Color Perception

Question 1

Primary (physical) qualities

Answer 1

Properties of matter such as size, shape, texture, etc.

Can be perceived by more than one sense.

Have the potential to produce the secondary qualities in our minds.

Question 2

Secondary (mental) qualities

Answer 2

Our mental of representing physical things. Not a property of matter.

Limited to one sense.

Qualia: entirely subjective property of an object

Question 3

Give an example of a qualia

Answer 3

The redness of the red of an object.

Question 4

The Steps in Color Perception

Answer 4

1. Detection: Light wavelengths must be detected first.
2. Discrimination: We have to be able to tell the difference between one wavelength (or a mixture of many) and another.
3. Appearance: We want to assign perceived colours to lights and surfaces in the world and have those perceive colours stable over time, regardless of different lighting conditions.

Question 5

Detection

Answer 5

Step 1 in colour perception.

Detect wavelengths of light.

This detection is done through 3 types of cones:

• S-cones (short wavelengths)
• M-cones (medium wavelengths)
• L-cones (long wavelengths)

Question 6

Discrimination

Answer 6

Step 2 in colour perception

Discrimination refers to be able to tell the difference between some wavelength (or a mixture of wavelengths) and another wavelength.

By the principle of univariance, we know that a single receptor cannot discriminate between some wavelength and another. It is thanks to the 3 types of cones that we can tell the difference. In this case, we consider a combination of reaction across the 3 cones to differentiate wavelengths.

Notice: this combination of activity remains constant across different intensities.

Question 7

Principle of Univariance

Answer 7

The principle that explains that a single cone would not make the difference between a given wavelength and another.

This also means that if we would have only one type of cone, we would only detect one colour.

Question 8

Based on discrimination, what would happen if a person wouldn’t have M-cones?

Answer 8

Since the colours under the M-cones are also under the S-cones and L-cones, the person will still detect them but not as well as a person with all three cones. For example, the colours that are usually under M-cones and L-cones would appear as one colour since it would only be L-cones who would detect them. Therefore, there would be some colour blindness.

Question 9

Color-Anomalous

Answer 9

Commonly known as “colour blindness”, most colour blind can still do discrimination between wavelengths, this discrimination is just different from the norm.

Some color-anomalous cases:

• Deuteranope
• Protanope
• Tritanope
• Cone monochromat

Question 10

Deuteranope

Answer 10

Due to the absence of M-cones.

Question 11

Protanope

Answer 11

Due to the absence of L-cones

Question 12

Tritanope

Answer 12

Due to the absence of S-cones

Question 13

Cone monochromat

Answer 13

Has only one cone type.

Question 14

Trichromacy

Answer 14

The theory that the colour of any light is defined in our visual system by the relationships of three numbers, which are the output of three receptor types

Question 15

Who discovered the trichromatic nature of colour?

Answer 15

Thomas Young (1773–1829) and Hermann Von
Helmholtz (1821–1894) independently
discovered the trichromatic nature of colour
perception using a colour-matching technique
developed by James Maxwell (1831–1879).
Three colours were required to match any other
colour. Two were sometimes insufficient.

Question 16

True or False

The combination of the wavelength for green and the wavelength for red from the M-cones and L-cones will give yellow

We can’t differentiate this “combined” wavelength from the “pure” wavelength of the colour yellow.

Answer 16

True

Question 17

Metamers

Answer 17

Different mixtures of wavelengths that look identical.

More generally, a pair of stimuli that are perceived as identical in spite of physical differences.

Question 18

Additive colour mixing

Answer 18

A mixture of lights.

Ex: Blue and yellow make white

Question 19

Subtractive Colour mixing

Answer 19

A mixture of pigments

Ex: Blue and yellow make green

Question 20

Nonspectral hues

Answer 20

Hues that can arise only from a mixture of wavelengths.

Question 21

Who notices that some colours were “legal” and some “illegal”?

Answer 21

Ewald Hering

• We can have bluish-green (cyan), reddish-yellow (orange), or bluish-red (purple).
• We cannot have reddish-green or bluish-yellow.

Question 22

Opponent color theory

Answer 22

The theory that the perception of colour depends on the output of three mechanisms, each of them based in a opponency of two colours.

Ex: red and gree, blue and yellow, etc

Question 23

Hue cancellation experiments

Answer 23

1. Start with a colour
2. Shine a light of some colour until you get some “pure” colour

Example:

• Blueish-green
• Shine red light and change the intensity until there is no more than blue.

Question 24

True or False

If you do hue cancellation over the whole spectrum you will manage to cancel all the hue.

Answer 24

False

If you do hue cancellation over the whole spectrum there are certain colours that can’t be cancelled with red/green or yellow/blue.

Question 25

Unique hue

Answer 25

Any four colours that can be described with one colour term:

red, blue, yellow, green

Question 26

True or False

The absolute level of activity of a cone is informative.

Answer 26

False

The absolute level of activity of a cone is uninformative this is because in general cones are very sensitive to light intensities in a wise range of wavelengths.

Question 27

True or False

The difference in activity between different types of cones is more informative than the absolute level of activity of one type of cones.

Answer 27

True

Example:

M-L or L-M compute red vs green

(M+L)-S or S- (M+L) compute blue vs yellow

M+L+S encodes general brightness

Question 28

Afterimage

Answer 28

A visual image is seen after the stimulus has been removed.

Caused by habituation (chromatic adaptation) of activated cones.

Question 29

Negative Afterimage

Answer 29

Afterimage whose polarity is the opposite of the original stimulus

Question 30

Colour Constancy

Answer 30

The tendency of a surface to appear the same colour under a fairly wide range of illuminants.

To achieve colour constancy we must discount the illuminant and determine what the true colour of the surface is regardless of how it appears.

Question 31

Illuminant

Answer 31

The light that illuminates the surface

Question 32

Discounting the illuminant

Answer 32

Calculating the visual spectrum across the visual field and subtracting from the patterns of cone activity.

Question 33

Deduction

Answer 33

• Intelligent guesses about the illuminant
• Assumptions about the light sources
• Assumptions about surfaces