Lecture 5 - colour perception

Question 1

What does colour do for us?

Answer 1

• colour enhances:
  1. perceptual organisation = process by which small elements become grouped perceptually into larger objects
  2. top-down control: when your cognition determines what you perceive
  3. automatic bottom-up processing

Question 2

What is colour - the visible light spectrum?

Answer 2

• different wavelengths of the light spectrum are perceived as different colours
• light does not contain colour
• colour is non veridical (not physically there)

Question 3

What is the perceived colour of short (and medium) wavelengths?

Answer 3

blue

Question 4

What is the perceived colour of medium wavelengths?

Answer 4

green

Question 5

What is the perceived colour of medium and long wavelengths?

Answer 5

yellow

Question 6

What is the perceived colour of long wavelengths?

Answer 6

red

Question 7

What are solid chromatic colours the result of?

Answer 7

• selective reflection = some wavelengths are reflected and others are absorbed

Question 8

What are solid achromatic colours the result of?

Answer 8

• equal reflection = all wavelengths are reflected

Question 9

What are transparent chromatic colours the result of?

Answer 9

• selective transmission = some wavelengths are transmitted and others are absorbed

Question 10

What are transparent achromatic colours the result of?

Answer 10

• equal transmission = all wavelengths are transmitted

Question 11

How is colour described - the 3 colour dimensions?

Answer 11

1. hue (colour value)
   - represented on the colour wheel
   - red, blue, green and yellow = pure/ primary colours
   - 200 hues can be discriminated by our eyes
2. saturation
   - the amount of white added to the colour
   - saturated vs non saturated colour
3. brightness
   - the amount of light reflected
   - the intensity of the light (bright vs dim)

Question 12

How are colours mixed?

Answer 12

• mixing light e.g. in a display
• mixing pigments e.g. in paints

Question 13

Theories of colour vision - trichromatic theory (Young-Helmholtz)?

Answer 13

• Helmholtz carried out colour matching experiments:
  -> task = combine 3 lights so that their colour matches the colour shown on the surface
  -> conclusion = full colour vision is base on the ability to combine 3 different wavelengths
• support for this theory = impaired colour vision in monochromats and dichromats

Question 14

What is monochromatism (colour blindness)?

Answer 14

• no functioning cones
• can only perceive white, grey, black

Question 15

What is dichromatism (colour deficiency)?

Answer 15

• only 2 types of cones
• protanopia = no L cones
• deuteranopia = no M cones
• tritanopia = no S cones

Question 16

Theories of colour vision - opponent process theory (Hering)?

Answer 16

• states that colour vision is caused by opposing responses generated by blue and yellow and by red and green
• looked at afterimages
• adapting to blue generates a yellow afterimage
• adapting to green generates a red afterimage
• adapting to black generates a white afterimage

Question 17

What are the 3 opponent channels?

Answer 17

1. black/ white
2. red/ green
3. blue/ yellow
   - respond in opposite ways to. different wavelengths

Question 18

Physiological evidence for the opponent process theory?

Answer 18

• opponent neurons in v1, v4 and inferotemporal cortex with single-opponent or double-opponent receptive fields
• neurons with single-opponent receptive fields are good at detecting uniform colour surfaces
• neurons with double-opponent receptive fields are good at detecting colour boundaries
• opponent neurons are responsible for perceptual experiences such as afterimages and simultaneous contrast