Task 3: Perceiving colour

Question 1

Colour of light

Answer 1

wavelengths in the visible spectrum

Question 2

Colour of objects

Answer 2

which wavelengths are reflected (for opaque objects) or transmitted (for transparent objects)

Question 3

Differences in pigment underlie

Answer 3

different absorption spectra => colour perception

Question 4

Subtractive colour mixture

Answer 4

mixing paints
- fewer wavelengths are reflected
=> only the WL that is in both colours appears

Question 5

Additive colour mixture

Answer 5

mixing lights
- more wavelengths are reflected
=> white

Question 6

The perceptual dimensions of colour

Answer 6

1. Hue = colour on spectrum (dominant wavelength)
2. Saturation or chroma = more white leads to a desaturated colour (spread of WL)
3. Value or brightness = value decreases as colour become darker (amplitude of WL)

Question 7

Trichromatic theory of colour vision (Young-Helmholtz)

Answer 7

• occurs at the receptor level (retina)
• offers ratio information (receptors respond to different WL)
• does not fully explain colour blindness (dichromacy)
• three colours : green cone, blue cone, red cone

Question 8

Colour matching experiments

Answer 8

• just amount of 3 different wavelengths mixed together in a comparison field until one single WL is perceived
• possible with 3 WL but not 2

Question 9

Metamerism

Answer 9

two different stimuli (different WL) are perceived as being the same

Question 10

Principle of unvariance

Answer 10

• a photon of light is absorbed => identity of WL is lost

- absorption of a single photon has the same effect no matter the WL or receptor type

Question 11

Different receptors can give different responses to the same wavelength because

Answer 11

different receptors have different pigments => different absorption spectra

Question 12

Population or distributed coding

Answer 12

analysis of stimulus by multiple types of receptor

• not ambiguous
• colour changes if ratio of activity between the 3 WL changes

Question 13

Opponent-process theory (Hering)

Answer 13

• neural level process (lateral geniculate nucleus)
• difference information
• colour experience is arranged into two opponent pairs
  Yellow -Blue
  Red-Green

Question 14

The two opponent pairs are linked through

Answer 14

lateral inhibition

Question 15

Complementary afterimage

Answer 15

seeing the opposite colour of the colour circle
1 - looking at red => L cones get used up
2- L cones less active => green afterimage seen on a white surface

Question 16

Hue cancellation procedure

Answer 16

how much of a opponent colour needs to be added to cancel all perception of the opponent colour (opponent pairs)

Question 17

Difference information

Answer 17

neurons integrate inhibitory (-) and excitatory (+) signals of receptors

Question 18

Opponent LGN-Neurons

Answer 18

• in the retina and lateral geniculate nucleus
• excitatory response to light from one part of the spectrum => inhibitory response to the opponent light
• cones send excitatory or inhibitory input to bipolar cell

Question 19

Types of opponent neutrons in the cortex

Answer 19

Single-opponent neurons

Double-Opponent neurons

Question 20

Single-opponent neurons

Answer 20

• colours within broad regions
• centre-surround antagonism
• ganglion cells, LGN, visual cortex
• best stimulus = higher firing rate

Question 21

Double-opponent neurons

Answer 21

• boundaries between colours
• colour contrast
• colour constancy
• visual cortex
• side by side receptive fields

Question 22

Chromatic adaptation

Answer 22

prolonged exposure to chromatic colour

- adaptation to red light => reduced sensitivity of L-cones => a less saturated red

Question 23

Chromatic adaptation leads to

Answer 23

colour constancy

Question 24

Colour constancy

Answer 24

perceiving the colours of objects as relatively constant even under changing illumination

Question 25

Actual reflected light depends on

Answer 25

object reflectance curve and illumination

Question 26

Other factors influencing the way we perceive colour

Answer 26

a) Surroundings (colour constancy works best when object is surrounded by many colours) b) Memory and prior knowledge (familiar objects have a more saturated colour than other objects that reflect the same WL)

Question 27

Dual-process theory

Answer 27

1. Trichromatic stage (photoreceptors) 2. Opponent-process stage (ganglion cells, LGN, VC) 3. Cortical processing

Question 28

"The dress" experiment explains

Answer 28

individual variability => perceived colour depends on the context Warm illumination = blue/black dress Cool illumination = white/gold dress