Color Vision

what allows us to identify and localize objects?

- chromatic contrast as well as luminance contrast

why do various animals employ color vision and body coloration?

- to increase sexual attraction/avoid predators/find prey

what is color dependent on for perception?

- Color is a wavelength-dependent perception, as the wavelength of a stimulus changes, the color of the stimulus also changes

what pathway is involved in processing color?

- parvocellular pathway

what pathway process color vision?

- ventral pathway (IT)

what photoreceptor is color coded by?

- cones

what is the trichromatic theory?

- theory of color vision that proposes three types of cones: red, blue, and green
- young suggested that color info is coded by a limited number of cone types, perhaps 3, and that the relative activity of these different cone types encode color

what is monochromacy?

- a person with only one photopigment has no color discriminated
- A person with one photopigment cannot make wavelength-based discriminations because different wavelengths can always be made to appear identical by adjusting their intensities
- individual is totally colorblind, unable to distinguish objects on the basis of wavelength alone

why can a person who has monochromic vision not see color?

- A person with one photopigment cannot make wavelength-based discriminations because different wavelengths can always be made to appear identical by adjusting their intensities

where do rods peak in patients with rod monochromacy?

- Rod monochromacy, where there are only rods, the rhodopsin contained in rods peaks at 507 nm.
• A patient with rod monochromacy, the 505-nm patch will appear bright, the 570-nm patch less bright, and the 600-nm patch will appear dim

why can patients who are monochromatic able to label colors correctly?

- A patient with monochromacy uses brightness to label colors.
• Monochromatic person cannot make wavelength based discrimination

why are individuals with dichromacy able to distinguish different colors?

- they are distinguishable solely because they are different wavelengths

what are metamers?

- any pair of stimuli that are perceived as identical in spite of physical differences

do dichromats have color discrimination?

- Dichromats have some degree of color discrimination, yet it is limited.
- A person with dichromatic vision is able to adjust the relative intensities of these wavelengths such that the two patches appear identical

what type of color vision do most humans have?

- trichromacy

What is trichromacy?

- Human with normal trichomacy have three cones of different wavelengths: S (short) cone, M (medium) cone, & L (long) cone

do trichromats show wavelength discrimination?

- Individuals with trichromacy show wavelength discrimination that is superior to those with dichromacy, but still limited
- Four wavelengths when divided into two patches, a person with trichromatic vision is able to adjust the relative intensities of these wavelengths such that the two patches appear identical.

what do most direct measurements of the photopigment spectra come from?

- come from electrophysiological recordings of individual photoreceptors and molecular genetics

what does each cone photopigment consist of?

- consist of the chromophore and an opsin

what is chromophore?

- The chromophore, which is identical for all cone photopigments, is retinal, an aldehyde derivative of retinol (vitamin A)

what is the function of a chromophore?

- absorbed light quanta and thus, initiate the procedure which leads to vision

what is an opsin?

- visually inert chain of amino acids interlaced into the disc membranes of the outer segment, that determines the absorption characteristics of the photopigment molecule

what chromosome are opsin genes found for M and L cones?

- For the M- and L-cones, the opsin genes were found on the X chromosome

what chromosomes are opsin genes found for S cones?

- located on chromosome 7

what chromosome are rhodopsin genes found on?

- found of chromosome 3

what does homologous nature of cone photopigment genes to the rhodopsin cone suggest?

- suggest that all four share the same ancestor

how similar are the M and L cones?

- M- and L-cone opsin genes are exceedingly similar to each other, showing 98% homology (i.e., 98% of the DNA sequence is identical),pointing to recent duplication.

how similar are S cones to M and L cones?

- the homology of the S-cone opsin gene to the M- and L-cone opsin genes is 40%

what are the three Grassman's Laws that follow trichromacy theory ?

- additive property
- scalar propery
- associative property

what is the additive property?

- According to the additive property, when the same radiation is added in an identical manner to two metamers, they remain metamers
.• If they were perfectly matched before we added the new light, they are still matched following this addition

whta is the scalar property?

- If the intensities of two metamers are increased (or decreased) by the same amount, they remain metamers

what is the associative property?

- A match will be maintained if one metamer is substituted for another metamer.

what are the three perceptual dimension of color?

- hue
- saturation
- brightness

what is hue?

- perception which is most closely associated with wavelength.
- Most monochromatic stimuli change hue slightly as their intensity is adjusted

what are unique hues?

- three wavelengths—478, 503, and 578 nm—that do not change hue as their intensity is increased are referred to as invariant wavelengths, and the hues associated with them—blue at 478 nm, green at 503nm, and yellow at 578 nm
- these hues look pure in that they do not appear to be mixed with another wavelength

what is desaturation?

- A desaturated color appears as though it has been mixed with white, it does not look bold or full of color

what is saturation?

-A saturated color, in comparison, appears to be full of color, it does not appear washed out like a desaturated color.

what is brightness?

- Brightness sensation closely follows the photopic luminance function
- Under photopic conditions, a 555-nm stimulus is perceived as brighter than other equal energy monochromatic stimuli.

is saturation wavelength dependent?

- The perception of saturation is wavelength-dependent, with a monochromatic stimulus of 570 nm appearing less saturated than a monochromatic stimulus of any other wavelength

what is the equation for colorimetric purity?

what does colorimetric purity show?

- This relationship quantifies the amount of white that has been added to a monochromatic light and, therefore, gives us information regarding the saturation of the sample.
- A monochromatic stimulus, which by definition has no white light added to it, is said to have a colorimetric purity of 1.
- A monochromatic 440 nm stimulus ( p = 1) appears more saturated than an equal combination of 440 nm and white light (p = 0.50)

can humans better discriminate one wavelength from another in certain regions in a spectrum than in others?

- yes

what is the difference in wavelength required so that the left and right side of the field (delta λ) no longer match a measure of?

- measure of wavelength discrimination

where is color discrimination best?

- best where the slopes of the cone absorption spectra change most rapidly with respect to each other
- A stimulus of 490 nm, for example, may be distinguishable from 495nm. But, a stimulus of 410 nm may be the same hue as 415 nm because hue discrimination is relatively poor in the short-wavelength region of the spectrum

What is the Bezold-Brucke shift?

- Most monochromatic stimuli slightly change hue as their intensity is adjusted.
• λc = control wavelength, λv = intensity slowly increased
• As the intensity of most monochromatic stimuli is increased (or decreased), the hue changes and, consequently, the wavelength must be changed to maintain a constant hue

what is an example of the Bezold-Brucke shift?

- When the intensity of 550 nm (greenish yellow)wavelength is increased, it appears to be of a longer wavelength (i.e., it appears more yellowish).
• Therefore, to maintain the initial hue appearance(greenish yellow), it is necessary to reduce its wavelength.
• The line that starts at 550 nm and tilts toward shorter wavelengths as the intensity increases.
• All stimuli that fall on this line, referred to as a hue contour line, have the same hue (greenish yellow).

what lines do not change their hue as intensity is increased?

- Nontilted hue contour lines, three wavelengths do not change hue as their intensity is increased
• The wavelengths, 478, 503, and 578 nm, are referred to as invariant wavelengths or invariant points.
• The hues associated with these wavelengths—blue at 478 nm, green at 503 nm, and yellow at 578 nm—are called unique hues

what is color constancy?

- refers to the approximately constant color appearance of objects as lighting conditions change

what does color constancy help us do?

- help in identifying object as light condition vary

is color constancy absolute?

- no
- As lighting conditions change, there are subtle, but important,changes in color appearance
.• For example, when viewing a natural landscape, for instance, lighting conditions provide a cue that allows us to gauge the time of day.

where did evidence of trichromatic theory come from?

- comes from color matchingand color mixing studies.
• Young and Helmholtz carried out experiments in which subjects adjusted the relative intensity of 1, 2, or 3 light sources of different wavelengths, so that the resulting mixture field matched an adjacent test field composed of a single wavelength.
• Subjects with normal color vision needed three different wavelengths (i.e., primaries) to match any other wavelength in
the visible spectrum
- led to hypothesis that normal color vision is based on the activity of three types of receptors, each with a different peak sensitivity

can several color perceptions be explained with trichromacy theory alone?

- Several color perception phenomenon cannot be explained by the trichromatic theory alone.
• For example, it cannot account for the complementary afterimages in which the extended inspection of one color will lead to the subsequent perception of its complementary color.
• Complementary afterimages are not explained by trichromatic theory, it is better explained by the opponent-process theory

what did Hering observe about after images?

- observed that a chromatic stimulus elicits an after imageof a complementary color.
• A red stimulus, for example, elicits a green afterimage, and a blue stimulus elicits a yellow afterimage

what is color processed by?

- processed by bipolar hue channels referred to as the red-green and blue-yellow channels.

how many signals can a channel send that it capable of coding?

- Channel can signal only one of the two attributes it is capable of coding.
• The red-green channel, for instance, is capable of signaling only red or green, but not both red and green simultaneously.
• The same is true of the blue-yellow channel—only blueness or yellowness is encoded, not both simultaneously
- Hering also hypothesized that brightness is coded by a separate white-black channel.

what theory best explains complementary afterimages?

- better explained by the opponent-process theory

what occurs when one member of the color pair is 'fatigued'?

- When one member of the color pair is "fatigued" by extended inspection, inhibition of its corresponding pair member is reduced.
- This increases the relative activity level of the unfatigued pair member and results in its color being perceived

what does trichromacy theory explain?

- explains color vision phenomena at the photoreceptor level

What does the opponent process theory explain?

- explains color vision phenomena that result from the way in which photoreceptors are interconnected neurally.

what is color opponency?

- Cell responds to one portion of the spectrum with excitation and another portion with inhibition, it is referred to as a color-opponent, or spectrally opponent, neuron

what is non-color opponent not inhibited by?

- Non-color opponent is not inhibited by any wavelength and responds to all spectral stimuli with excitation.
• This neuron does not have color-coding capabilities; it is monochromatic.

does trichromacy theory address the manner in which info is encoded by the nervous system?

- no

what plays a critical role in coding color information?

- opponent processing

where are color opponent neurons present in and what does it reveal?

- The presence of color opponent neurons in the retina and LGN reveals that receptoral information (trichromacy) is encoded in an opponent fashion at postreceptoral levels

how are three classed of cones arranged to produce spectrally opponent neurons?

- The three classes of cones are "wired" together to produce spectrally opponent neurons

when does postreceptoral antagonism first occur?

- This postreceptoral antagonism first occurs very early in the retina, at the level of the bipolar cells in primates

in what fashion is the trichromatic signal encoded?

- Human color vision is trichromatic, with the trichromatic signal encoded in an opponent fashion

what is hue information encoded by?

- encoded by color opponent (red-green parvo and blue-yellow konio) neurons

what is brightness information encoded by?

- t is not clear whether brightness information is encoded by these neurons or by a separate class of noncolor opponent (magno) cells

what does munsell color appearance system allow us to do ?

- allows us to describe colors with a great deal of specificity along three physical dimensions: hue, chroma, and value

what is hue related to?

- related to the wavelength of the stimulus.

what is chroma related to?

• Chroma is related to colorimetric purity and related to the saturation of the color sample

what does value refer to?

- Value refers to the reflectance of the sample and is related to brightness

what is the munsell color appearance system?

- The Munsell system can be conceptualized as a cylinder containing various color samples.
• Hue varies continuously along the perimeter of the cylinder, with the primary hues being red, yellow, green, blue, and purple, and these are further subdivided into intermediaries.
• Chroma is measured as a radius from the center of the cylinder and ranges from 1 to 14, with 14 denoting the highest purity.
• Value increases along the vertical dimension, with 0 indicating the least reflectance and 10 the most.
• The Munsell system is laid out in a book, with each page containing a number of samples (Munsell Chips)

what is the CIE color specification system?

- a color space model created by the International Commission on Illumination known as the Commission Internationale de l'Elcairage (CIE).
• It is also called as the CIE XYZ color space
- A color is specified by the relative amounts of three primaries, which when mixed together, produce the color

what is CIE color specification system based off of?

- based on the trichromacy of vision

are CIE primaries real?

- CIE primaries are not real, and they represent a mathematical transformation of real primaries, and this transformation is performed to avoid specifying negative quantities of primaries

what is tristimulus value in CIE?

- The quantity of each primary required for a match is referred to as the tristimulus value
(slide 34)

how is chromaticity diagram constructed in CIE?

- constructed by converting the tristimulus values to relative units, referred to as chromaticity coordinates
- The following equations show the relationship between the tristimulus values and the chromaticity coordinates

how are spectral hues arranged in CIE?

- arranged along the arc of the perimeter of the diagram, which is referred to as the spectral locus

what is CIE used for?

- useful for calculating color mixtures

what is excitation purity?

- The relative distance of the sample along the line that connects W to the perimeter of the CIE diagram is a measure of its colorimetric purity, and is referred to as the excitation purity.

what color is produced when complimentary colors are mixed together?

- white

how would we determine complimentary color using CIE?

- To use the CIE diagram to determine the complement for a given wavelength, simply connect the wavelength, through white, to the opposite side of the CIE diagram
- When 490 and 600 nm wavelengths when mixed together, produce white.

what is planckian locus?

- Sources of various color temperatures form an arc within the CIE diagram, referred to as the Planckian locus.
• The samples that fall on this locus, including the various standard illuminants, can be considered to be variants of white
.• An illuminant with a lower color temperature (e.g.,illuminant A) appears reddish white, while an illuminant with a higher color temperature (e.g.,illuminant C) appears bluish white.

what are the 4 characteristics of CIE color Specification System?

1. The CIE diagram specifies the relative amounts of primaries necessary to matcha color sample.
2. The CIE primaries do not really exist—they are imaginary.
3. The CIE system uses imaginary primaries to avoid the use of negative quantities.
4. Any set of color matching functions, real or imaginary, can be converted to another set of real or imaginary functions.