Color Vision

Trichromatic Theory of Color Vision

Thomas Young, in 1802, proposed a (alternate model) theory of color perception ▪ that required only three kinds of color receptors & ▪ that the relative activities of these different cone types encode color

The genes that code for the opsins of the human photopigments have been identified and isolated with recombinant DNA techniques ▪ For the M- and L-cones, the opsin genes were found on the ___-chromosome.

X-chromosome

▪ The physiological basis for color constancy has not been fully understood (Zeki, 1993). ▪ Color constancy is not _______________. As lighting conditions change, there are subtle, but important, changes in color appearance. ▪ When viewing a natural landscape, for instance, lighting conditions provide a cue that allows us to gauge the time of day

absolute

According to the opponent-color theory, the perception of color can be explained by the relative ___________ of the red-green, blue-yellow, and brightness channels

activity The model correctly predicts the locations of the unique hues, ▪ which are located at the crossover points of the red-green and blue- yellow channels ▪ At these points (e.g., 578 nm), only one of the two channels is active, resulting in the perception of a "pure" hue (e.g., yellow).

Color Vision may have evolved in primates by conferring a competitive ________________ to monkeys in the detection and selection of tropical fruits and leaves

advantage

Because receptors are sensitive to some degree to the entire spectrum, it is misleading to refer to cones as red, green, or blue receptors ▪ why?

all receptors are simply light receptors

The three classes of cones are "wired" together to produce spectrally opponent neurons. ▪ This postreceptoral antagonism first occurs very early in the retina, at the level of the ______________ cells in primates

bipolar cells

Subjective perception of the luminance of the visual stimulus (target)

brightness

Light quanta are absorbed by the ___________________, initiating the series of events leading to vision.

chromophore

Each molecule of cone photopigment consists of the (1) ______________ and an (2) _________.

chromophore; opsin

The gene for the S-cone photopigment is located on chromosome ____, and the gene coding Rhodopsin is found on chromosome ____.

chromosome 7, chromosome 3

Much of support for trichromatic theory comes from psychophysical _____________________ experiments

color matching

The finding of two L-cone photopigments was not surprising because it had been known for some time ▪ that people with normal color vision do not all show the same ___________________ data

color matching

The brain responds to the stimuli that are produced when incoming light reacts with the several types of _________ photoreceptors in the eye.

cone

Early investigators hypothesized that ▪ each color is coded by a ____________ that is uniquely responsive to that color ▪ For example, if a person were looking at fall foliage, ▪ the retinal image of red leaves would stimulate red receptors, ▪ yellow leaves stimulate yellow receptors, and so on for every color

cone Because humans are capable of perceiving thousands of colors, thousands of different cone types would be required at all retinal locations. ▪ As other stimulus characteristics must be encoded, this is not plausible

humans are monochromats when they are fully ________________________ and vision is governed only by the rods, all of which contain the same photopigment

dark-adapted

The perception of saturation is wavelength _____________, ▪ with a monochromatic stimulus of 570 nm (yellow) appearing less saturated than a monochromatic stimulus of any other wavelength.

dependent

one lacking M-cones is a ___________________

deuteranope

Other researchers, however, believe that both hue and brightness are encoded by the same cells. ▪ In this (sometimes referred to as the ________________________) opponent color neurons do double-duty by coding both hue and brightness information

double-duty hypothesis

A stimulus of 540 nm has a hue of __________, a stimulus of 570 nm has a hue of ___________

green; yellow

The cone photopigment genes are _________________ to the rhodopsin gene, suggesting that all four share the same ancestor

homologous

Quantitative data in support of color opponency comes from _____________________ experiments

hue cancellation ▪ For a sample wavelength, the observer adjusts the amount of red or green and blue or yellow that is necessary to cancel the hue sensation produced by the sample wavelength. ▪ By cancel=that hue (red or green and blue or yellow) is added to the sample until it turns white. ▪ This procedure, when repeated for many wavelengths across the visible spectrum, can be used to derive opponent color functions-the red- green and blue-yellow channels (figure slide 77)

Although it is common to use the terms color and hue interchangeably, color is a much broader term that includes _______________________________________________

hue, saturation, and brightness

CIE - French name- Commission internationale de l'éclairage-The International Commission on Illumination ▪ CIE color specification system is based on the trichromacy of vision ▪ A color is specified by the relative amounts of three primaries, which when mixed together, produce color. ▪ CIE primaries are not real but ______________ ▪ Represents mathematical transformation of real primaries ▪ Conversion to avoid specifying negative quantites of primaries

imaginary

If all receptors have the same photopigment, only one primary is needed to match all other lights. Such an observer is described as a ___________________

monochromat

Some investigators believe that the L - M and S - (L+M) cells are devoted exclusively to coding hue and have no role in coding brightness information. ▪ In this view, the encoding of brightness is performed by the _______________________ cells

noncolor opponent

The _________________, especially the lens, absorb many of the shorter wavelength photons before they reach the retina and so these lights seem less effective for vision; ▪ one can allow for this by readjusting the number of photons delivered to the cornea

ocular media

Each class of cones has a different __________

opsin

How long has color vision been studied?

over 2 centuries

Colorimetric purity, a physical property of the stimulus, is given, by the following formula

p = L l / L l + L w Where p = colorimetric purity L l = luminance of the monochromatic light Lw = luminance of the white light that is combined with the monochromatic light

Brightness sensation closely follows the ___________ luminance function.

photopic

Monochromacy - only one kind of ____________________

photoreceptor

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

postreceptoral

The light(s) that are combined to make a match are referred to as ___________, and the number of _______________ corresponds to the number of dimensions.

primaries

A person lacking L-cones is termed a _______________

protanope

According to Hering Color Opponent Theory, color is processed by bipolar hue channels referred to as the __________________ and ___________________ channels.

red-green; blue-yellow ▪ 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. ▪ For example, the red-green channel 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.

To understand trichromatic color vision let us first consider ▪ why a person with only one class of retinal photoreceptors has no color discrimination ▪ This is the case for persons with a hereditary color anomaly called _______________________________.

rod monochromacy

▪ Fig 8.2 B shows the same receptor functions, but now in terms of equal numbers of photons at each wavelength delivered to the cornea. ▪ Sensitivity to the shorter wavelengths seems greatly depressed

slide 37 ▪ This is an artifact caused by the usual procedure in psychophysical studies of color vision in which the light stimulus is measured at the cornea

A human's perception of colors is a ________________ process

subjective ▪ In essence different people may see the same illuminated object or light source in different ways.

Individuals with trichromacy show wavelength discrimination that is _________________ to those with dichromacy, but still limited.

superior

A normal adult has three classes of cones and is a trichromat ▪ to match all possible lights, that individual needs to mix __________ primaries in the additive mixing field.

three

The concepts underlying trichromacy are the same as those for dichromacy, ▪ the only difference being that there are ____________ different photopigments with overlapping absorption spectra rather than two.

three

It is well established that most humans have ____________________ vision.

trichromatic

Grassman's laws of metamers describes the general characteristics of __________________ ▪ The three laws summarize some basic principles of metameric color matching

trichromatic vision

The RGB color model is an additive color model in which red, green, and blue light are added together in various ways to reproduce a broad array of colors ▪ Figure: Three primaries are adjusted to make a match ▪ Repeated to obtain color matching functions ▪ The quantity of each primary required to make a match is referred as ____________________ ▪ Note the wavelength is negative in 450-550 nm to make a match ▪ The CIE system was designed to overcome this

tristimulus value

one lacking S-cones (a very rare person) is a ________________

tritanope

▪ We know color is closely related to the _____________ of light

wavelength ▪ As we change the wavelength of the stimulus, its color also changes

Hering also hypothesized that brightness is coded by a separate __________________ channel

white-black

Nathans determined the absorption spectra of human photopigments by transfecting tissue culture cells with the genes encoding the photopigments (Merbs and Nathans, 1992). ▪ The S- and M-cone photopigments peaked at ______ and ______ nm, respectively ▪ Two variants of L-cone photopigments were found, with one peaking at ______ nm and the other at ______ nm

▪ 426 and 530 nm ▪ 552 and 557 nm

The most direct measurements of the photopigment absorption spectra come from: These data confirm and extend those obtained through other techniques such as

▪ Electrophysiological recordings of individual photoreceptors and ▪ Molecular genetics ▪ Color Matching Experiments and ▪ Retinal Densitometry

In Color Science, each stimuli (color) is specified along three perceptual dimensions:

▪ Hue ▪ Saturation ▪ Brightness ▪ Attributes are not independent of each other

Why should we study color vision?

▪ It is one of the most important aspects of our sense of vision. ▪ Sometimes we evaluate the color vision of patients to see if they meet occupational requirements ▪ We need a basic knowledge of color vision to diagnose color vision anomalies (color blindness, etc.) ▪ Counseling patients ▪ Color vision testing helps diagnose certain diseases, such as optic neuritis

Things that have contributed to our understanding of color vision:

▪ Psychophysics ▪ Anatomy ▪ Neurophysiology ▪ Molecular Genetics ▪ Clinical Studies

(2) Opsin, a visually inert chain of amino acids interlaced into the disc membranes of the outer segment, that

▪ determines the absorption characteristics of the photopigment molecule.

Modern Models of Color Vision In Summary:

▪ primate color vision is trichromatic, ▪ encoded in an opponent fashion. ▪ This occur very early in the visual system, at the level of the bipolar cells in the primate. ▪ Although it is established that hue information is encoded by color opponent (red-green parvo and blue-yellow konio) neurons, it is not clear whether brightness information is encoded by these neurons or by a separate class of noncolor opponent (magno) cells

The normal human retina has four distinct populations of receptors:

▪ rods and three types of cone whose spectral sensitivities are shown (slide 36)

For wavelengths longer than about ______ nm, however, ▪ the S-cones are so insensitive compared with L- and M-cones that these longer wavelengths effectively stimulate only the latter two types of cones ▪ thus, anyone with normal color vision is a ______________ when the spectrum is limited to middle and long wavelengths

- 520 nm -dichromat

Munsell Color Appearance System Allows us to describe colors with great deal of specificity along three dimensions ▪ ______- Wavelength of the stimulus (perimeter of the cylinder) ▪ Primary Hues- ____________________________________ ▪ ________- Colorimetric Purity or Saturation (radius of the cylinder) ▪ Ranges __________ ▪ _________- Reflectance-related to brightness (Vertical Dimension of the cylinder) ▪ _________ (0=indicating least reflectance)

- Hue - red, yellow, green, blue and purple - Chroma - 1-14 (14 highest purity) - Value - 0 to 10

As discussed in Chapter 2, the retinocortical visual projection is organized along at least three major pathways: the parvo, magno, and konio pathways. ▪ The parvo cells manifest ________________ opponency ▪ whereas konio show ________________ opponency. ▪ Magno cells are _______________ opponent

- red-green - yellow-blue - noncolor (or weakly color opponent).

▪ A cone is isolated and exposed to monochromatic light, and the amount of energy required to elicit a criterion response is determined as a function of _________________ ▪ The reciprocal of this threshold function is the _____________________ of the cone. ▪ The cone spectra obtained using this procedure are very similar to those derived through psychophysical matching experiments.

- wavelength - spectral sensitivity

To most adults, light of ▪ _____ nm looks yellow, ▪ _____ nm looks green, and ▪ _____ nm looks red

-590 -550 -630

-If two primaries are needed the observer is a ______________ -If three are needed the observer is a _________________

-dichromat -trichromat

The two patches appear identical because they result in the same number of _______________________________ by each of the three photopigments. -They are __________________

-quantal absorptions -metamers

Given at least four wavelengths, ▪ divided into two patches ▪ a person with trichromatic vision is able to adjust the ________________________ of these wavelengths such that the two patches appear ________________

-relative intensity -identical

A monochromatic stimulus, which by definition has no white light added to it, is said to have a colorimetric purity of ___

1

In comparison, the homology of the S-cone opsin gene (to the M- and Lcone opsin genes) is ____%.

40%

The wavelengths of maximal sensitivity of the different cone types are approximately ___________, ___________, and __________ nm

430, 530, and 560 nm

Under photopic conditions, a ________-nm stimulus is perceived as brighter than other equal energy monochromatic stimuli

555 nm

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

98%

Change in Hue as Intensity is changed (Monochromatic stimuli) ▪ Both sides are initially identical ▪ The right side, remains constant throughout the experiment. ▪ During the experiment, the intensity of lv is slowly increased. ▪ If its hue changes as the intensity increases, its wavelength is adjusted so that it continues to match the hue of lc

Bezold-Brucke Phenomenon Read Slides 66-69

refers to the constant color appearance of objects as lighting conditions change (Land, 1964). ▪ E.g Shirt color in indoor and outdoor lighting, Apricot color

Color Constancy ▪ Color constancy assists us in identifying objects as lighting conditions vary. ▪ The world would be rather confusing if the color properties of objects changed dramatically as the lighting conditions varied.

the ability of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit

Color Vision

________________ color appears as though its been mixed with white ______________ color appears to be full of color

Desaturated Saturated

that perception which is most closely associated with wavelength

Hue

Grassman's first law

I. Additive property of metamers. - If the same wavelength is added to two metamers, they remain metamers

Grassman's second law

II. Scalar property of metamers ▪ If you increase (or decrease) the intensity of two metamers by the same factor, they remain metamers.

Grassman's third law

III. Associative property of metamers ▪ Metamers can be substituted into other metamers, and the color perception will remain the same

L and M cones oppose each other to produce ________________________ (red-green) cells.

L-M opponent

The X-chromosome typically has one copy of the ____-cone opsin gene and one or more copies of the ____-cone opsin gene.

L-cone; M-cone

If presented with two stimuli that differ in wavelength, could a subject with only one photopigment (e.g., rod monochromat) tell them apart, based on wavelength alone?

No

does the presence of multiple copies of the M-cone Opsin gene affect color matching?

No ▪ The role of the extra copies is not understood at this time

▪ Towards the end of 19th century, German physiologist, Ewald Hering, ▪ made important observations regarding the perception of colors ▪ He noted that red and green are not seen simultaneously. The same holds true for blue and yellow. ▪ Hering also observed that a chromatic stimulus elicits an afterimage that is of a complementary color. ▪ For instance, a red stimulus elicits a green afterimage and a blue stimulus elicits a yellow afterimage.

Opponent Color Theory

Once a photopigment absorbs a quantum of light, all information regarding its wavelength is lost

Principal of Univariance

(1) The chromophore, which is identical for all cone photopigments, is _______________, an aldehyde derivative of retinol (Vitamin A).

RETINAL

S-cones are opposed by an addition of L and M-cones to produce ________________________ (Blue-Yellow) cells.

S - (L+M) opponent

The curves are for a situation in which the same numbers of photons at each wavelength is delivered to the retinal receptors; the cones are labeled _______________

S, M, L

given by the ratio of activity of the opponent color to noncolor opponent cells

Saturation