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Chapter 4

Chapter 4

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University of Toronto St. George

Chapter 5 The Perception of Colour • The apparent colour of a bit of the world is correlated with the wavelengths of light reaching the eye from that bit of the world – Most of the light we see is reflected light • Some surfaces will absorb the light that hits them o The more light absorbed, the darker the surface appears • Like pain, color is the result of the interaction between physical stimulus with a particular nervous system • Colour is created by our minds – it doesn’t actually exist • Colour helps us localize objects 3 Steps to colour perception - DDA – Detection, Discrimination and Appearance 1. Detection – wavelength must be detected • We have 3 types of cone receptors, these receptors differ in the types of photo pigment they’re sensitive to o S (short-wavelength cones- blue) 420nm o M (medium- wavelength cones- green) 535 nm o L (long-wavelength cones- red) 565 nm • These cones detect photopic light 2. Discrimination – we must be able to tell the difference between one wavelength (or a mixture of wavelengths) and another • The problem with univariance – The fact that an infinite set of different wavelengths- intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make color discrimination based on wavelength o Univariance explains the lack of colour in dimly lit spaces (we struggle to discriminate colours) because we only have one type of rod • The trichromatic solution – The theory that the color of any light is defined in our visual system by relationships of three numbers – the outputs of three receptor types now known to be the three cones (Young- Helmholtz Theory) Uses different cones to respond to different wavelengths and intensities of light • Metamers – Different mixtures of wavelengths that look identical. More generally, any part of stimuli that are perceived as identical in spite of physical differences – The key point is that the nervous system only knows what the cones tell it – If the mixture of “red” plus “green” lights produce the same cone output as the single wavelength of “yellow” light, then the mixture and the single wavelength will look identical • Maxwell’s Colour matching experiment – three mixing lights are needed to match any reference light • Additive Colour mixing – a mixture of lights. If light A and B are reflected from the surface to the eye, in the perception of color the effects of those two lights add together • Subtractive Colour mixing – a mixture of pigments. If pigments A and B mix, some of the light shinning on the surface will be subtracted by A and B. Only the remainder contributes to the perception of colour. • “Yellow” is equivalent to a mix a long wavelength and a medium wavelength (ex) red and green • A single photoreceptor shows different responses to lights of different wavelengths (ex) S- cones peak when exposed to blue light but not when exposed to green light • Neural basis for light discrimination – o We compute the differences between cone responses to transmit information to the brain  The nervous system computes the differences (L–M) and ([L+M] – S)  (L–M) discriminates information about colour  Combining L and M is a good measure of the intensity of light o Cone-opponent cells in the retina and LGN  Lateral Geniculate Nucleus – A structure in the thalamus, part of the midbrain, that receives input from the retinal ganglion cells and has input and output connections to the visual cortex  Cone-Opponent Cell – A cell type, found in the retina, LGN and visual cortex that in effect, subtracts one type of cone input from another 3. Appearance – We want to assign perceived colors to lights and surfaces in the world and we want those perceived colors to go along with the object (blood is red) and not to change dramatically as the viewing conditions change (blood should remain red in the sun and a shadow) Metamers • If you
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