PSYCH 1XX3 Lecture Notes - Lecture 13: Magnocellular Cell, Visual Acuity, Retina
Colour Perception
Introduction to Colour Perception
- The reason we see different colours is because the objects reflect certain wavelength of the visible
spectrum, and these wavelengths trigger specific patterns of response in our brains that give rise to
subjective experience of colour
Evolution of Colour Vision – Why and How
Not all living things see in colour
- Bulls don’t respond to the red colour, instead the motion of the moving cloth
- Primates are the only mammals that see in colour
o Birds, fish, reptiles and insects have excellent colour vision
- Primates benefit from colour vision when foraging for food
o Primates have thee types of cones in the eye that are used for colour vision
The functions of Colour Vision in Different Species
- Other animals can also see colours that we don’t see, such as
infra red and UV
- Colour vision is critical for mate selection in birds
o The colour of a mates’ feathers are indicators of
health
o This type of colouration can help determine mates,
but also stay inconspicuous to predators that can’t see
the certain wavelength of colour that birds can
- Bees see ultraviolet hues on flowers that act as nectar maps
Colour Mixing
Intro to Colour Mixing
- We process colour similar to how artists mix colour pigments
o Humans only need a few colour receptors whose activity can be combined in various
proportions to make every colour
o Primary Colour
▪ The three colours that can be combined in various proportions to make every colour
spectrum
▪ There are no colours that can be mixed together to make primary colours
Subtractive Colour Mixing
- Subtractive Colour Mixing
o When coloured pigments selectively absorb some wavelengths and reflect others
o Every reflective surface absorbs (or subtracts) the colours that it does not reflect
▪ Adding other pigments to that surface alters the combination wavelengths subtracted
▪ Involves a combination of pigments reflecting the sum of wavelengths that are not
absorbed
- With subtractive colour mixing, the primary colours are cyan, magenta and yellow, because these three
colours can be mixed in various proportions to make all the colors of the rainbow
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Additive Colour Mixing
- Additive Colour mixing
o When coloured lights add their dominant colour to the mixture
- This is how our visual system processes colour, by adding the effects of different wavelengths together
in our nervous system
- With additive colour missing the primary colours are red, green and blue
o The complimentary colour of blue is yellow, for red its bluish-green, and for green it’s a reddish-
purple
o With additive colour mixing, when you mix a primary colour and its complementary colour you
get white or grey
- Our visual system fuses colour if they are close enough together
o TV color is just a combination of tiny dots of primary colours
Theories of Colour Vision
Two Theories of Colour Vision
- Trichromatic Theory
o Proposes that the retina contains different kinds of cones,
that are each maximally sensitive to different wavelengths
of light
o Photoreceptors respond to a gradient of different
wavelengths
▪ A given receptor will respond to other
wavelengths, just not as much as it would to its
peak wavelength
o The retina contains three different types of cones
▪ This theory follows from empirical observations about primary colour mixing
▪ It proposed that you only need three different kinds of photoreceptors to see all the colors
of the visible spectrum
Elegance of Theory
Problems with Theory
Fits with additive colour mixing
Yellow seems to be a primary colour
Physical evidence for three types of cones
Certain pairs of wavelengths produce the
experience of white
Why is yellow the afterimage of blue?
The same colour can appear lighter or darker
depending on the surrounding colours.
- Opponent- Process Theory of Colour Vision
o Each colour receptor is made up of a pair of opponent colour processes
▪ Each receptor is capable of being in two opponent states and it can only be in one of
those states at a time
▪ The ability to see blues and yellows is mediated by a blue-yellow opponent receptor
▪ Receptor for dim and bright light; these brightness detectors are excited by lights of any
wavelength
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