colour perception Flashcards
why is colour important?
- We could operate without colour, but most people wouldn’t want to live without colour.
what do we use colour for?
- Decoration - as we like looking at colour/certain pattern of colours
- Distinguishing food between good and bad
where does colour come from?
- 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.
cone photoreceptors
- Human trichromacy - three cone types, maximally sensitive at short (s), middle (m), and long (l) wavelengths
evolution of cone types
- Trichromacy evolution related to foraging for ripe fruit/berries
- E.g., Regan, Julliot, Simman, Vienot, Charles-Dominique and Mollon, 2001
bare skin - socio-sexual signals from blood oxygenation
- monochromatic primates (one cone type)
- dichromatic primates (two cone types: ‘short’ and ‘medium’)
genetic colour vision deficiency
- 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
- Only one cone (or no cones, only rods)
- Also acquired colour vision deficiency (ageing, drugs, hormones)
a cure for colour vision deficiency?
· 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)
human tetrachromacy
· 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)
cone opponency
· 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
colour-opponent cells in the LGN
· Parvocellular = L/(L+M) (cherry-teal)
· Koniocellular = S/(L+M) (violet-lime)
- Magnocellular = luminance (black-white
lilac chaser illusion
· 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.
colour at the cortex
· 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)
central achromatopsia
· 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
memory colour
· 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