Vision IV Colour Vision Chapter 11 in Chaudhuri 1 1 Overview of Topics Overview of Topics "Avoid vertebrates because they are too complicated, avoid colour vision because it is much too complicated, and avoid the combination because it is impossible."!!!!advice given to his students by!!!!nobel-prize winner H. Keffer Hartline Evolution of colour vision Physics of Colour Biological Foundations of Colour Vision Perceptual Aspects of Colour Vision 2 3
Evolution of Vision Eyes evolved 500 Myr ago in the Cambrian Explosion Figure shows a sequence of increasingly complex eyes All exist in creatures today and are thought to be the precursors of advanced chambered eyes like ours Evolution of Multiple Cone Types Early fish ( 360 Myr ago) already had 4 cone types! Our nocturnal rat-like mammal ancestors had just 2, as do many modern mammals Some primates, including humans, have 3 Many birds and fish have 4 or more, probably aiding them mainly in mate selection 4 5 Functions of Colour Vision Aids in segmenting the scene into objects This makes it easier to find food/prey and avoid predators via camouflage breaking Helps to classify objects, often along subtle dimensions (ripe/unripe, healthy/unhealthy) Aids in feeding and mate selection Colour Vision Helps Segmentation 6 7
Colour Vision Helps Segmentation Crouching Tiger 7 8 Crouching Tiger Hidden Dragon 8 9
Hidden Dragon Plants Take Advantage of Colour Vision 9 10 Plants Take Advantage of Colour Vision Plants Take Advantage of Colour Vision 10 10
Classification Frugivorous monkeys use colour vision to pick out ripe fruit from green foliage Classification Colour is important in mate selection, especially amongst birds and fish, where it is an indicator of male health. Folivorous monkeys use colour vision to pick out new red leaves from old green ones 11 12 Classification Colour is important in mate selection, especially amongst birds and fish, where it is an indicator of male health. Hot Classification Colour is important in mate selection, especially amongst birds and fish, where it is an indicator of male health. Hot Not 12 12
Classification Colour is important in mate selection, especially amongst birds and fish, where it is an indicator of male health. Hot Not Evolution of Colour Vision Among Zebra Finches, the ladies like a nice bright orange cheek patch 12 Due to different evolutionary pressures, different species have radically different colour vision capacities (and radically different sensory capacities in general) 13 Evolution of Colour Vision Evolution of Colour Vision Due to different evolutionary pressures, different species have radically different colour vision capacities (and radically different sensory capacities in general) Due to different evolutionary pressures, different species have radically different colour vision capacities (and radically different sensory capacities in general) 13 13
Questions What are some of the functions of colour vision? How would you explain red to a cat? Colour Science 14 15 A Little Light Philosophy Is the colour in the object, or in the observer? A classic bad question. It is in both. Colour experiences result from an interaction between physical properties of objects and light, and the physiology of an observing organism Light (reprise) A form of electromagnetic (EM) radiation (along with gamma rays, UV light, radio, etc.) EM radiation varies by: Wavelength Intensity Polarity 16 17
Wavelength Wavelength Abbreviated λ (lambda) Measured in units of distance, such as angstroms or nanometers (nm, 1x10-9 ) Visible light (to humans) ranges from 400 nm to 700 nm Variations in wavelength give rise to colour experience, but relationship is complex. 18 19 Wavelength Wavelength Long λ (perceived as red) Medium λ (perceived as green) Short λ (perceived as blue) 19 19
Laser Lights Example Emission Spectra A laser emits a single wavelength, so is a nice simple model to look at first (analogous to a pure tone in hearing) Its colour appearance will vary across the spectrum with wavelength 20 21 Example Emission Spectra Colour Circle The colour circle shows the colour experience produced by each individual λ These are called spectral colours, as they can be produced by a single λ Purple is a special case, an example of a nonspectral colour. No single λ can produce a nonspectral colour experience 21 22
Nonspectral Colours Questions Spectral colours can be produced by a single λ Non-spectral colours can only be produced by a mixture of λs. Examples: Purple: Mix 400 nm and 700 nm lights Pink: Mix 460 nm and 600 nm (white) with a bit of 700 nm (red) How many λs in a laser light? What about all other lights? What is a spectral colour? What about a nonspectral colour? 23 24 Additive Colour Mixes A colour circle can also be used to show the appearance of a mix of λs Draw a line between the 2 λs being mixed If the λs are of equal intensity, the midpoint shows the appearance Otherwise, pick a point along the line at a proportional distance from the two components Additive Colour Mixes A colour circle can also be used to show the appearance of a mix of λs Draw a line between the 2 λs being mixed If the λs are of equal intensity, the midpoint shows the appearance Otherwise, pick a point along the line at a proportional distance from the two components 25 25
Additive Colour Mixes A colour circle can also be used to show the appearance of a mix of λs Draw a line between the 2 λs being mixed If the λs are of equal intensity, the midpoint shows the appearance Otherwise, pick a point along the line at a proportional distance from the two components Complementary Colours Any 2 λs across the centre from one another are complementary colours Mixing them in equal proportions produces an achromatic colour experience (i.e., black-grey-white) Achromatic colours are examples of nonspectral colours 25 26 Complementary Colours Complementary Colours Any 2 λs across the centre from one another are complementary colours Mixing them in equal proportions produces an achromatic colour experience (i.e., black-grey-white) Achromatic colours are examples of nonspectral colours Any 2 λs across the centre from one another are complementary colours Mixing them in equal proportions produces an achromatic colour experience (i.e., black-grey-white) Achromatic colours are examples of nonspectral colours 26 26
Full Spectrum White Sunlight and other full spectrum white light sources produce equal numbers of photons across the λs spectrum Thus they produce all the spectral colours mixed together One can think of this as all of the pairs of complementary colours mixed together, such that an achromatic colour experience occurs Full Spectrum White Sunlight and other full spectrum white light sources produce equal numbers of photons across the λs spectrum Thus they produce all the spectral colours mixed together One can think of this as all of the pairs of complementary colours mixed together, such that an achromatic colour experience occurs 27 27 Other Ways to Make White Fluorescents look white because they combine a few different λs that together are complementary (refer back to the emission spectrum) Any two sets of λs that produce the same colour experience are said to be metamers Thus, all pairs of complementary λs are examples of metamers, because they all produce white Other Ways to Make White Fluorescents look white because they combine a few different λs that together are complementary (refer back to the emission spectrum) Any two sets of λs that produce the same colour experience are said to be metamers Thus, all pairs of complementary λs are examples of metamers, because they all produce white 28 28
Metamers Any two lights with different λ contents that produce the same colour experience are called metamers Example: Yellow can be produced by either 1) 600 nm light alone 2) 550 nm (green) + 650 nm (red) Monitors & The RGB Colour Space Monitors produce colours by varying the intensity of three phosphors in each pixel The phosphors are red, green, and blue, abbreviated RGB Each one typically has 256 levels of intensity (0-255) 29 30 Monitors & the RGB Colour Space Each phosphor type emits a range of λs, no just one (they re not lasers) Monitors & the RGB Colour Space Each phosphor type emits a range of λs, no just one (they re not lasers) the colours they individually produce are in the body of the circle, not the edge The colours they can mix to produce are limited to those inside the RGB triangle (e.g., no laser green, and few orange shades) the colours they individually produce are in the body of the circle, not the edge The colours they can mix to produce are limited to those inside the RGB triangle (e.g., no laser green, and few orange shades) R=255 G=000 B=000 31 31
Monitors & the RGB Colour Space Each phosphor type emits a range of λs, no just one (they re not lasers) the colours they individually produce are in the body of the circle, not the edge The colours they can mix to produce are limited to those inside the RGB triangle (e.g., no laser green, and few orange shades) R=255 G=128 B=128 Monitors & the RGB Colour Space Each phosphor type emits a range of λs, no just one (they re not lasers) the colours they individually produce are in the body of the circle, not the edge The colours they can mix to produce are limited to those inside the RGB triangle (e.g., no laser green, and few orange shades) R=255 G=255 B=255 31 31 Monitors & the RGB Colour Space Each phosphor type emits a range of λs, no just one (they re not lasers) the colours they individually produce are in the body of the circle, not the edge The colours they can mix to produce are limited to those inside the RGB triangle (e.g., no laser green, and few orange shades) Not One λ, But Many Natural lights have a range of λs, each with its own intensity. Only laser lights approximate a single λ A light s subjective colour is determined (in part) by its intensity spectrum 31 32
Example Intensity Spectra Example Emission Spectra Intensity 33 34 Intensity Spectra of RGB Phosphors Additive Primary Colours Red, Green and Blue are known as the additive primaries Nearly any colour can be made by adding them in the right proportions 35 36
Questions What s an emission spectrum? What s an intensity spectrum? What are metamers? Colour & Objects To this point, we ve mostly discussed emitted lights But most light we see is reflected from objects As we ve discussed previously, there are two main types of reflection: Specular & Body 37 38 Body Reflection Specular Reflection Occurs when photons enter an object, and are dispersed by its molecules Light is bounced off in all directions as hemispheric wavefronts This is primarily responsible for the colour appearance of objects Occurs when photons bounce off the surface of an object Light is bounced off at angle opposite the angle of incidence This is primarily responsible for the degree of glossiness of an object 39 40
Reflectance Spectra Reflectance Spectra To precisely describe an object s reflection properties, we use a reflectance spectrum This shows the % of photons of each wavelength that are reflected To know what colour the object will appear to have, we also need to know the emission spectrum of the light source shining on it Will a tomato always look red? 41 42 Emission, Reflectance, & Colour Signal The photons that come off an object are the product of the object s reflectance spectrum and the illuminant s emission spectrum This, along with the physiology of the visual system, determines the colour appearance Emission, Reflectance, & Colour Signal The photons that come off an object are the product of the object s reflectance spectrum and the illuminant s emission spectrum This, along with the physiology of the visual system, determines the colour appearance Sunlight 43 43
Emission, Reflectance, & Colour Signal The photons that come off an object are the product of the object s reflectance spectrum and the illuminant s emission spectrum This, along with the physiology of the visual system, determines the colour appearance Sunlight Emission, Reflectance, & Colour Signal The photons that come off an object are the product of the object s reflectance spectrum and the illuminant s emission spectrum This, along with the physiology of the visual system, determines the colour appearance Sunlight 43 43 Subtractive Colour Mixing Rules of colour mixture are different when mixing coloured objects (e.g., dyes, inks, paints) This is because objects absorb photons rather than emitting Subtractive Colour Primaries The subtractive colour primaries are cyan, magenta and yellow Note how these are the secondary colours from additive colour mixing, and vice versa 44 45
Subtractive Colour Mixing Subtractive Colour Mixing Just as with additive colour mixing, one can add the subtractive primaries to produce a range of colours. However, it can be hard in practice to produce a deep black colour Therefore, printers tend to add black* and work in CMYK colour space Just as with additive colour mixing, one can add the subtractive primaries to produce a range of colours. However, it can be hard in practice to produce a deep black colour Therefore, printers tend to add black* and work in CMYK colour space * Note that black is denoted by K because B is taken (for blue in RGB colour space) 46 * Note that black is denoted by K because B is taken (for blue in RGB colour space) 46 Questions What s the relationship between emission spectrum, reflectance spectrum, and colour spectrum? If you mix all the paints in your paint set, what colour do you get? Why? A Perceptual Colour Space: HSV RGB and CYMK represent the physics of colour. A more psychologically-based colour space is HSV: Hue (red, orange, violet): What we normally think of when we say colour Saturation (red vs. pink): Purity, vividness High = vivid. Low = washed out Value (bright red vs. dark red): aka brightness or lightness or intensity 47 48
Relating the Physical & Perceptual Hue, saturation and value each relate to a characteristic of a light s energy spectrum (or colour signal ) Hue: Position along spectrum Saturation: Narrowness of spectrum Value: Total area of spectrum Hue Spectral Position Moving the colour signal up/down the λ spectrum changes the perceived hue 49 50 Saturation Spectral Width Making the colour signal narrower increases saturation (vivid), making it wider decreases saturation (washed out) Value Spectral Area Making the colour signal larger in area makes the colour brighter/lighter.3.2. 1 Hue 2. 51 52
The HSV Colour Space The HSV Colour Space 53 53 Questions The difference between blue and red is a difference in, while the difference between red and pink is a difference in. A laser light exhibits maximum. Theories of Colour Vision 54 55
Theories of Colour Vision Thesis: Trichromacy, there are three colour mechanisms. Antithesis: Colour-opponency; No, there are four colour mechanisms in opposing pairs Synthesis: Modern colour vision models show that both trichromacy and opponency are correct at different stages. Trichromatic Theory of Colour Vision Proposed by Young and Helmholtz (1800s) Suggested that three different receptor mechanisms are responsible for colour vision Behavioral evidence: Colour-matching experiments show that observers need three wavelengths to match a comparison field to a test field 56 57 Colour-matching Experiment Colour-matching Experiment 525 nm 400 nm 525 nm Test field 600 nm 550 nm Comparison field Test field 600 nm 550 nm Comparison field Observer adjusts the intensity of the three colours that combine to make the comparison field, trying to match the test field. It turns out this can be done with any three wavelengths in the comparison field 58 For dichromats with only two cones, only two wavelengths are needed to match the test field (these two greens look the same to them). 59
Physiological Evidence for Trichromatic Theory Researchers measured absorption spectra of visual pigments in receptors (1960s) They found pigments that responded maximally to: Short wavelengths (440nm) Medium wavelengths (530nm) Long wavelengths (560nm) Later researchers found genetic differences for coding proteins for the three pigments (1980s) Opponent-Process Theory of Colour Vision Proposed by Hering (1800s) Said that colour vision is caused by opposing responses generated by blue/yellow, green/red, and white/black. Behavioural evidence: Colour afterimages and simultaneous colour contrast show the opposing pairings Types of colour blindness are red/green & blue/yellow 60 61 62 62
62 62 Table 7.3 Results of afterimage and simultaneous contrast demonstration 62 63
Opponent-Process Theory of Colour Vision Opponent-process mechanism proposed by Hering Three mechanisms - red/green, blue/yellow, and white/black The pairs respond in an opposing fashion, such as positive to red and negatively to green (correct, except for white/black) These responses were believed to be the result of chemical reactions in the retina Figure 7.19 The three opponent mechanisms proposed by Hering. 64 65 Physiology of Opponent-Process Figure 7.19 The three opponent mechanisms proposed by Hering. Researchers performing single-cell recordings found colouropponent neurones in the retina, LGN and V1. These respond in an excitatory manner to one of four colours an inhibitory manner another of the four colours. Organized along red-green and yellow-blue axes (approximately). There are also achromatic (no colour preference) neurones in these areas. 65 66
Synthesis Responses of opponent cells in the monkey s lateral geniculate nucleus. These cells respond in opposite ways to blue and yellow (B+Y- or Y+B-) and to red and green (G+R- or R+G-). Both theories are correct. Each describes physiological mechanisms in the visual system Trichromatic theory explains the responses of the cones in the retina Opponent-process theory explains responses of colour-opponent neurones in retina, LGN and V1. 67 68 Questions What are some sources of evidence for trichromatic theory? What is some evidence for opponent process theory? Which one is correct? How Do Cones Provide Information About λ? Does each cone somehow transmit info about the λ of the photons it absorbs? No. A photoreceptor responds the same way when it absorbs a photon, regardless of that photon s λ This is known as the principle of univariance Thus, a receptor can only signal how many photons it has absorbed, not their λs 69 70
How Do Cones Provide Information About λ? For any given colour signal, the cones types will absorb different #s of photons Because of univariance, we must compare across cone types to get information about λ It is the relative differences in the firing rates of cones that provide information about λ That is, this is a cross-fibre coding scheme How Do Cones Provide Information About λ? 71 72 How Do Cones Provide Information About λ? How Do Cones Provide Information About λ? Colour Signal Colour Signal 72 72
How Do Cones Provide Information About λ? How Do Cones Provide Information About λ? 73 73 How Do Cones Provide Information About λ? How Do Cones Provide Information About λ? 73 73
How Do Cones Provide Information About λ? Physiological Basis of Metamers Cones don t provide perfect information about λ, or there would be no metamers Metamers occur because many patterns of λs can produce the same pattern of activation across cone types Some of trichromatic theory s strongest evidence was its ability to explain metamers. 73 74 Physiological Basis of Metamers Questions How do cones provide information about λ? Why do metamers happen? 75 76
Why 3 Cone Types? Monochromacy: 1 cone type provides no colour vision due to univariance Dichromacy: 2 cone types provides crude colour vision (red/green OR blue/yellow) Trichromacy: 3 cones types provides better colour vision (red/green AND blue/yellow) More cones provides better colour discrimination, but at the expense of acuity Why Not More Cone Types? More cones would provide better colour discrimination Might allow frequency analysis, as with sound But each receptor type would have low density, thus low acuity 77 78 Why Not 16 Cone Types? 1 Receptor Type Monochromacy: True colour blindness. All λs look the same hue. One receptor type cannot provide any information about λ independent of intensity Advantage is high acuity and/or sensitivity (e.g. rod system in humans) "The eye has no sense of harmony in the same meaning as the ear. There is no music to the eye." -Helmholtz 79 80
2 Receptor Types Dichromacy: Provides a single dimension of colour vision. Information about λ can be obtained by comparing S cone to L cone. Long λs will activate L cone more, while short λs will activate S cone more. With one photoreceptor type, two wavelengths of light might produce different outputs... 81 82 With one photoreceptor type, two wavelengths of light might produce different outputs......or they might produce the same outputs. It depends on the intensity (number of photons) at each λ. Thus, a one-cone system can t differentiate between intensity and wavelength differences. 82 83
...or they might produce the same outputs. It depends on the intensity (number of photons) at each λ. Thus, a one-cone system can t differentiate between intensity and wavelength differences. 83 84 3 Receptor Types Allows multiple dimensions of colour sensitivity In humans, red/green and blue/yellow Questions Why can one cone type not provide colour vision? What are the two basic dimensions of human colour vision? 85 86
Colour Deficiency Most humans are normal trichromats with S, M, and L cones having typical absorption spectra But other forms of colour vision exist in humans, including: Rod Monochromacy Cone Monochromacy Dichromacy Anomalous trichromacy Unilateral dichromacy. L, M, & S subtypes (aka Protan, Deutan and Tritan) Rod Monochromacy Extremely rare ( 1:33000) Only rods and no functioning cones Perceive only in white, grey & black tones True colour-blindness Poor visual acuity ( 20/150) Very sensitive to bright light ( day blind ) 87 88 Cone Monochromacy Extremely rare (incidence unknown) One type of cone (plus rods). 3 varieties: S, M, or L-cone monochromacy. Individuals have generally normal vision but can discriminate no hues. Three types: Dichromacy Protanopia: Missing L cones Deuteranopia: Missing M cones Tritanopia: Missing S cones 89 90
Dichromacy: Protanopia Missing L cone 1% of males and.02% of females! Individuals see short-wavelengths as blue Neutral point occurs at 492nm ( halfway between M and S cone peaks) Above neutral point, they see yellow Dichromacy: Deuteranopia Missing M cone 1% of males and.01% of females Individuals see short-wavelengths as blue Neutral point occurs at 498nm ( halfway between peaks of S and L cones) Above neutral point, they see yellow 91 92 Dichromacy: Tritanopia Missing S cone.002% of males &.001% of females Individuals see short wavelengths as green Neutral point occurs at 570nm ( halfway between peaks of M and L cones) Above neutral point, they see red 93 94
Anomalous Trichromacy Note error in text 3 cone types, but pigments are not isolated (e.g., L cones may have some M cone pigment in them, or vice versa) Need 3 λs to match any colour, but their proportions are different than for normal trichromats. Poorer hue discrimination 94 95 Anomalous Trichromacy Comes in three types: No neutral point. Protanomaly (L cones): 1%,.01% Deuteranomaly (M cones): 6%,.4% Tritanomaly (S cones):.01% & Pseudoisochromatic Plates Ishihara pseudoisochromatic plate One method for testing for colour deficiency. Why is it made of bubbles of different shades? 96 97
Cap-arrangement Tests Cap-arrangement Test Farnsworth-Munsell DM-15 cap arrangement test Task is to place coloured caps in a smooth progression of colours. Colour-deficient observers make characteristic patterns of mistakes 98 99 Questions If your friend seems to have terrible fashion sense, mixing orange pants with a green shirt, he most likely suffers from: a) Being male b) Being a hipster c) Deuteranomalia d) All of the above Physiological Basis For Colour Opponency Trichromacy is correct at the level of the retina: We have 3 cone types But behavioural evidence shows that we have 4 colour primaries in opponent pairs: Red/Green and Blue/Yellow How can this be? 100 101
Colour-Opponent Neurones Colour-Opponent Neurones Found in retina, LGN and V1, these neurones are excited by one range of λs and inhibited by another Four types exist: R+/G-, G+/R-, B+/Y-, Y+/B- How do these arise from the L, M, and S cones? 102 103 From Trichromacy to Opponency From Trichromacy to Opponency 104 105
Achromatic Cells While some RGCs are colour opponent, many are not These are the ON/OFF and OFF/ON cells we saw earlier These most likely add up L and M cone responses in both centre and surround (recall that S cones have little to do with luminance perception) From Trichromacy to Opponency 106 107 Questions What kinds of cones are found in the surround of a B/Y RGC s receptive field? What kinds of cones are found in the surround of an ON/OFF RGC receptive field? Colour Processing in Cortex Not fully understood There are colour opponent cells in V1 and beyond There are also double-opponent cells 108 109
The Isoluminance Problem Isoluminance means same luminance For instance, red and green stripes can be made to have the same luminance We can see the edge between such stripes, but how? Achromatic cells would not respond, and neither would colour opponent cells Colour-Opponent Cells Don t Respond at Isoluminance 110 111 Double-Opponent Cells In V1, cells are found that combine spatial and chromatic opponency For example, R+G-/G+R- cells are excited by red light on centre and green on surround Other configurations, exist: G+R-/R+G- B+Y-/Y+B- Y+B-/B+Y- Most are found in cytochrome oxidase blobs Perceptual Aspects of Colour Vision Difference detection Colour contrast Colour constancy 112 113
Wavelength Discrimination Thresholds Saturation Discrimination Thresholds 114 115 Colour Contrast Colour Contrast The background of an object (among many other things) can affect colour perception The background of an object (among many other things) can affect colour perception 116 116
Colour Contrast The background of an object (among many other things) can affect colour perception 116 117 117 117
Colour Constancy Perception of colours is relatively constant despite changing light sources Sunlight has approximately equal amounts of energy at all visible wavelengths Tungsten lighting has more energy in the longwavelengths Objects reflect different wavelengths from these two sources The Mystery of the Green Sweater Figure 7.24 The reflectance curve of a sweater (green curve) and the wavelengths reflected from the sweater when it is illuminated by daylight (white) and by tungsten light (yellow). 118 119 Possible Causes of Colour Constancy Chromatic adaptation - prolonged exposure to chromatic colour leads to: Receptors adapt when the stimulus colour selectively bleaches a specific cone pigment Sensitivity to the colour decreases adaptation occurs to light sources leading to colour constancy Observers shown sheets of coloured paper in 3 conditions: Baseline - paper and observer in white light Observer not adapted - paper illuminated by red light; observer by white Uchikawa et al. Observer adapted - paper and observer in red light 120 121
122 122 Uchikawa et al. Results showed that: Baseline - green paper is seen as green Observer not adapted - perception of green paper is shifted toward red Observer adapted - perception of green paper is slightly shifted toward red (i.e, partial colour constancy) 122 123
Possible Causes of Colour Constancy Hue Depends on Intensity Grey World assumption: Visual system may discount overall hue of scene. Evidence: Colour constancy works best when an object is surrounded by many colours Memory and colour: Past knowledge of an object s colour can have an impact on colour perception Memory for colour is poor, so we don t notice slight changes caused by illumination changes 124 125 Hue Depends on Intensity Questions What are some possible causes of colour constancy? Which of these does the Uchikawa experiment demonstrate? 125 126
THE END (of the course!) 127 127