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Chapter 9

Sensation and Perception Psych 367 Chapter 9.docx

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Douglas Wylie

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Chapter 9 Colour Introduction to colour What are some functions of colour vision - Colour serves important signalling functions both natural and contrived by humans - Colour perception greatly facilitates the ability to tell one object from another and especially to pick out objects within scenes - Tanaka and Presnell: o Demonstrated that by asking observers to identify objects which appeared either in their normal colours, like yellow banana, or inappropriate colours like purple banana, o Observers recognized the appropriately coloured objects faster What colours do we perceive - People are presented with many different colours and asked to describe them, they describe all of than when they are allowed to use all four of the terms o red yellow green blue o But cant describe them when the terms are omitted o Short wavelength end of the spectrum is blue , green in the middle, and yellow then red at the long wavelength end - Colours brown and purple, which are called extraspectral colours because they do not appear in the spectrum - People can discriminate between about 200 different colours across the length of the visible spectrum - We can create even more colours by changing the intensity to make colours brighter or dimmer, or by adding white to change colours saturation - Adding white decreases colour saturation Colour and wavelength - Perception of colour is associated with the physical property of wavelength - Spectrum is from 400nm to 700nm - 400- 450 appear violet - 450-490 blue - 500-575 green - 575- 590 yellow - 590-620 orange - 620-700 red Reflectance and transmission - Colours of objects are largely determined by the wavelengths of light that are reflected from the objects into our eyes - Reflectance curves: plots of the percentage of light reflected versus wavelength - Some wavelengths are reflected more than others, as for the coloured paints and tomato, we called them chromatic colours or hues - selective reflectance : The property of reflecting some wavelengths more than other which is characteristic of chromatic colours - achromatic colours: When light reflection is similar across the full spectrum, this is contains no hue, (white gray black) - Things that are transparent such as liquids, plastics and glass, chromatic colour is created by selective transmission, meaning that only some wavelengths pass through the object or substance Mixing lights - Blue (short) + yellow (long) equals white - All the wavelengths that hit a surface are reflected into an observers eye - When coloured lights are superimposed o All of the light that is reflected from the surface by each light en alone is also reflected when lights are superimposed - the adding together light therefore contains short, medium and long wavelengths, which result in white - mixing light is called additive colour mixture mixing paints - blue blob absorbs long wavelengths - yellow blob absorbs short wavelengths - when paints are mixed o both paints still absorb the same wavelength they absorbed when alone, so the only wavelengths reflected are those that are reflected by both paints o blue (short) and yellow (long) = green (medium) - paint mixing is called subtractive colour mixing - most paints reflect a broad band of wavelengths wavelengths do not have colour - these colours are created by our perceptual system - the connection between wavelength and the experience we called colour is an arbitrary one - many animal perceive either no colour or greatly reduced palette of colours compared to humans trichromatic theory of colour vision - trichromatic theory of colour, states that colour vision depends on the activity of three different receptor mechanisms behavioural evidence for the theory - helmholtz’s colour matching experiment o observers adjusted the amount of three different wavelengths of light mixed together in a comparison field until the colour matched the colour of a single wavelength test light - key findings o correctly adjudging the proportion of 3 wavelengths in the comparison field, it was possible to match any wavelength of test light o normal colour vision cannot match all wavelengths in the spectrum with only two wavelengths - people who are colour deficient and therefore cant perceive all colours in the spectrum, can match the colours of all wavelength in the spectrum by mixing only two other wavelengths the theory: vision Is trichromatic - Young: proposed the trichromatic theory of colour based on the finding that people with normal colour vision need at least 3 wavelengths to match any wavelength in test field - Light of a particular wavelength stimulates the three receptor mechanisms to different degrees, and the pattern of activity in the three mechanisms results in the perception of a colour Physiology of trichromatic theory Cone pigments - Researchers were able to measure the absorption spectra of three different cone visual pigments with maximum absorption in the o Short 419nm o Medium 531nm o Long 558 nm - All visual pigments are made up of a large protein component called opsin and a small light sensitive component called retinal - Differences in the structure of the long opsin part of the pigments are responsible for the three different absorption spectra Cone responding and colour perception - Thinking of wavelengths as causing certain patterns of receptors responding helps us to predict which colour should result when we combine lights of different colours - This result is interesting because the lights in the two fields are physically different (contain different wavelengths) but they are perceptually identical ( they look the same) - metamerism : This situation in which two physically different stimuli are perceptually identical, and the two identical fields in a colour matching experiment are called metamers - The reason metamers look alike is that they both result in the same pattern of response in the three cone receptors - Thus even though the lights in these two fields are physically different, the two lights result in identical physiological responses and so are identical, as far as the visual system is concerned Are three receptor mechanism necessary for colour vision - According to trichromatic theory, a lights wavelength is signalled by the pattern of activity of three receptors mechanism - Colour vision is possible with two receptors types but not with one - A person with only one visual pigment can match any wavelength in the spectrum by adjusting the intensity of any other wavelength - Adding a second pigment makes it possible to distinguish between wavelengths independent of light intensity - When we increase the illumination on Barbra, we see that the pattern of receptor activation caused by the dress is still different from the pattern on Mary’s dress - Adding the second pigment cause mary and barbras dresses to have different effects, even when we change the illumination - So colour vision becomes possible when there are two pigments - The ratios of response caused by the two pigments are the same for a particular wavelength, no matter what the intensity. - The ratio for the 550nm light is always 2 to 1 - Thus the visual system can use this ratio information in determine the wavelength of any light - There are people with just two types of cone pigments o Dichromats, do see colours, just as our calculations predict, but they see fewer colours that people with 3 pigments trichromates Colour deficiency - Most problems with colour vision involve only a partial loss of color perception called colour deficiency and are associated with problems with the receptors in the retina - Daltonism: describe colour deficiency - This had been determined by colour vision tests like ishihara plates - Another way to determine the presence of colour deficiency is by using the colour matching procedure to determine the minimum number of wavelengths needed to match any other wavelength in the spectrum - Monochromat: can match any wavelength in the spectrum by adjusting the intensity of any other wavelength o A monochromat needs only one wavelength to match any colour in the spectrum and sees only in shades of gray - Dichromat: needs two wavelengths to match all other wavelengths - Anomalous trichromat: need three wavelengths to match any wavelength, just as normal trichromat does but mixes these wavelengths in different proportion from a trichromat o Not as good as normal at discriminating between wavelengths that are closer together - To determine what a dichromat perceives, we need to locate a unilateral dichromat: a person with trichromatic vision in one eye and dichromatic in the other Monochromatism - Monochromatism is a rare form of colour blindness - Usually hereditary - Usually have no functioning cones - There vision has the characteristics of rod vision in both dim and bright lights - Colour blind - Poor visual acuity - Sensitive to bright lights that they often must protect their eye with dark glasses during the day Dichromatism - Experience some colours through a lesser range that trichromats - Most common kinds o Protanopia and deuteranopia are inherited through gene in X chromosome - Protanopia o Affects 1% of people o Perceives short wavelengths light as blue, and as wavelength increases, the blue becomes less saturated until at 492nm its gray o Neutral point: wavelength at which the protanope perceives gray o At wavelengths above gray, they perceive yellow, which becomes increasingly saturated as wavelength increases - Deuteranopia o Affects about 1% of males o Perceives blue at short wavelengths and yellow at long o Neutral point at 498 - Tritanopia o Very rare o Blue at short wavelengths o Red at long o Neutral point at 570 Physiological mechanism of receptor based colour deficiency - Monochromats have no colour vision because they have just one type of cone or no cones - Protanope are missing the long
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