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Chapter 9

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PSYC 2390
Naseem Al- Aidroos

PSYC 2390 Chapter 9 – Perceiving Color - Color is one of the most obvious and pervasive qualities in our environment - We pick favourite colors (blue is the most favoured), associate it with emotions (turn purple in rage, red in shame etc.), imbue colors with meanings (red for danger, purple for royalty) - Color blindness can occur either due to cortical injury after birth or due to genetic absence of one or more cone receptors at birth - People born color blind are not so disturbed by their blindness as they have never experienced color Functions of Color Vision: - Helps identify and classify things - It helps facilitate perceptual organization, process by which small elements become grouped perceptually into larger objects - Helps differentiate one object from another and pick out objects from scenes (due to contrast) - It has been hypothesized that monkey & human color vision may have evolved for the express purpose of detecting (picking) fruit (from a garden), considering color blind people have difficulty doing so and have to pick them up by feeling/looking at their shape - Red, yellow, green, blue and their combinations are all the colors that we perceive - People describe the color of most objects in terms of these 4 colors, but can’t when 1 of the 4 is omitted. - Colors such as orange, violet, purple and brown are not needed to achieve these descriptions - The 4 colors are so basic because they are available in the visible spectrum whereas the others are extraspectral colors (usually a mix of 2 or more of the 4 basic colors) - Changing the intensity of colors by adding or removing white (which is equal amounts of all wavelengths across the spectrum) is called saturation (makes it darker), or desaturation (makes it lighter) - By changing the wavelength, intensity and saturation, we can create over a million discriminable colors, of which only a fraction are a part of our everyday world Color and Wavelength: Reflectance and Transmission:- - The colors of objects are determined by the wavelengths of light that are reflected from the objected to our eyes - Reflectance curves: plots of the % of light reflected vs wavelength for a number of objects (figure 9.5) - Black and white reflect all wavelengths across the spectrum, but blue, green, yellow & red reflect only some - Chromatic colors or hues: colors that reflect some wavelengths more than others. This property is called selective reflection - Relations between color and their wavelengths: Blue – short, Green – medium, Red – long , Yellow – long and medium, White – long, medium and short - Achromatic colors: colors with no hue, reflect all wavelengths, like white and black PSYC 2390 Mixing lights:- - If a blue light is projected on a white surface, and a yellow light is superimposed on the blue area, the superimposed area appears white (will not happen on mixing colors in a palette), because the two spots projected on the white surface, all of the wavelengths that hit the surface are reflected into an observer’s eye - All the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed. Thus the superimposed light contains short (blue), long and medium (yellow) wavelengths resulting in white light - Additive color mixture: adding up wavelengths during mixing of lights Mixing Paints: - When paints are alone, they absorb all wavelengths (short, medium & long), but reflect only the wavelength of their respective color i.e. short for blue (with little bit of medium), long and medium for yellow etc. - Subtractive color mixture: When paints are mixed, both paints still absorb the same wavelengths, but reflect only those that are reflected by both paints in common, i.e. on mixing blue and yellow, only medium wavelength is common, which is reflected resulting in a green color. - Paints reflect a wide range of wavelengths allowing the color mixing affects that take place - The colors of objects are associated with which wavelength is reflected (for opaque objects) or transmitted (for transparent objects) According to Newton, color is not part of the wavelengths in the visible spectrum(they are just energy rays) , but part of our perceptual system Trichromatic Theory of Color Vision: - Thomas Young and Hermann von Helmhotlz (19 century) stated that color vision depends on the activity of 3 different receptor mechanisms. - This theory was first proposed by Young and later refined by Helmholtz and is therefore also called Young-Helmholtz theory of color vision Behavioural evidence: - Helmholtz’s color matching experiments: observers adjusted the amounts of 3 wavelengths of light mixed together in a ‘comparison field’ until the color of the mix matched the color of a single wavelength in the ‘test field’ ex: mix 420. 560 and 640nm to make a color match 500nm - Finding: People with normal color vision cannot match all wavelengths in the spectrum with only 2 wavelengths. Color blind people couldn’t match the colors of all wavelengths in the spectrum by mixing only 2 other wavelengths - According to the theory, light of a particular wavelength stimulates the 3 receptor mechanism to different degrees and the pattern of activity in the 3 mechanisms result in the perception of color - Each wavelength is therefore represented in the nervous system by its own pattern of activity in the 3 receptor mechanisms PSYC 2390 Physiology of theory: - Cone pigments: 3 cone visual pigments were identified; short (419nm), medium (531nm) and long (558nm) [abbreviated as S,M & L]- made up of large protein component called opsin and a small one called retinal - Differences in the structures of these proteins are responsible for the 3 different mechanisms - Different patterns of firing of the 3 cones result in our perception of different colors. Eg: Blue is largely signalled by S receptor, smaller response by M, and an even smaller response by L. - Metamerism: when two physically different stimuli are perceptually identical (as seen in color matching expt). Mixing of 530nm and 630nm will result in L & M signals, same as in 580nm - Metamer: the two identical fields in a color matching experiment, due to the same pattern of response in the 3 cone receptors Are the 3 receptor mechanisms necessary for color vision? - Color vision is possible with 2 receptors but not with one. - A person with only 1 visual pigment (monochromat) can match any wavelength in the spectrum by adjusting the intensity of any other wavelength (i.e. by adjusting intensity of a light of 550nm can make it identical to 590nm). Hence people with 1 receptors cannot tell the difference between two objects based on the wavelengths they reflected - Adding a second pigment (dichromat), which increasingly identifies another range of wavelength, even on changing the intensity, one can differentiate between two different objects. Because the ratios of response by the 2 pigments for a particular wavelength are same irrespective of intensity Color Deficiency: - Color deficiency: partial loss of color vision, associated with problems with receptors on retina - Daltonism: John Dalton’s description of abnormal color perception, used to describe color deficiency - Ishihara plates: test for color deficiency; plate in which normal vision people see the number 74, but color deficient people don’t - Color matching procedure for color deficiency: revealed that monochromats - can match any wavelength in the spectrum by adjusting intensity, dichromats - need 2 wavelengths to identify color and an anamolous trichromat – need 3 wavelengths to watch any wavelength. However, they mix different proportions of wavelengths than a trichromat and is not as good at discr
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