Colour Vision

Reflectance Curve

a plot showing the percentage of light reflected from an object versus wavelength

Blue

Colour perceived by trichromats at short wavelengths of visible light spectrum

Green

Colour perceived by trichromats at medium wavelengths of visible light spectrum

Red

Colour perceived by trichromats at long wavelengths of visible light spectrum

Yellow

Colour perceived by trichromats when medium and long wavelengths of visible light are seen together

White

Colour perceived when long, medium and short wavelengths of visible light are seen together

Spectral sensitivity

The sensitivity of visual receptors to different parts of the visible spectrum

Rhodopsin

A light-sensitive pigment found in the rod cells that is formed by retinal and opsin. Absorbs best at 500nm wavelengths.

Short Wavelength Pigment

Light-sensitive pigment found in the cone cells with a maximum absorption at 419 nm (~420nm)

Medium Wavelength Pigment

Light-sensitive pigment found in the cone cells with a maximum absorption at 531 nm (~530nm)

Long Wavelength Pigment

Light-sensitive pigment found in the cone cells with a maximum absorption at 558 nm (~560nm)

Spectral sensitivity curve

The function relating a subject's sensitivity to light to the wavelength of the light. These indicate that rods and cones are maximally sensitive at 500 nm and 560 nm, respectively.

What is the Opponent-Process Theory?

The theory that opposing retinal processes enable color vision.

How does the Opponent-Process Theory explain color vision?

It explains that some neurons are stimulated by one colour and inhibited by its opposite (red/green, blue/yellow and white/black). The theory refers to the response of neurons connected to the cones further up in the visual system than the retina.

Trichromatic Theory is also known as:

The Young-Helmholtz theory

Additive color mixing

The creation of colors that occurs when lights of different colors are superimposed.

What happens when blue and yellow lights are superimposed in additive color mixing?

It leads to white light.

Subtractive color mixing

The creation of colors that occurs when paints of different colors are mixed together.

How does mixing blue and yellow paints lead to green?

The color perceived is made up only of the wavelengths that are reflected by both pigments.

Brightness

The perception of the intensity of light waves.

Saturation

The perception of the purity of colour. Mediated by the intensity of the dominant wavelength relative to the total visible light spectrum.

Hue

the dimension of colour perception that is determined by the wavelength of light

Who first conducted colour-matching experiments?

James Maxwell

What theory do colour-matching experiments provide behavioral evidence for?

Young-Helmholtz theory

What do participants adjust in colour-matching experiments?

Amounts of three wavelengths in a comparison field to match a test field of one wavelength

What theory states that all colour vision is based on the differential sensitivity of cones to three principal colours?

The Trichromatic Theory

What are the three principal colours referred to by the trichromatic theory of colour vision?

Red, green, and blue

What does the Trichromatic Theory say about cells in the visual system?

It explains the response of cones in the retina to different wavelengths of light

From his experiments, Maxwell concluded that:

Colour vision depends on three receptor mechanisms, each with different spectral sensitivities

What were the findings of the colour matching experiments?

When adjusting 3 wavelengths in the comparison field, any reference colour could be matched. When adjusting 2 wavelengths, only some reference colours could be matched.

Color perception is based on:

The response of the three different types of cones.

Combinations of the responses across all three cone types lead to

perception of all colours

Monochromats

People with only one type of light-sensitive pigment. Usually because they lack functional cones cells in the retina. A very rare hereditary condition.

How do monochromats perceive the world?

They cannot perceive any colour and see in shades of grey meaning they have true colour blindness. They also have poor visual acuity and are sensitive to bright lights.

Dichromats

People with only two types of light-sensitive pigment. Most commonly linked with a genetic defect on the X chromosome. Comes in three major forms.

How do dichromats perceive the world?

See chromatic colours, but cannot distinguish among all colours

Trichromats

People with all three types of light-sensitive pigment.

How do trichromats perceive the world?

Can discriminate among all wavelengths across the visible light spectrum

Who proposed the Opponent-Process Theory of Color Vision?

Ewald Hering

Researchers performing single-cell recordings in the 1950s found that:

Opponent neurons are located in the retina and in brain areas involved in visual processing. Such neurons respond in an excitatory manner to one end of the spectrum, and an inhibitory manner to the other.

Protanopia

A form of red-green dichromatism caused by a lack of the long-wavelength cone pigment. Affects 1% of males and .02% of females.

Deuteranopia

A form of red-green colour dichromatism caused by lack of the medium-wavelength cone pigment. Affects 1% of males and .01% of females.

Tritanopia

A form of dichromatism thought to be caused by a lack of the short-wavelength cone pigment. Extremely rare. Affects .002% of males and .001% of females.

How do people with protanopia perceive colour?

Neutral point occurs at 492nm. See short wavelengths as blue. Above neutral point, see yellow.

How do people with deuteranopia perceive colour?

Neutral point occurs at 498nm. See short wavelengths as blue. Above neutral point, see yellow.

How do people with tritanopia perceive colour?

Neutral point occurs at 570nm. See short wavelengths as blue. Above neutral point, they see red.

Trichromacy

The theory that the color of any light is defined in our visual system by the representation and combination of three primary colours (red, green and blue)

Anomalous trichromats

Are in possession of all three photopigments, but the absorption spectrum of one of the pigments has been shifted to respond to an overlapping part of the wavelength spectrum with another so that they have trouble distinguishing between hues.

Protanomalous trichromat

L cone shifted to shorter wavelength - red, orange, green look similar (greener) reds may appear dimmer

Deuteranomalous trichromat

M cone shifted to longer wavelengths - red, orange, green look similar (redder)

Principle of Univariance

The principle that absorption of photons of light in any cone cell results in the same response regardless of the wavelength of the light