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Chapter 7

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Department
Psychology
Course
PSYCH 2E03
Professor
Gautam Ullal
Semester
Fall

Description
Psych 2E03: Sensory Processes Chapter 7: Perceiving Colour Introduction to Colour - What are some functions of colour vision:  Colour serves important signalling functions, both natural and contrived by humans  Colour helps facilitate perceptual organization  Facilitates the ability to tell one object from another and to see objects against a varied background  Monkey and human colour vision may have evolved for the express purpose of detecting fruit  Helps us recognize and identify things we can see easily - How can we describe colour experience:  We can describe all the colours we can discriminate by using the terms red, yellow, green, blue, and their combinations  The order of the four basic colors in the colour circle – blue, green, yellow, and red – matches the order of the colours in the visible spectrum  We can create more colours by changing the intensity to make colours brighter or dimmer, or by adding white to change a colour’s saturation  White: equal amounts of all wavelengths across the spectrum - What is the relationship between wavelength and colour perception:  Spectrum stretches from 400nm to 700nm  The colours we perceive in the spectrum are related t their wavelength  Colour of objects are determined by the wavelengths of light reflected from the objects  Reflectance curves: plots of the percentage of light reflected versus wavelength  Chromatic colors or hues: some wavelengths are reflected more than others  Selective reflection: property of reflecting some wavelength more than others  Achromatic colours: when light reflection is flat across the spectrum  Selective transmission: in the case of things that are transparent, only some wavelength pass through the object or substance  Our perception of an object’s colour can be influenced by the background on which an object is seen Trichromatic Theory of Colour Vision - Colour vision depends on the activity of three different receptor mechanisms based on colour matching - Behavioural evidence for the theory:  Colour-matching experiments: observers adjusted the amounts of three different wavelengths of light mixed together in a comparison field until the colour of this mixture matched the colour of a single wavelength in a test field  By correctly adjusting the proportions of three wavelengths, it was possible to make the comparison and test fields match  A person with normal colour vision needed to use at least three wavelengths in order to match all wavelengths in the spectrum. People with normal colour vision could not, however, match all wavelengths in the spectrum with only two wavelengths. - The theory – vision is trichromatic:  Colour vision depends on receptor mechanisms, each with different spectral sensitivities  According to this theory, 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  Each wavelength is therefore coded in the nervous system by its own pattern of activity in the three receptor mechanisms - Physiology of trichromatic theory:  Cone pigments:  Researchers were able to measure the absorption spectra of three different cone visual pigments  Different types of pigments have different sequences of the small molecular groups called amino acids that make up the opsin molecule  Differences in the opsin’s amino acids are what cause different pigments to have different absorption spectra  Cone responding and colour perception:  If colour perception is based on the pattern of activity of these three receptor mechanisms, we should be able to determine which colours will be perceived if we know the response of each of the receptor mechanisms  Thinking of wavelengths as causing certain patterns of receptor responding helps us to predict which colours should result when we combine lights of different colours  Metamerism: situation in which two physically different stimuli are perceptually identical  Metamers: two identical fields in a colour matching experiment due to the fact that they both result in the same pattern of response in the three cone receptors  Are three receptor mechanisms necessary for colour vision:  Colour vision is possible with two receptor types but not with one  The reason one receptor type won’t work is that absorption of a photon causes the same effect no matter what the wavelength, principle of univariance  When there is just one visual pigment, we can make any two wavelengths cause the same response by adjusting the intensity of one of them  It is not possible to adjust the intensities to cause the same responses in two pigments simultaneously  Addition of a third pigment, although not necessary for creating colour vision, increases the number of colours that can be seen across the visual spectrum Mixing Coloured Paints - Additive colour mixture: all of the wavelengths in each light reach the eye and stimulate the cone receptors - Subtractive colour mixture: each paint in the mixture subtracts wavelengths and decreases the light reaching the cone receptors Colour Deficiency - Colour deficiency: partial loss of colour perception which is associated with problems with the receptors in the retina - Ishihara plate: colour vision test - Monochromat: can match any wavelength in the spectrum by adjusting the intensity of any other wavelength. Thus, a monochromat needs only one wavelength to match any colour in the spectrum - Dichromat: needs only two wavelengths to match all other wavelengths in the spectrum - Anomalous trichromat: needs three wavelengths to match any wavelength, just as a normal trichromat does. However, the anomalous trichromat mixes these wavelengths in different proportions from a trichromat, and an anomalous trichromat is not as good at discriminating between wavelengths that are close together - Unilateral dichromat: person with trichromatic vision in one eye and dichromatic vision in the other eye - Monochromatism:  Rare form of colour blindness that is usually hereditary and occurs in only about 10 people out of 1 million  No functioning cones; therefore, their vision has the characteristics of rod vision in both dim and bright lights.  See everything in shades of lightness  Poor visual acuity and are sensitive to bright lights - Dichromatism:  Protanopsia and deuteranopia are inherited through a gene located on the X chromosome  Protanopia:  Affects 1% of males and 0.02% of females  Perceive short-wavelength light as blue, and as wavelength is increased, the blue becomes less and less saturated until at 492 nm, the protanope perceives gray (neutral point)  Above the neutral point, perceives yellow with becomes more satura
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