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PSYCH 1XX3 Chapter Notes -Visual Acuity, Two-Streams Hypothesis, Thalamus

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Colour Perception (Week 6)
Introduction to Colour Perception
The reason we can see different colours is because the objects and surfaces we look at
reflect certain wavelengths of the visible spectrum.
These reflected wavelengths trigger specific patterns of responses in the brain that give
rise to our subjective experience of colour
The subjective experiences relates to how the different wavelengths of light affect certain
cones in our retina
The composition of wavelength reflected by an object is determined by its reflectance
and the wavelength composition if the illuminating light
Colour constancy is the ability of colour vision to correct the variation in overall
illumination(brightness) so that an object’s colour appears the same all the time
Evolution of Colour Perception: Why and Who
A bull does not react to the red cap, the bulls response is more or likely to the motion of
the cap
Colour vision in mammals is limited to primates, including humans (cats, dogs, and fish
see in shades of grey).
Primates colour vision is well suited to distinguish red and yellow against a green
background, helping us to find fruit in bushes. The ability to see colour gives us an
advantage to detect predators and prey against a background
The Functions of Colour Vision in Different Species
Many birds, fish, reptiles and insects are able to see colour that we don’t see, including
colours at the UV end of the spectrum
The colour of a birds feather indicates to other potential mates their health
Variation of Colour Vision
Humans, bees, and macaque monkeys are trichromats
Bees are sensitive to ultraviolet light
Goldfish, pigeons, and ducks have eyes containing four types of receptors
Rabbits, cats, and squirrels are dichromatic(only two types of receptors)
Any animal that is monochromatic lacks chromatic vision entirely, and must resolve
object by differences in brightness and contrast
Colour Mixing
Only a few receptors types are needed that can be combined in various proportions to
make every conceivable colour
The three primary colours can be combined in various proportions to make every colour
in the spectrum
Primary colours cannot be further reduced into other colours
Subtractive Colour Mixing
Applies to the mixing of pigments, dyes, or paints and it is called ‘subtractive’ because
every surface colour absorbs (or subtracts) the colours it does not reflect. Adding other
pigments to the surface alters the combination wavelengths subtracted
The complementary colour of red was green; for yellow it was purple; and blue it was
If a primary colour is mixed with its complementary colour you get brown
Summary: involves a combination of pigments reflecting the sum of wavelengths that are
not absorbed
Additive Colour Mixing
The mixing of coloured lights
Coloured lights add their dominant wavelengths to the mixture as opposed to
subtracting wavelengths out
This is how our visual system processes colour, by adding the effects of different
wavelengths together in the nervous system
The primary colours with additive colour mixing are red, green and blue , because these
three colours can be added together in various proportions to make all the colours we
The complementary colour of blue is yellow; for the red is a bluish-green(teal); and for
green the complementary colour is a redish-purple
When every a primary colour is mixed with its complementary colour you get grey or
The mixing of blue and yellow produces grey
Trichromatic Theory (Young-Helmholtz theory)(Retina)