colour perception

Question 1

why is colour important?

Answer 1

• We could operate without colour, but most people wouldn’t want to live without colour.

Question 2

what do we use colour for?

Answer 2

• Decoration - as we like looking at colour/certain pattern of colours
  
  • Distinguishing food between good and bad

Question 3

where does colour come from?

Answer 3

• Electromagnetic spectrum spans from cosmic and gamma rays to radio waves and theres this little slice in the middle which we call visible light and it so happens that there’s a little slight of electromagnetic spectrum which are eyes are sensitive to.
  
  • If you take white light, you can split it with a prism and you will get lights of lots of different wavelengths.
  • The brain is doing a lot of work, so interprets those signals from the outside world and turn it into a full colour experience.

Question 4

cone photoreceptors

Answer 4

• Human trichromacy - three cone types, maximally sensitive at short (s), middle (m), and long (l) wavelengths

Question 5

evolution of cone types

Answer 5

• Trichromacy evolution related to foraging for ripe fruit/berries
  
  • E.g., Regan, Julliot, Simman, Vienot, Charles-Dominique and Mollon, 2001

Question 6

bare skin - socio-sexual signals from blood oxygenation

Answer 6

• monochromatic primates (one cone type)
• dichromatic primates (two cone types: ‘short’ and ‘medium’)

Question 7

genetic colour vision deficiency

Answer 7

• Monochromats:
  
  • Only one cone (or no cones, only rods)
    · Dichromats:
  • Protanopia - lack L cone i.e., long wavelength
  • Deuteranopia - lack M cone i.e., medium wavelength
  • Tritanopa - lack s cone i.e., short wavelength
    · Anomolous trichromats:
  • Deuteranomoly (M cone shifted towards L)
  • Protanomoly (L cone shifted to M)
    · Overall 8% men, <1% women gentic deficiency
• Also acquired colour vision deficiency (ageing, drugs, hormones)

Question 8

a cure for colour vision deficiency?

Answer 8

· Gene therapy turns dichromat into trichromat!
· Dichromatic male squirrel monkeys
· Red opsin gene, virus & DNA injected into some cones
· Can see colours previously not seen!
· Brain able to use new signal even though circuitry not in use early on in life
· (Mancuso et al., 2009 – the Neitz’s lab)

Question 9

human tetrachromacy

Answer 9

· Some women have four cone types!
· Usual 3 cone types & shifted red or green cone type
· Does an extra cone mean they can see more colours?
· Psychophysical tests & genetic analysis
· Only one woman behaviourally tetrachromatic
· Still need cortical processing of extra signal
· (Jordan, Deeb, Bosten & Mollon, 2010)

Question 10

cone opponency

Answer 10

· Output from three cones combined & contrasted to give three cone-opponent channels:
- L/(L+M): “red-green”
- S/(L+M): “blue-yellow”
- L+M: “black-white”
· They are more accurately expressed as:
- L/(L+M): “cherry-teal”
- S/(L+M): “violet-lime”
- L+M: achromatic (or luminance axis

Question 11

colour-opponent cells in the LGN

Answer 11

· Parvocellular = L/(L+M) (cherry-teal)
· Koniocellular = S/(L+M) (violet-lime)
- Magnocellular = luminance (black-white

Question 12

lilac chaser illusion

Answer 12

· Adaptation - prolonged exposure to a sensory stimulus reduces sensitivity.
· Opponent coding - After a short period of staring at the central cross the gap from the missing magenta spot is replaced by its opponent colour – green.

Question 13

colour at the cortex

Answer 13

· Patches of cells (“blobs”) responsive to colour at primary visual cortex (V1)
· Other areas of visual cortex process colour (e.g., V2,V4/V8)
· Sent to temporal cortex (ventral processing stream –“what” pathway)

Question 14

central achromatopsia

Answer 14

· Damage to small cortical region, loss of colour perception
· Humans with lesions in extrastriate visual cortex (e.g., V4/V8)
· Functioning cones, can record activation at V1 in response to colour
· BUT, things don’t appear coloured
· Can affect one visual field
· Illustrates importance of cortical processing
· e.g., see Cowey & Heywood, 1997

Question 15

memory colour

Answer 15

· Some objects have a typical colour (e.g. banana) that we learn from experience and therefore expect.
· Bananas are typically yellow, so to make a banana look grey it is necessary to add a little extra blue, to counteract the memory colour.
· The error is not present for a simple circle of colour
· Hansen, Olkkonen, Walter & Gegenfurtner (2006) Nature Neuroscience

Question 16

aesthetic response to colour

Answer 16

· Why do we prefer some colours more than other?:
- Biological components theory (Hulbert and Ling, 2007)
- Ecological valence theory (Palmer and Schloss, 2010)

Question 17

ecological valence theory

Answer 17

· Colour preference due to colour-object associations
· WAVE (how good/bad objects associated with that colour are)
· Palmer and Schloss, 2010

Question 18

aesthetic preference for colour patterns

Answer 18

· Colour in natural scenes tends to be dominated by blue-yellow variation
· This is also the type of colour pattern which people like the most.