Vision IVaix1.uottawa.ca/~ccollin/PCLWebsite/Teaching_files/CLR_Slides10FoP11... · Colour Vision 1 Vision IV Chapter 11 in Chaudhuri 1 Overview of Topics 2 "Avoid vertebrates because

Colour Vision

1

Vision IVChapter 11 in Chaudhuri

1

Overview of Topics

2

"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

Overview of Topics

• Evolution of colour vision

• Physics of Colour

• Biological Foundations of Colour Vision

• Perceptual Aspects of Colour Vision

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

4

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

5

• 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

Functions of Colour Vision

6

Colour Vision Helps Segmentation

7

Colour Vision Helps Segmentation

7

Crouching Tiger

8

Crouching Tiger

8

Hidden “Dragon”

9

Hidden “Dragon”

9

Plants Take Advantage of Colour Vision

10


10


10

Classification

Frugivorous monkeys use colour vision to pick out ripe fruit from green foliage

Folivorous monkeys usecolour vision to pick out new red leaves from old green ones

11

ClassificationColour is important in mate selection, especially amongst

birds and fish, where it is an indicator of male health.

12


birds and fish, where it is an indicator of male health.Hot

12


birds and fish, where it is an indicator of male health.Hot Not

12


birds and fish, where it is an indicator of male health.Hot Not

Among Zebra Finches, the ladies like a nice bright orange cheek patch

12

Evolution of Colour Vision

Due to different evolutionary pressures, different species have radically different colour vision capacities

(and radically different sensory capacities in general)

13




13




13

• What are some of the functions of colour vision?

• How would you explain “red” to a cat?

Questions

14

Colour Science

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

16

Light (reprise)

• A form of electromagnetic (EM) radiation (along with gamma rays, UV light, radio, etc.)

• EM radiation varies by:

• Wavelength

• Intensity

• Polarity

17

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

Wavelength

19

Wavelength

19

WavelengthLong λ (perceived as red)

Medium λ (perceived as green)

Short λ (perceived as blue)

19

Laser Lights

• 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

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 non-spectral colour experience

22

Nonspectral Colours

• 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)

23

• How many λs in a laser light? What about all other lights?

• What is a spectral colour? What about a nonspectral colour?

Questions

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

25






25






25

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

26





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

27

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

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

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

Metamers

• Any two lights with different λ contents that produce the same colour experience are called metamers

• Example: Yellow can be produced by either1) 600 nm light alone2) 550 nm (green) + 650 nm (red)

29

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)

30

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)

31





R=255 G=000 B=000

31





R=255 G=128 B=128

31





R=255 G=255 B=255

31





31

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

32

Inte

nsity

33

Example Intensity Spectra

34

Example Emission Spectra

35

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

36

• What’s an emission spectrum? What’s an intensity spectrum?

• What are metamers?

Questions

37

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

38

Body 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

39

Specular Reflection

• 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

40

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

41

Reflectance Spectra

Will a tomato ∴ always look red?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

43




Sunlight

43




Sunlight

43




Sunlight

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

44

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

45


• 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


• 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

• 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?

Questions

47

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”

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

49

Hue ≈ Spectral Position

Moving the colour signal up/down the λ spectrum changes the perceived hue

50

Saturation ≈ Spectral Width

Making the colour signal narrower increases saturation (vivid), making it wider decreases saturation (washed out)

. 1.2.3

51

Value ≈ Spectral Area

Making the colour signal larger in area makes the colour brighter/lighter

2 .Hue

52

The HSV Colour Space

53

The HSV Colour Space

53

• 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 _______.

Questions

54

Theories of Colour Vision

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.

56

• 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

Trichromatic Theory of Colour Vision

57

Colour-matching Experiment

Test field 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

400 nm

550 nm600 nm

525 nm

58

Colour-matching Experiment

Test field Comparison field

For dichromats with only two cones, only two wavelengths are needed to match the test field (these two greens look the same to them).

550 nm600 nm

525 nm

59

• 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)

Physiological Evidence for Trichromatic Theory

60

• 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

Opponent-Process Theory of Colour Vision

61

62 62

62 62

62

• Table 7.3 Results of afterimage and simultaneous contrast demonstration

63

• 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

Opponent-Process Theory of Colour Vision

64

• Figure 7.19 The three opponent mechanisms proposed by Hering.

65

• Figure 7.19 The three opponent mechanisms proposed by Hering.

65

• Researchers performing single-cell recordings found colour-opponent 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.

Physiology of Opponent-Process

66

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-).

67

• 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.

Synthesis

68

Questions

• What are some sources of evidence for trichromatic theory?

• What is some evidence for opponent process theory?

• Which one is correct?

69

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

70


• 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

71


72


Colour Signal

72


Colour Signal

72


73


73


73


73


73

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.

74

Physiological Basis of Metamers

75

Questions

• How do cones provide information about λ?

• Why do metamers happen?

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

77

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

78

Why Not 16 Cone Types?

"The eye has no sense of harmony in the same meaning as the ear. There is no music to the eye." -Helmholtz

79

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)

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.

81

With one photoreceptor type, two wavelengths of light might produce different outputs...

82

With one photoreceptor type, two wavelengths of light might produce different outputs...

82

...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

...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

85

Questions

• Why can one cone type not provide colour vision?

• What are the two basic dimensions of human colour vision?

86

• 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.

Colour Deficiency

87

L, M, & S subtypes(aka Protan, Deutan and Tritan)

• 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”)

Rod Monochromacy

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.

89

Dichromacy

• Three types:

• Protanopia: Missing L cones

• Deuteranopia: Missing M cones

• Tritanopia: Missing S cones

90

• 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: Protanopia

91

• 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

Dichromacy: Deuteranopia

92

• 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

Dichromacy: Tritanopia

93 94

94

Note error in text

Anomalous Trichromacy

• 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

95

Anomalous Trichromacy

• Comes in three types:

• Protanomaly (L cones): 1% ♂, .01%♀

• Deuteranomaly (M cones): 6% ♂, .4% ♀

• Tritanomaly (S cones): .01% ♂ & ♀

• No neutral point.

96

Pseudoisochromatic Plates

• Ishihara pseudo-isochromatic plate

• One method for testing for colour deficiency.

• Why is it made of bubbles of different shades?

97

Cap-arrangement Tests

• 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

Cap-arrangement Test

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 maleb) Being a hipsterc) Deuteranomaliad) All of the above

100

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?

101

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

Colour-Opponent Neurones

103

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)

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?

108

Colour Processing in Cortex

• Not fully understood

• There are colour opponent cells in V1 and beyond

• There are also double-opponent cells

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

110

Colour-Opponent Cells Don’t Respond at Isoluminance

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

112

Perceptual Aspects of Colour Vision

• Difference detection

• Colour contrast

• Colour constancy

113

Wavelength Discrimination Thresholds

114

Saturation Discrimination Thresholds

115

Colour Contrast

The background of an object (among many other things) can affect colour perception

116

Colour Contrast


116

Colour Contrast


116 117

117 117

• 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 long-wavelengths

• ∴ Objects reflect different wavelengths from these two sources

Colour Constancy

118

• 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).

119

The Mystery of the Green Sweater

• 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

Possible Causes of Colour Constancy

120

• 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

• Observer adapted - paper and observer in red light

Uchikawa et al.

121

122 122

122

• 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)

Uchikawa et al.

123

• “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

Possible Causes of Colour Constancy

124

Hue Depends on Intensity

125

Hue Depends on Intensity

125

Questions

• What are some possible causes of colour constancy?

• Which of these does the Uchikawa experiment demonstrate?

126

127

THE END (of the course!)

127

Documents

Vision IVaix1.uottawa.ca/~ccollin/PCLWebsite/Teaching_files/CLR_Slides10FoP11... · Colour Vision 1 Vision IV Chapter 11 in Chaudhuri 1 Overview of Topics 2 "Avoid vertebrates because