W7 - Colour perception ✅

Question 1

What is colour good for?

Answer 1

1. Scene Segmentation: Variations in colour often signal object boundaries
2. Camouflage: Animals disguise themselves by colour markings
3. Perceptual Organisation: Our visual system uses colour to group elements in a scene
4. Strong evolutionary force (depends on species)
5. Food identification (e.g. ripe fruits, correct leaves, harmless or poisonous berries/animals)

Question 2

What is meant by colour?

Answer 2

The electromagnetic spectrum covers 400nm of visible light band.

Different frequencies have different hues, from red -> violet

Humans can see between 390-750nm

Question 3

How can objects have colours?

Answer 3

• Different objects absorb and reflect different wavelengths of light -> ‘colour’
• The colour also depends on the light source

Question 4

What is meant by: hue, intensity and saturation?

Answer 4

• Hue: wavelength of the light reflected
• Intensity: how bright reflected light is
• Saturation: how much white light is mixed in with the pure hue (spectral purity)

Question 5

What are the 2 theories of colour perception?

Answer 5

Trichromatic Theory: 3 cone receptors combined responses account for all colours
- (S)hort-cones: blue
- (M)edium-cones: green
- (L)ong-cones: red
-> Colour you perceive depends on the activity of these cones -> e.g. see red light - more activity in red cone

Opponent Process Theory:
3 opposing process picks out the colours present in nature
1. Red-green
2. Yellow-blue
3. Black-white
-> Explanation for the colour perception of the brain (Lateral Geniculate Nucleus - LGN) and cortical cells
-> send information from retina to LGN -> LGN sends projection to primary visual cortex

Question 6

What is the supporting evidence for Trichromatic Theory?

Answer 6

• Three primary colours combine to produce all possible colours
• Three forms of colour blindness -> evidence for fault in different cones
• A mixture of green and red light produces
  same perception of yellow colour as monochromatic yellow light (metamersim)
• Yellow after-image of blue object (fatigue of blue cones, more activity in red & green cones)

Question 7

What is meant by tapetum lucidum?

Answer 7

Relflective tissues behind the retina to reflect back light that missed the photoreceptors -> allow animals to see better in the dark when there is less light

Question 8

What are the critique for both colour perception theories?

Answer 8

Explanatory gap
Different perception of colours/tastes

Question 9

What is the supporting evidence for Opponent process theory?

Answer 9

1. Non-existence of certain colours (e.g. bluish-yellow)
2. Colour confusions in colour blindness (e.g. red and green)
3. Complementary afterimages (e.g. yellow after-images of blue objects)
4. Colour context effects: colour perception changes based on the accompanying colours/knowledge

Question 10

What is meant by colour blindness?

Answer 10

First described by John Dalton (suffered from it) -> colour deficiency or confusion

2 forms:

1. Anopias: insensitive of L, M or S wavelengths of light (missing a type of cone)
   - Protanopia: L-cone
   - Deuteranopia: M-cone
   - Tritanopia: S-cone
   => all are more common in males (except for tritanopia)
2. Anomalies: Misalignment of L or M in trichromats (distribution or deficiency)
   - Protoanomaly: L-cone (e.g. need more ‘red’ in ‘red-green’ mixture to match ‘yellow’)
   - Deuteranomaly: M-cone (need more ‘green’ in ‘red-green’ mixture to match ‘yellow’)

Question 11

How does different forms of colour blindness supports both colour perception theories?

Answer 11

1. Trichomatic theory: Anopia points to 3 cone types.
2. Opponent process theory: people who have trouble with RED also have trouble with GREEN