Colour Perception Flashcards
Primate Colour Perception
especially well-suited for distinguishing red and yellow against a green background
- helpful for foraging
- detecting predators or prey, determining ripeness of fruit, richness of soil, sunset to predict weather
Colour Perception in Other Animals
can see other colours that we can’t (UV spectrum)
- birds: signals of health in colour of feathers
- bees: responds to specific patterns that we are unaware of for foraging (nectar maps)
Colour Mixing
few receptor types whose activity can be combined in various proportions to make every conceivable colour
Primary Colours
the three colours that can be combined in various proportions to make every colour in the spectrum
-base colour - cannot be reduced
Subtractive Colour Mixing
when coloured pigments selectively absorb some wavelengths and reflect others
- kindergarten paint mixing
- primary colours are red, yellow and blue
Complementary Colours
opposite respective primary colour, always makes brown when mixed
Additive Colour Mixing
when coloured lights add dominant colour to the mixture
- used in our nervous system
- primary colours are red, green, and blue - used together in different proportions to make all the different colours that we see
- complementary colour mixing gets grey or white
Trichromatic Theory
proposes that the retina contains 3 different kinds of cones
Empirical Observations of Colour Mixing
you can match all of the colours of the visible spectrum by the appropriate mixing of 3 primary colours, therefore you only need three types of receptors
-human eye has three types of cones, spectrally selective photopigments maximally respond to primary colours
Elegance of Trichromatic Theory
fits with additive colour mixing
physiological evidence for 3 types of cones
Problems with Trichromatic Theory
yellow seems to be a primary colour
complementarity
after image - why is yellow the afterimage of blue?
Opponent Process Theory
each colour receptor is made up of pairs of opponent colour processes
- each receptor is capable of being in one of 2 opponent states, but can only be in one state at a time
- green/red and blue/yellow
- bright/dim receptors are excited by every wavelength
Elegance of Opponent Process Theory
can explain why after image is the complementary colour
- why mixtures of wavelengths appear white
- fits with why we can imagine some colours and not others
In the Retina
Trichromatic Theory
- 3 component receptors or cones that are maximally responsive to a certain wavelength
- red, green, blue
- response of receptors differentially affect what happens further down the line
Ganglion Cells and Onward
Opponent Process Theory
-red/green, blue/yellow, light/dark
Red Light (Example)
Stimulates red cone, excite red/green ganglion cells, signal that stimulus is red
Yellow Light (Example)
equally stimulates red and green cones, red cones excites red/green ganglion cells while green cone inhibits red/green ganglion cells and excites blue/yellow ganglion cell
- red/green signals cancel out
- blue/yellow is excited and signals that stimulus is yellow
Afterimage
is the complementary colour of the thing you just stared at
Rate of Fire
signals to the brain what colour is being seen
-faster(excites) or slower(inhibits) than baseline means different colours
Rebound Effect
when a receptor is excited/inhibited for a prolonged period of time, the same colour will go into the opposite state when you stare at a neutral colour
High Resolution Channels
one cone to one ganglion cell
-small receptive field that, when stimulated, will cause the ganglion cell to increase or decrease its firing rate
Low Resolution Channels
(away from fovea) many cones to one ganglion cell
-large receptive field that causes ganglion cell to increase or decrease firing rate
Receptive Fields
Donut shaped
- respond to colour in the centre-surround fashion
- increases rate of firing if strikes in the middle portion (red)
- decreases rate of firing if strikes outer ring (green)
- response is strongest if both of the above is true, weakest if the opposite is true
Higher Level Colour Processing
sent to the LGN after ganglion cells
has 6 layers
Magnocellular
first two layers of LGN
-processing form, movement and depth
Parrocellular
next 4 layers of LGN
-colour processing from red/green cone and finer detail
Koniocellular
sublayers of LGN
-info from blue cones to primary visual cortex
Cytochrome Oxidase (CO) Blobs
regions of cytochrome oxidase containing neurons that are distributed at roughly equal intervals over the primary visual cortex
- neurons respond exclusively to colour information, show little/no response to shape, orientation or movement
- oval in shape
- arranged into columns that project down into layers 2 and 3 (and less so in layers 5 and 6) of the primary visual cortex
- respond in opponent fashion
- passed into visual association areas (analyzed further in ventral stream)
Total Colour Blindness
everything is in shades of grey
-vary rare
Protanopia
red/green colour blindness
- cones filled with photopigments for green
- no way of responding differently to green and red
- lacking photopigments, not cones
- can tell the difference if there is a slight difference in the way each colour absorbs light
Deuteranopia
Red/green colourblindness
-cones filled with photopigments for red
no way of responding differently to green and red
-lacking photopigments, not cones
-can tell the difference if there is a slight difference in the way each colour absorbs light
Tritanopia
Yellow/blue colour blindness
- extremely rare
- equally present in both males and females
- blue cones are either lacking or defective
- see reds/greens/greys
