subjects perspective, these two patches of light, which are physically different, appear identical
Two stimuli that appear identical, but are physically difference
Metamers
How do dichromat a limited amount of color discrimination?
A patch of Ya always looks different than a patch of Yb
- a patch of Yb may be matched by the proper combination of Ya and Yc
- given at least three wavelengths divided into two patches, a dichromat is able to adjust th relative intensities of these wavelengths until the patches appear identical
Three difference photopigment with overlapping absorption spectra rather than two
Trichromatic
Mono, di, or trichromacy shows superior wavelength discrimination
Trichroamcy
Given at least four wavelengths divided into two patches, a person with _____vision is able to adjust the relative intensities of these wavelengths such that the two patches appear identical
Trichromatic vision
-the two patches appear identical because they result int he same number of quantal absorption’s by each of the three photopigment, they are metamers
For three wavelengths divided into two patches, a person with ______ can match the patches, while a person with _____ cannot
Dichromacy
Trichromacy
This demonstrates the super color discrimination found in trichromacy
-there are limitations in trichromatic vision
Limitations to trichromacy
If given four wavelengths divided into two patches, a person with trichromatic vision will be abel to match the patches
Each molecule of cone photopigment consists of
The chromophore and an opsin
Cone chromophore
Ie retinal, an aldehyde derivative of retinol (vitamin A)
- identical for all cone pigments
- absorption of a light quantum by the chromophore initiates a series of events leading to vision
Cone opsin
Interlaced into the disc membrane of the outer segment
- determines absorption characterics of photopigment molecule
- visually inert chain of amino acid
- each class of cones has a differnt opsin
M and L opsin genes are found on the ____ chromosome
X
-consistent with the sex linked inheritance of color vision deficiencies in which either the M or L cone photopigment is missing or altered
The S cone photopigment gene is located on
Chromosome 7
The rhodopsin gene is found on
Chromosome 3
The cone photopigment genes and the rhodopsin gene
They are homologous
-suggests that all 4 share the same derivation
Which cone opsins are the the most similar
M and L
-indicative of recent duplication
The homology of the S cone opsin gene to the M and L cone opsin gene is
40%
How many copies of the L and M cone opsin gene does the X chromosome typically have
One copy of the L cone opsin and one or more copies of the M cone opsin gene
Does having more than one copy of the M cone opsin gene affect color matching?
No
-there role is not understood at this time
S cone absorption
426
M cone absorption
530
L cone absorption
Two variants
- one at 552
- one at 557
Three perceptual dimensions
Hue
Saturation
Brightness
Hue
Laypersons term for color
Saturation
Fullness or purity of color
Physical dimension of hue
Wavelength
Saturation physical dimension
Both wavelength and the amount of white in the mixture
Brightness physical; dimensions
Radiance
Perception that is most closely assocauted with wavelength
Hue
- 540=green
- 570=hue of yellow
Most monochromatic stimuli change _____ slightly as their intensity is adjusted
Hue
- there are however three wavelengths that do not change hue as their intensity is increased
- invariant wavelengths
Invariant wavelgnthns
Three wavelengths that do not change hue as their intensity is increased
- blue at 478
- green at 503
- yellow ar 578
Hues associated with invariant waverlgntsh are called
Unique hues
Invariant blue
478
Invariant green
503
Invariant yellow
578
How can we predict effect of increased intensity on hue
Stimuli with a wavelength that is shorter than unique green appear more blue as their intensity is increased
- stimuli longer than 503 appear more yellow as their intensity is increased
- a possible physiological basis for this phenomenon is provided by opponent color theory
Desaturated color
Pastel, appears to be mixed with white and is not as bold
Saturated color
Appears full in color
Monochromatic stimuli are more _____ than stimuli that are mixed with white
Saturated
Saurtation is _____ dependent
Wavelength
Brightness sensation closely follows the _________ function
Photopicluminance function
Under photopic conditions, a ______ stimulus is perceived as brighter than other equal energy monochromatic stimuli
555
Bipartite
Split field
Regions of best wavelgnth discrimination are at about ____and ____
495 and 590
-two wavelengths are differnt hues even if their wavelengths are just a few nanometers differnt
Where is hue discrimination poor
In the short wavelgnth spectrum
What is the wavelgnth discrimination curve sometime called
W curve of color discrimination
Why do wavelgnth discrimination thresholds vary across the spectrum
It may be color discrimination is best where the slopes of the cone absorption spectra change most rapidly with respect to each other
Color constancy
Whether viewed indoors under incandescent or fluorescent lights or outdoors in natural sunlight, the shirts color appear almost the same
Assist us in IDing objects as lighting conditions vary
Color constancy
What is color constancy advantageous for
For animals gather fruit in different lightings
Physiological basis for color constancy
Dunno
Is color constancy absolute
No
As lighting conditions change, these are subtle, but important, changes in color appearance
Opponent color theory
Red and green are not seen simultaneously, colors are either red or green, not red-green
-same for blue and yellow
Chtormatic stimulus elicits an after image of a complementary color
_red=green afterimage, blue=yelllow afterimage)
Opponent color theory conclusions
Color is processed by bipolar hie channels referred to as the red-green and blue-yellow channels
-by bipolar, it is meant that at any given instant, the channel can signal only one of the two attributes it is capable of coding
-
Electrpphysiological evidence for color opponency
- discovered horizontal cells in the fish retina that are excited by certain wavelengths of light and inhibited by others
- a form of color opponency
- horizontal cells are graded
- AP in the LGN, clamp next to cells
Once cell is isolated in color opponency
The neurons response, as measured by its frequency of APs, is plotted as a function of wavelength
Cell has color coding capabilities (electrophysiological evidnce for color opponency)
If the neuron is excited by a stimulus, we know the stimulus must be a long wavelgnth, reverse for short wavelgnth
Since the cell repsonds to one portion of the spectrum with excitation and another portion with inhibition, it is referred to as
Color opponent
Non-color opponent cells
Not inhibited by any wavelgnth and responds to all spectral stimuli with excitation
- does not have color coding capability
- 55 peak
Tells us that three classes of cones are sufficiency to explain color matching data
Trichromacy
Reveals that receptoral information is encoded in an opponent fashion at postreceptoral levels
Presence of color opponent neurons in the retina and LGN
L and M cones popsicle each other to produce
L-M opponentcells
S cones are opposed by an addition of L- and M-cones to produce
S-(L+M) opponent cells
Re-green cells found in
Fovea
Parvo
Red-green
Konio
Yellow blue
Magno
Non-color opponency
International authority on light, illumination, color, and color spaces
CIE
Primate color vision
Trichromatic, with the trichromatic signal encoded in an opponent fashion
- may occur very early in the visual system, at the level fo the bipolar cells in the primate
- established that hue information is encoded by color opponent neurons
CIE color system is based on
Trichromacy of vision
A color is specified by the relative amounts of
Three primaries, which when mixed together, preduce the color
Calculating a color
Coordinating Z is calculated by subtracting the sum of x and y from 1
X, y, z all add up to y
What is the chromaticity CIE diagram is useful for what
Determining the result of mixing different wavelgnth stimuli together
What is the result of mixing together equal amounts of 510 and 560nm?
530nm
-draw line connecting 560 and 510, draw a line from the W across the midway point of that line and then up and that gives you your wavelength
Excitation purity
Tells if its closer to white (lower) or closer to the color (higher up)
A/(a+b)
How to find the wavelgnth for a colors when you mix one part of a color and 3 parts of another color
Draw the line 3/4 of the way closer to the color with 3 parts from the W point.
Complimentary colors, when mixed together, give you
White
How do you find complement color on the graph?
Just draw a lien from the given wavelgnth through white onto the other side
The CIE diagram specifies the relative amount of primaries necessary to
Match a color sample
CIE primaries
Are imaginary
The CIE system uses imaginary primaries to
Avoid the use of negative quantities
Any set of color-matching functions, real or imaginary, can be converted to
Another set of real or imaginary functions
Color is perceived along the perceptual dimensions of
Hue, saturation, and brightness
