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Colour Perception .pdf

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Joe Kim

th Psychology 1XX3 Notes – Colour Perception – Mar 7 , 2010 Evolution of Colour Perception: Who Has Colour Vision?  Many birds, fish, reptiles, and insects have excellent colour vision. Among mammals, however, colour vision is limited to primates, including humans.  This means that your dog and cat, along with the bull, can only see in shades of grey. The Functions of Colour Vision in Different Species:  How did primates end up evolving the ability for colour vision? Primate colour vision is especially well suited to distinguish red and yellow against a green background.  This adaptation helps immensely with foraging for fruit in the bushes and trees.  In this way, one possible biological advantage of colour vision for primates is the ability to detect colourful objects in the wild.  Additionally, the ability to perceive colour is an important advantage for detecting predators or prey against a background, determining the ripeness of fruit, the richness of the soil, or even using the colour of sunsets as a means to predict weather. The Functions of Colour Vision in Different Species:  Colour vision is also an important part of the lives of many non-mammalian species. Many birds, fish and insects are able to see colours that we don’t see at all, including colours at the UV end of the spectrum.  For birds, the colour of a potential mate's feathers provides signals to other birds about how healthy that bird is, and can influence how likely that bird is chosen as a mate.  This type of colouration system would help the birds communicate with each other about how sexy and healthy they are, while still remaining inconspicuous in the forest to potential predators that are unable to see this bright colouration.  Next time you see a bee in a flower garden, don’t assume that the beautiful red rose you see must be irresistible to the bee. 1'he bee might he attracted to flowers that look dull to human eyes, because it is responding to specific patterns and colours on the flower that we are unaware of.  Some flowers have adapted patterns on the petals that are invisible to us, but serve as "nectar maps” to the bee. Colour Mixing: Introduction:  The human eye processes colour information using principles that have been known to artists for centuries. You don’t need to have millions of colour receptors to deal with every conceivable colour out there in the world.  Instead, you just need a few receptor types whose activity 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 are like a base colour, they cannot be reduced into other colours.  There are actually two different types of colour mixing: additive and subtractive. Def’n of Subtractive Colour Mixing: When coloured pigments selectively absorb some wavelengths and reflect others  Subtractive colour mixing applies to the mixing of pigments, dyes, or paints, and it is called ‘subtractive’ because every reflective surface absorbs (or subtracts) the colours that it does not reflect. Adding other pigments to that surface alters the combination wavelengths subtracted.  So a blue object looks blue to us because all wavelengths are being absorbed by the object except short, blue waves, which are being reflected back to our eye and making the object look blue.  So when we mix two pigments, all wavelengths are being absorbed except those that the two pigments jointly reflect.  It may help to think about what would happen if we shine a white light through both a yellow filter and then a blue filter and look at the remaining light on a white screen. You would see that it looks green.  This is because when the light hits the yellow filter all the short waves (the blues and purples) are being absorbed, or subtracted out, and only the longer (green, yellow, orange and red) waves are allowed to pass through. When these longer waves hit the blue filter, it absorbs the longest waves (yellow, orange and red) and what is left over is a middle band of wavelengths that is transmitted by both pigments. This middle band that is left over is green.  With subtractive colour mixing the primary colours are red, yellow and blue, because these three colours can be mixed in various proportions to make all colours in the rainbow.  The complementary colour of red was green, for yellow it was purple, and for blue it was orange. If you mixed a primary colour with its complementary colour, you always got brown. Def’n of Additive Colour Mixing: When coloured lights add their dominant colour to the mixture.  The coloured lights add their dominant wavelength 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 our nervous system. With additive colour mixing, the primary colours are red, green and blue, because these three colours can be added together in various proportions to make all the different colours that we see.  If you made a colour circle using additive colour mixing, you’d find that the complementary colours are also different: the complementary colour of blue is yellow, for red the complementary colour is a bluish-green, and for green the complementary colour is a reddish-purple.  With additive colour mixing, whenever you mix a primary colour with its complementary colour you get grey or white.  Take a blue and yellow filter, but instead of passing the light through each filter successively, overlap the coloured lights from each one onto the reflective surface.  Because each coloured light adds its dominant wavelength to the mixture, you find that now blue and yellow do not make green when added together; but instead they make grey.  This is because grey light is the sum of complementary colours (in this case, blue and yellow). Theories of Colour Vision:  Def’n of Trichromatic theory: Proposes that the retina contains three different kinds of cones.  The trichromatic theory of colour vision is based on the proposal that the retina contains three different kinds of receptors that are each maximally sensitive to different wavelengths of light.  This very influential theory has a long history, first proposed by Thomas Young in 1802, and later modified by Hermanvon Helmholtz in 1866. Sometimes it’s referred to as the Young-Helmholtz theory.  This theory follows from empirical observations about primary colours and colour mixing; namely, that it’s possible to match all of the colours of the visible spectrum by the appropriate mixing of 3 primary colours.  Thus, the trichromatic theory postulated that you only need three different types of receptors to discriminate all the colours of the visible spectrum. Although this theory was developed purely from behavioural data, we now know that the human retina is indeed equipped with three types of cones which contain spectrally selective photopigments that are maximally responsive to wavelengths of light that correspond to the primary colours red, green and blue.  ‘maximally responsive’: this means that a given receptor will respond to other wavelengths, just not as much as it would to its peak wavelength.  When you perceive yellow, this is because the red and green cones are equally stimulated. White is what you see when all three receptors types are stimulated.  However, some things didn't quite fit the theory. First, yellow seemed to be a primary colour and not a mixture of red and green. When people were asked to describe the most basic colours, yellow was usually included as one of them, even by young children.  Furthermore, the trichromatic theory could not explain the law of complementarity, that certain pairs of wavelengths produce the experience of white.  Finally, there was the phenomenon of the complementarity of afterimages: why do you see a yellow afterimage when you stare at a blue stimulus? The Opponent-Process Theory: Def’n: Each colour receptor is made up of a pair of opponent colour processes.  In 1920, Hering formulated the opponent-process theory of colour vision. Like the trichromatic theory, the opponent- process theory argues that there are three classes of receptors, but unlike the trichromatic theory, the opponent-process theory posits that each of these receptors is made up of a pair of opponent processes.  Each receptor is capable of being in one of 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 receptors, greens and reds are mediated by green-red opponent receptors, and 3 type of opponent receptor distinguished bright from dim light; these brightness detectors are excited by lights of any wavelength.  The new opponent- process was very successful at explaining how a mixture of wavelengths from complementary colours (like blue and yellow or red and green) appear white; it also explained why the afterimage of a visual stimulus is the complementary colour.  It also fit logically with the fact that most people can easily imagine a yellowish- red or a bluish-green colour; but it‘s more difficult to imagine a reddish-green or a bluish- yellow colour.  According to the opponent-process theory, these colour pairs are opposite and occur from differential activation of the same receptor type; accordingly, it would be impossible for a single red-green receptor to be active in both the red and green states simultaneously. Both Theories Needed to Explain Colour Perception:  Both of these theories are needed to explain colour perception. Hurvich and Jameson elaborated the theories in 1955 and proposed that there are three component receptors or cones in the retina that are each maximally responsive to light of a certain wavelength, just as the trichromatic theory suggested.  The three cones are maximally responsive to red, green and blue. The response of these receptors differentially affect what is happening further down the line in the brain, where things are organized as the opponent-process theory would have predicted. The opponent pairs are red-green, blue- yellow, and light-dark.  The combination of the two theories says that the output of the cones is input for the next layer of colour processing in the retina, which is organized in an opponent fashion.  Colour coding continues in this opponent arrangement up to the brain's visual processing systems.  For example, a red light would stimulate a red cone in the retina, which would then excite the red-green ganglion cells that are organized in an opponent fashion, and this excitation of the red-green ganglion cell would signal the brain that the stimulus is red.  A green light, however, would stimulate a green cone, which
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