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Colour Perception.docx

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PSYC 2390
Lana Trick

What is Colour? 10/6/2012 11:12:00 AM What is Colour?  Nanometer (nm)  1 / 1 billion m  Amplitude  from midline to peak  Wavelength  from peak to peak Purpose of Colour:  Signaling function (ripe banana)  Helps facilitate perceptual organization (tell one object from another, picking out an object from a scene) o Colour vision evolved for the purpose of detecting fruit?  Helps us recognize objects  Colour researchers consider red, yellow, green and blue to be basic colours o Extraspectral colours: do not appear in spectrum (pink) Three Dimensions:  Hue: wavelength of the light o See short wavelength light (blue, 400nm) and long wavelength light (red, 700nm) o Black absorbs most light o White reflects most light  Grey in between  All contain no hue, “achromatic colours) o Green absorbs all colours and reflects green (shown in reflectance curves)  Selective reflection: reflecting some wavelengths more than others  Selective transmission: only some wavelengths pass through substance (e.g. cranberry juice and wavelength red), for transparent objects  Brightness: amplitude o Short amplitude (dim) vs. high amplitude (bright)  Saturation: how pure the colour is o Max saturation = monochromatic light (light of only 1 colour, only 1 wavelength)  pure o Min saturation = mixtures of all lights (e.g. white, grey, black [desaturated]) o Most colours we see in day to day life are desaturated or unsaturated  Less saturated: Olive green (green+brown), pink (red+white) Wavelength Does NOT Have Colour:  Colours that we see in response to different wavelengths are not contained in rays of light themselves o They are created by our perceptual system o E.g. Mr. I  does not experience colours but still receiving the same stimulation Mr. I:  Painter that went colour blind after a concussion  Caused by cortical injury vs. having colour blindness since birth due to genetic absence of one or more cone receptors Newton’s Insight 10/6/2012 11:12:00 AM Newton’s Insight About Colour  “figured out the say colour worked”  The man of the millennium  Playing with prism o If you put white light through prism see all the colours of the rainbow o Realized when we see something as white (sunlight) white is made up of all wavelengths and is a construction of our mind o Colour is created by our minds o We see equal amounts of all different wavelengths and call it white  Took prism, blocked out all colours and only blue light went through nd 2 prism o Only blue light was observed  once divided, cannot divide them any further Trichromatic Colour Theory 10/6/2012 11:12:00 AM Young and Helmholtz’s Trichromatic Colour Theory  Basic knowledge now but at the time people did not believe  We cannot have a separate colour receptor for every colour we see  Realized we only have 3 (red, green and blue cones) o Knew this long before we could see the cones  Helmholtz took up his work 100 years later than Young  Trichromatic theory: what we see when we see light o Mix light by taking different projectors of 3 colour receptors o Depends on 3 receptors that are each sensitive to different parts of the spectrum o Can fool visual system (mixing of light) o When you mix light  additive colour mixture (adding more wavelengths, all of the light that is reflected from the surface by each light when alone is also reflected when the lights are superimposed)  Mixing paint works in exact opposite way  Has pigments in it  subtractive colour mixing  Red reflects red and absorbs blue and green  Green absorbs red and blue and reflects green  You are reducing number of wavelengths that are reflected into your eye  When mixed, both paints still absorb the same wavelengths they absorbed when alone so the only wavelengths reflected are those that are reflected by both paints in common Behavioural Evidence: Colour Matching Experiments Comparison Field and Colour Matching Experiments  Observers adjusted the amounts of 3 different wavelengths of light mixed together in comparison field until colour matched colour of single wavelength (they were metamers)  Was found that you could make any colour in the spectrum by adjusting the portions of the 3 wavelengths o People (normal colour vision) also cannot match all wavelengths with only 2 wavelengths o Colour deficient people can match all colours of spectrum with 2 wavelengths o Red + green + blue = white light o Red + green = yellow  Equal activity of red and green cones and not much activity of blue cones  Metamers look the same because they each have the same activation of the 3 cone receptors (perceptually identical, physically different) Optical Light Mixing: Seurat  Farther back you are, you see different colours  Create colours you cannot make via paint by manipulating light (Dots of colours if you are close, when far can see colours that cannot be created with paint mixtures) Neuropsychological Evidence: 3 Types of Cones: Relative Number of Each Type of Cone  3 different photochemicals  3 cones o retinal + opsin but opsin differs (slight difference in amino acid sequence)  Photochemical eryfarolabe (red catching chemical) o Receives long wavelength best  Photochemical chlorolabe (green catching chemical) o Receives medium wavelength best  Photochemical cyanolade (blue catching chemical) o Receives short wavelength best o Relatively uncommon compared to red and green cones o Why?  Chromatic aberration: light of different colours through a lens, bends differently  Red, green have close focal points and blue light has different focal point that falls in front of green an red  Blue light is always slightly out of focus  Lens will bend different colours differently  Blue wavelengths will bend more Positioning of Different Types of Cones in the Retina  In center of fovea (fine detail), don’t want any blue cones (will make things slightly out of focus) o Therefore have no blue cones, just red and green cones  Right outside fovea, will have blue cones  Colour perception is different in different parts of your eye o No colour receptors in periphery Colour Deficiencies: Trichromats (NORMAL COLOUR VISION)  Don’t need 3 cone receptors to see colour rd o 3 cone receptor allows us to see more colours in the spectrum Dichromats (most common, colour deficient)  Only have and can only produce 2 different kinds of photochemicals therefore can only pick up 2 out of 3  Sometimes people can go through entire life without knowing they are colour deficient o E.g. Dalton  getting Ph.D. and wore scarlet suit, thought it was a nice colour o Could not see cherries  Not as if you can’t see them, just cannot see the differences between them (cannot distinguish)  Ishihara Colour Plates  way to test dichromats o Test questions people will get correct if colour blind and not colour blind  Independent of light intensity  can recognize a difference in colour regardless of light  In colour matching experiments  needs 2 wavelengths to recreate any colour in spectrum  Protanopia and deuteranopia are most common, inherited through chromosome X (sex-linked, more common in males than females)  Protanopia o Effects 1% males and 0.02% females o Missing the red cone o Unilateral dichromats  colour deficient in only 1 eye  Help tell us how colour deficient people see the world  Very rare o If protanope range of blue and yellow and middle is neutral point  In midpoint is grey (492 nm)  Deuteranopia o Effects 1% males and 0.01% of females o Missing the green cone o See shades of blue to yellow but midpoint in slightly different place (498 nm) o Have trouble distinguishing red from green (both protanopes and deuteranopes)  Tritanopia o Most rare (0.002% males and 0.001% females) o Born without blue cones o See blue-y green to red and in midpoint see grey at about 570 nm o Have trouble distinguishing blue form yellow Monochromats (colour blind)  Cannot see colour at all can only see brightness  If in colour matching experiment  see black white and shades of grey, only need one wavelength to match any colour in spectrum  Usually hereditary, occurs in 10 / 1 million  Rod monochromats o Doesn’t have any cones (cones are colour cells) o Also have trouble seeing fine detail o Rods are also sensitive to light therefore often uncomfortable in daylight conditions  Cone monochromats o Have 1 type of cone (cannot make all 3 photochemicals) o Not uncomfortable in daylight conditions o If had only red cones, cannot see red or sometimes any colour  Because whole colour system is on relative response  If only had red cone, how would you know the difference between red and white? Red and yellow?  If only have red cones, yellow is created via equal activation of red and green therefore cannot see yellow  Can only see black white and shades of grey because
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