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colour perception (2).docx

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McMaster University
Gautam Ullal

November 12 , 2012 Psych 2E03: Sensory Processes Colour Perception (2) Theories of colour vision and Colour Blindness - Trichromatic - opponent Trichromatic Theory of Colour Vision - ratio of activity of three receptors each sensitive to different wavelength of light S (short), M (middle) and L (long) wavelength sensitive cones as a function of wavelength - three types of receptors - rhodopsin and iodopsin: opsin protein are different in rod and cones, which is what allows to see blue, green, and red Evidence for trichromatic theory - colour pigments: amino acid substitutions in the opsin molecules - slight replacements or substitutions - more pink dots is more differences - closer in the spectrum there is less differences Distribution of colour-specific cones on retina - adaptive optics images: by using this technic we can see the distribution in the retina - dilate pupils, use laser to shoot against retina and rebound, what comes back is picked up by a sensor to tell us what type of absorbance is there - if you compare one human to another there is no definite ratio - there are more cones for medium wavelength than long wavelength, mainly seen near the fovea - s-types least in number and mainly in periphery Adaptive optic-images of green and red colour blind Ishihara plate for testing colour deficiency - person with normal colour vision sees a 74 when the plate is viewed uder standardized illumination - ishihara plate as perceived by a person with a form of red-green colour deficiency Normal Colour vision (trichromat) - only stimulations Vision in Protanopia - absence of pigment for long wavelength - most common missing pigment is red - complement colours: red and green - sees in blue, yellows and greys Vision in Deuteranopia - absence of pigment for middle wavelength - blue, yellow, and greys Chromosome for individual opsin (human) - rhodopsin: chromosome 3 - cyanolabe: chromosome 7 - chlorolabe and erythrolabe: X chromosome - while gene for cyanolabe is on different chromosome (an autosome) - genes for erythrolabe and chlorolabe are juxtaposed on X chromosome (98% identical) - gene for green and red pigment is on the same chromosome (same arm) and the neucleotides have significant overlap (juxtaposed) - therefore its not surprising that some people lean more towards red or green which leads to colour weakness Inheritance of colour defects - M/L colour defects: X linked recessive (most common of the colour defects) - S colour defects: autosomal dominant (very rare) chromosome 7 - Achromatopsia (no colours perceived): autosomal recessive (very rare), also called complete rod monochromatism, no cones - Further, the most common M/L colour defect is protanomaly (deficiency of red-cones) not protanopia (absence of red cones) patients confuse red for green - Protanomaly: tend to seem some red as green or green as red Colour vision in monoch
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