Anomalies of Color Vision

Color Vision

the ability to discriminate light solely based upon wavelength, independent of intesnity

The perceived color of an object is influenced by __

a vast number of parameters

• Photometric: Dominant wavelength

• Psychological: __

hue

• Photometric: Dominant wavelength

• Psychological: __

brightness

• Photometric: Dominant wavelength

• Psychological: __

saturation

Trichromatic Theory of Color vision

any given color can be matched by a suitable mixture of three ‘primary’ colors termed trichromacy

Evidence for trichromacy comes from

early experiments color mixture

Who attributed to the trichromatic theory of color vision?

Young and Helmholtz

Color opponent theory

the three cone receptors do not send independent and separate signals to the brain, instead their outputs are compared in two color opponent and non color opponent achromatic channel

Who attributed to the color opponent theory

physiologist Hering

Color vision is now explained by a combination of both

the Young-Hemholtz trichromatic and Hering Opponent Color theories

We now know that the Trichromatic theory is applicable at the level of the __

photoreceptors

We now know that the color opponent theory applies to the to the __

subsequent stages (beyond the level of photoreceptors)

Normal color vision can be defined as the ability of a subject to make discriminations based solely on ___ without factoring in the light ___

wavelength composition; intensity

The photometric equivalent of saturation is __

purity

The psychological equivalent of wavelength is _

hue

As you add more white light to a color it becomes __

desaturated

The theory that there are three cone photoreceptors that have a peak sensitivity to different wavelengths in the visible spectrum is referred to as the __ theory

trichromatic

The blue-yellow color opponent channel compares the output of the S-cones with __ cones

combined output of L+M

The Principle of Univariance: A single photoreceptor by itself is __

color blind

The Principle of Univariance: Once a quantum of light has been absorbed by a photoreceptor all information pertaining to its __ is lost

wavelength

The Principle of Univariance: Thus, it is possible to match the response of a photoreceptor to a light of lower intensity at a preferred wavelength to a light of higher intensity at a __

less efficient wavelength

Most of us if tested for color vision will show __

small deviations from the so-called norm

These deviations from the norm arise from differences in the __

transmission of ocular media, variations in the photoreceptor mosaic and/or small differences in the cone photopigments

However, in some patients the response to color is significantly different from the norm and they fall clearly outside the __

normal limits

Color vision deficiencies can be classified as

congenital (genetic or inherited) or acquired (secondary to disease)

Approximately __ of the general population has a congenital color anomaly

4.5%

About __ of males have a congenital color defect

8%

About __ of females have a congenital color defect

0.5%

Protan

(Red-defective) anomaly with Erythrolabe or L cone

Deutan

(Green-defective) anomaly with Chlorolabe or M cone

Tritan

(Blue-defective) anomaly with Cyanolabe or S cone

The prefix Proto means

primary

The prefix Deuto means

secondary

The prefix Trito means

tertiary

People with normal color vision are referred to as __

Trichromats

Trichromats have __ functional cone types

three

People missing one of the three cone types are termed __

Dichromats

Dichromatism is considered a more absolute form of __

color blindness

Depending on the class of cone missing, Dichromats are classified as

protanopes, deuteranopes, and tritanopes

Protanopes

missing L-cone

Deutranopes

missing M-cone

Tritanopes

missing S-cones

Note that the suffix for Dichromats is

-opia or -ope

__ have all three cone types but there is a deficiency or anomaly in one of the photopigments

Anomalous trichromats

Note that the suffix for anomalous trichromats is

-anomaly or - anomalous

Example: An anomalous trichromat with a deficiency in the L-cone is said to have __

Protanomaly, or called a protanomalous trichromat

Percentage of Deuteranomaly for males

5%

Congenital Red-Green color vision defects are prevalent in __ males and __ females

8%, 0.5%

A patient with three functional cones, has a deficiency in the L cone. Name the color vision defect -

protanomaly or Protanomalous trichromat

A patient is entirely missing the M-cone. Name the color vision defect

deuteranopia

A patient who is either missing or has a deficiency in the S-cone is said to have a __ defect

tritan

The most prevalent congenital color vision anomaly in the male population is __

deuteranomaly or deuteranomalous trichromacy

There are differences in the processing of __ by color anomalous subjects in comparison to trichromats

chromatic information

There are differences in the processing of chromatic information by color anomalous subjects in comparison to trichromats, these differences are seen in the following aspects

spectral sensitivity, wavelength discrimination, confusion lines, saturation

Spectral Sensitivity: The perceived brightness of colors depends upon the __

spectral sensitivity of a subject

Spectral Sensitivity: Given an equal energy spectrum, those wavelengths to which a subject is more sensitive, appear __

brighter

Spectral Sensitivity: Under photopic conditions spectral sensitivity can be measured by presenting a __, with a standard wavelength in one half and a test wavelength in the other half

bipartite field

Spectral Sensitivity: The subject is required to match the __ of the two halves

brightness

Peak photopic sensitivity at __

555nm

Luminance function - V(lambda) function

Luminous efficiency plotted as a function of wavelength - V(lambda) function represents the addition of the M and L cone inputs

The V(lambda) function in represents the addition of __

M and L cone inputs

The V(lambda) function in Dichromats (think, Deuteranopes and Protanopes), the absence of one of these two types of cones causes a __ in the function

shift

In Anomalous Trichromats the V(lambda) function will again show a displacement but ___

to a lesser degree

The V(lambda) function of a protanope (missing L-cone) shows a marked displacement towards the __ wavelengths

shorter

In the V(lambda) function of a Protanope, the peak of the curve is closer to __

540nm

In the V(lambda) function of a Protanope, this displacement leads the protanopic subject to have very poor sensitivity to __

longer wavelengths

(V(lambda) function of a Protanope) In the real world, this implies that the protanopic subject either has a great deal of difficulty or is unable to perceive __ (brake lights, tail lights etc.)

red objects

The V(lambda) function of a Deuteranope shows a displacement towards the __ wavelengths

longer

In the V(lambda) function of a Deuteranope, the peak of the curve shifts to __

560nm

The V(lambda) function of a Deuteranope - the relatively small shift in the curve implies that the spectral sensitivity of Deuteranopes is very similar to __

Trichromats

V(lambda) function in Anomalous Trichromats: Anomalous trichromats also exhibit a displacement in their V(lambda) functions, although to a __

lesser extent

V(lambda) function in Anomalous Trichromats: Protanomalous trichromats show a displacement of the curve towards the __ wavelengths (although to a lesser degree in comparison to protanopes)

shorter

V(lambda) function in Anomalous Trichromats: Deutranomalous trichromats show a very small displacement of the function towards the __ wavelengths

longer

A __, missing M-cone shows two peaks corresponding to the S-cone and L-cone

deuteranope

A __, missing L-cone shows two peaks corresponding to the S-cone and M-cone

protanope

The __ is a measure of hue discrimination

wavelength discrimination function

The wavelength discrimination function is measured by presenting a bipartite field with both sides illuminated by __ light of the same wavelength

monochromatic

When the luminance is balanced in the wavelength discrimination function the wavelength of one side of the field is varied until the subject reports a __

difference in appearance

The classic “W” shaped due discrimination function. The minimum change in wavelength is measured for each wavelength. Discrimination is best for two wavelengths at __ and _

490nm and 590nm

Wavelength discrimination: Sensitivity to wavelength change is low at both ends of the spectrum where only one __ mechanism is operating

cone

Wavelength discrimination: It is highest near 490nm and 590nm, where at least two cone types are well __

stimulated

Both Protanopes and Deuternaopes show relatively good wavelength discrimination in the region of __

490nm

This ability for both Protanopes and Deuteranopes show relatively good wavelength discrimination in the region of 490nm diminishes as we approach the __ wavelengths

longer

In the region beyond 545nm Red-Green dichromats are unable to appreciate changes in wavelengths as they are essentially __ in this region

color blind ?

In the region beyond 545nm: Red-Green dichromats have just __ functional cone beyond 545nm

one

In the region beyond 545nm: The Deuteranope has the __ cone

L

In the region beyond 545nm: The Protanope has the __ cone

M

In the region beyond 545nm: Therefore, two stimuli both greater than 545nm can be told apart by red-green dichromats only on the basis of the __ of the stimuli but not based upon their hue

luminance (brightness)

Tritanopes show good wavelength discrimination in the __-wavelength region because they have two functional cones (M&L) in this region of the spectrum

long

Tritanopes show very poor discrimination in the region roughly between __ because the spectral sensitivities of the M & L cones is nearly identical in this region

450nm to 475nm

Define spectral sensitivity

It tells you how “bright” different wavelengths in the visible spectrum appear to a subject

The photopic spectral sensitivity (Vlambda) function departs significantly from the normal in this class of color defective individuals __

protans (protanopes and protanomalous trichromats)

The V(lambda) function of the protanopes shifts towards the __ wavelengths, this causes the __ wavelengths, to appear dark

shorter; longer (red-end)

If you had just one cone class, you would perceive different wavelengths as __

same hue but varying in brightness

Protanopes and Deuteranopes operate with a single cone class beyond __ nm. This is because the ___ ends here

540-545; S-cone spectral sensitivity

Since dichromats have limited wavelength discrimination, they __

confuse certain colors

Protanopes, deuteranopes and tritanopes each have their characteristic pattern of __

color confusions

This confusion is represented within the __ as color confusion lines

CIE color space

Confusion lines represent the locus of points (on the CIE color space) that cannot be distinguished based on their __

chromaticities