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Chapter 5

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Matthias Niemeier

Chapter 5: The Perception of Colour Three Steps to Colour Perception 1. Detection: Wavelengths must be detected 2. Discrimination: We must be able to tell the difference between one wave length (or mixture of wavelengths) and another 3. Appearance: We want to assign perceived colours to lights and surfaces in the world. We want those perceived colours to go with the specific object and not change dramatically as the viewing conditions change Step 1: Color Detection - We have 3 types of cone photoreceptors which differ in the photopigment they carry resulting in different sensitivities to different wavelengths - We have only 1 type of rod photoreceptor which peaks at about 500 nm - S-Cones: A cone that is preferentially sensitive to short wavelengths; colloquially (but not entirely accurately) known as the “blue cone”. Picks up wavelengths at about 420 nm - M-Cones: A cone that is preferentially sensitive to middle wavelengths; colloquially (but not entirely accurately) known as the “green cone”. Peak at about 535 nm - L-Cones: A cone that is preferentially sensitive to long wavelengths; colloquially (but not entirely accurately) known as the “red cone”. Peaks at about 565 nm - Photopic: Referring to light intensities that are bright enough to “saturate” the rod receptors (that is, to drive them to their maximum responses). Cones work at Photopic (daylight) levels. - Scotopic: Referring to light intensities that are bright enough to stimulate rod receptors but too dim to stimulate the cone receptors Step 2: Colour Discrimination The Problem of Univariance: The fact that an infinite set of different wavelength-intensity combinations can elicit exactly the same response from a single type of photoreceptor. One photoreceptor type cannot make colour discriminations based on wavelength - Ex. Lights of 450 and 625 nm each elicit the same response from the photoreceptor - Univariance explains the lack of colour in dimly lit scenes. There is only 1 type of rod photoreceptor with only 1 type of photopigment, rhodopsin. Thus they all have the same sensitivity to wavelengths, and therefore can’t discriminate between colours - We can detect differences between wavelengths or mixtures of wavelengths because we have more than 1 kind of cone photoreceptor Trichromatic Theory of Colour Vision (or Trichromacy): The theory that the colour of any light is defined in our visual system by the relationship between 3 numbers – the outputs of 3 receptor types now known to be the 3 cones. Also known as the Young- Helmholtz Theory - M cones, S cones, and L cones may all respond more or less intensely to different wavelengths and therefore be the cause of colour differentiation Metamers: Different mixtures of wavelengths that look identical. More generally, any pair of stimuli that re perceived as identical in spite of physical differences - Usually we do not see single wavelengths, almost everything we see emits a combination of wavelengths - Example picture on pg 121 - Ex. if you put a red and a green light on a piece of paper so they mix Say for the red light your M cones may produces 80 units of activity, 40 units in the L cones, etc. Then for the green light your M cones produce 40 units and the L cones produce 80 units. The amount of units in total for the M cones and L cones are the same, although the units are distributed differently between the 2. So these colours may in turn come together and make for example, yellow. So if 2 sets of lights produce the same responses, they are metamers and must look identicle in terms of response units *Mixing wavelenths of light does not change the physical wavelengths (Ex. 400nm of light mixed with 450nm of light does not make a wavelength of 850nm or the average of the two. They are still distinct and separate entities) * Maxwell discovered that only 3 mixing colours of light at different intensities are needed to match any one colour. Therefore 3 different colour mechanisms (primary colours) limit the human experience of colour Additive Colour Mixture: A mixture of lights. If light A and light B are both reflected from a surface to the eye, in the perception of colour the effects of those 2 lights add together Subtractive Colour Mixture: A mixture of pigments. If pigments A and B mix, some of the light shining on the surface will be subtracted by A, and some by B. Only the remainder contributes to the perception of colour - Explains why even though red and green in terms of wavelengths can make yellow, when you paint red and green together it makes brown *if all wavelengths for all 3 colours are equal it produces white light From Retina to Brain: Repackaging the Information - The nervous system computes 2 differences. L-M and [L+M]-S - Combining L and M signals give information about light intensity - The difference between L and M tell the differences between amount of blood in the skin (therefore people can see blushing and paleness) Cone-Opponent Cells in the Retina and LGN Lateral Geniculate Nucleus (LGN): 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 - Have receptive fields with center-surround organization - Some retinal and ganglion cells are excited by the L-cone onset in their center and inhibited by M-one onsets in their surround - 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 - The retina and LGN contain cells that have repackaged wavelength information into cone-opponent difference signals that constrain our ability to see differences between regions Step 3: Colour Appearance - Because we have 3 different types of cone photoreceptors, light reaching any part of the retina will be translated into 3 responses, one for each population of cones - If the light rays reflecting off 2 surfaces produce the same set of cone responses, the 2 surfaces must and will appear to be exactly the same colour (they will be metamers) Colour Space: The three-dimensional space, established because colour perception is based on the outputs of 3 cone types, that describes the set of all colours Achromatic: Referring to any colour that lacks a chromatic (hue) component. Black, white, or gray Hue: The chromatic (colourful) aspect of colour (red, blue, yellow, and so on) Saturation: The chromatic strength of a hue. - Ex. White has zero saturation, pink is more saturated, and red is fully saturated Brightness: The perceptual consequence of the physical intensity of a light Opponent Colours Opponent Colour Theory: The theory that perception of colour is based on the output of 3 mechanisms, each of the resulting from an opponency between 2 colours: red vs green, blue vs yellow, and black vs white - Ex there is no such thing as a reddish-green or bluish-yellow because those colours oppose each other - Unique Hue: Any of four colours that can be described with only a single colour term: red, yellow, green, blue. Other colours (e.g. purple or orange) can be described as compounds (reddish blue, reddish yellow). - Hugh Cancellation: To attempt to determine how much of one opponent colour is needed to eliminate the other opponent colour Colour in the Visual Cortex - Cells in the LGN seem to be cone opponent cells Double-Opponent Cells: A cell type, found in the visual cortex, in which one region is excited by one cone type, combination of cones, or colour and inhibited by the opponent cones or colour (eg. R+/G-). Another adjacent region would be inhibited by the first input and excited by the second (thus in this example, R-/G+) - Conveys information about chromatic edges Single-Opponent Cell: Another way to refer to
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