W2 L2 - Colour perception

where does the colour come from

  • high frequencies and wavelengths

  • 800 - 380 nanometers in wavelengths we can see - were sensible to

  • experience of colour is within our brain


cone photoreceptors

  • active in bright light

  • 3 types

  • maxillary sensitive - where wavelengths peak

  • short (blue), middle (green), long (red) wavelengths


evolution of cone types

Theory 1

  • trichomacy - state of having 3 cone types

  • evolved as a response to need of fruits in forest

  • in the past people were dichromatic (had only 2 crone types - short and long)


Theory 2

  • positioning of peak of wavelengths are exactly to detect the difference between oxygenated and deoxygenated skin

  • so that we are able to detect blood, health of our species

  • STUDY

  • bare skin: sócio-sexual signals from blood oxygenation

  • dichromatic species have more fur than trichromatic species (less fur → skin on show, social signal)


genetic colour vision deficiency

  • monochromats - only 1 cone or no cones only rods

  • bichromats - lack 1/3 cones

- protonopia: lack L cone

- deuteranopia: lack M cone

- tritanopia: lack S cone

  • anomalous trichromats

- deuteranomoly: M cone shifted towards L

- protanomoly: L cone shifted towards M

  • generally caused by genetic differences but is also caused by drugs, ageing and hormones

  • 8% of men and <1% of woman have a colour vision deficiency


a cure for colour vision deficiency?

  • STUDY

  • gene therapy in squirrel monkeys

  • took long wavelengths (red) of genes which is not present in males

  • inverted it into some cones by use of a virus

  • turned monkey from dichromatic to trichromatic

  • brain able to use new signal even through circuitry not in use early on in life


human tetrachromacy

  • some woman have four cone types evoked

  • usual cone types 3 and shifted red or green cone types

  • does extra cone mean more colour?

  • psychophysical tests and genetic analysis


cone opponency

  • combine 3 cones and contrast to give 3 cone-opponent channels

  • L/(L+M): “cherry-teal” (supports theory 2 oxygene in blood)

  • 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) - also carries movement


memory colour

  • obj with particular colour - brain adds colour automatically

  • STUDY

  • banana on screen - made banana grey and background blue to counteract the memory colour - ppts said banana was yellow

  • top-down perception


aesthetic response to colour

  • we prefer some colours to others

  • ecological valence theory says that it is because what we associate the colour with - Palmer and Schloss

  • if we ask people what objects/ images they like the best - can predict persons favourite colour

  • colour in natural scenes tend to be dominated by blue-yellow variation - is also the type of colour people like the most - this might be culturally-mediated


colour constancy

  • brains ability to subtract the illumination and recover the true surface colour