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C82COG - Perception > Human colour perception > Flashcards

Human colour perception Flashcards

1
Q

Why is colour information important to the visual system?

A
  • Aids the discrimination and detection of objects
  • Assessment of the suitability of food to eat
  • Scene segmentation
  • Aids visual memory
  • Provides an aesthetic component to vision
  • Used in nature to signal mating availability or for camouflage.
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2
Q

Why do objects appear coloured?

A

Because they reflect different wavelengths of light from different regions of the visual spectrum – colour is a property of the neural apparatus which detects the reflected light.

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3
Q

What frequencies is the visual light spectrum?

A

380-750nm

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4
Q

What does colour perception require?

A

The correct photoreceptors and neurons – without these we get very different impressions of colour.

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5
Q

How does perception of colour arise?

A

From the ability of certain light rays to evoke a particular pattern of neural responses in our eye and visual cortex.

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6
Q

Define hue.

A

The quality that distinguishes red from blue, etc.

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7
Q

Define brightness.

A

The perceived intensity of light

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8
Q

Define saturation.

A

The characterisation of colour as ‘pale’ or ‘vibrant’.

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9
Q

Define metamers.

A

Sensory stimuli which are physically different but perceptually equivalent.

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10
Q

What are colour metamers?

A

Physically different combinations resulting in perceptually identical colours.

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11
Q

Give an example of a colour metamer.

A

An orange light is indistinguishable from a yellow and red light combined.

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12
Q

Why do metamers look the same?

A

The visual system is generating identical neural responses to visual stimuli despite them being physically very different – the physical property which makes them different is not being encoded in the visual system.

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13
Q

Where does the connection between the physical properties of light and our perception of colour most likely lie?

A

In the photoreceptors in the retina, the first stage in the processing of visual information.

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14
Q

Why do we look at the properties of cones rather than rods to determine how we perceive colour?

A

Rods are colour-blind – we don’t perceive colour in scotopic/dark conditions.

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15
Q

What are photopigments?

A

The parts of the photoreceptors that absorb light.

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16
Q

What parts of the cones, specifically, are involved in colour perception?

A

Photopigments.

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17
Q

What is represented in a photoreceptor’s spectral sensitivity function, or scotopic luminosity curve?

A

The probability it will absorb photons at different wavelengths.

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18
Q

Hypothetically, if a photoreceptor had a single photopigment which absorbs about 25% of light at wavelength A (λA) and 50% at λB, what kind of neural response would it have if the intensity of λA and λB were the same?

A

A differential response – different levels of absorption.

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19
Q

Hypothetically, if a photoreceptor had a single photopigment which absorbs about 25% of light at wavelength A (λA) and 50% at λB, what kind of neural response would it have if the intensity of λA was 2x the intensity of λB?

A

An identical neural response to both, as they create the same level of absorption.

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20
Q

What is the principle of univariance?

A

Any single photopigment is “colour blind” since an appropriate combination of wavelength and intensity can result in an identical neural response.

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21
Q

What are individuals with a single pigment called, and what do they experience?

A

Monochromats, they experience everything in shades of grey.

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22
Q

How do we make the crucial differentiation between wavelength and intensity?

A

Through a comparison of signals from two or more cone classes, each with a different spectral sensitivity.

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23
Q

How does the number of cone classes affect wavelength discrimination?

A

As a general rule, the more cone classes, the better the discrimination.

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24
Q

What is dichromacy?

A

Having 2 cone types (many non-primate mammals who rely heavily on sound and smell).

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25
Q

What is pentachromacy?

A

Having 5 cone types (many birds who rely heavily on vision).

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26
Q

How many cone types do humans have, and what is this called?

A

3 - trichromacy.

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27
Q

What are the three different cone types in humans?

A

Blue/short, green/middle, and red/long.

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28
Q

What are the maximum wavelength absorptions for S, M and L sensitive cones?

A

S - 420nm
M - 530nm
L - 565nm

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29
Q

Who proposed the trichromatic theory of vision?

A

Thomas Young in 1802, long before we were able to make any direct measurements of spectral absorption.

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30
Q

What was proposed by the trichromatic theory of vision?

A

That humans had 3 types of photoreceptor (cone) each sensitive to a different part of the visual spectrum.

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31
Q

What method is used to directly measure spectral absorption?

A

Microspectrophotometry.

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32
Q

What did Young base his theory on?

A

The fact that he could produce a wide range of colours by mixing three primary colours.

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33
Q

Who later quantified the trichromatic theory, and what is it now known as?

A

Herman von Helmholtz , the Young-Helmholtz theory .

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34
Q

What fundamental principles of colour perception did Helmholtz’s studies define?

A
  1. Normal human observers have three cone photoreceptors which have a broad spectrum tuning to long (L), medium (M) and short (S) wavelengths.
  2. The balance of neural activity in each of these receptors is sufficient to represent the vast array of natural colours we encounter.
35
Q

Describe the retinal topography of the 3 cone classes in terms of comparative quantities.

A

There are much fewer S cones (blue) than L (red) or M cones (green)

36
Q

What cone class is absent from the fovea?

A

S cones.

37
Q

What cone classes are randomly distributed within the mosaic?

A

L and M.

38
Q

What is a common feature of cone type distribution?

A

“Clumping” of cones of a single type.

39
Q

Describe individual differences in the geographical layout of L and M cones.

A

Very large - some individuals have an L:M ratio of 4:1 whilst others show approx. equal amounts of each cone type.

40
Q

What did Roorda and Williams (1999) do?

A

Adapted the eye to intense lights to selectively stimulate each of the photopigments and then imaged them using a high performance optical system, in order to get the first images of the 3 cone types in the living human retina.

41
Q

What did Roorda and Williams (1999) find?

A

That the pattern of trichromatic cones differs greatly in each individual.

42
Q

What is the difference between chromatic and achromatic vision?

A

Chromatic is colour, and achromatic is luminance.

43
Q

What did Hering’s colour-opponent theory suggest?

A

Red opposes green, and blue opposes yellow. Reflected in colour-opponent ganglion cells.

44
Q

Describe the formation of chromatic pathways.

A

m(g), l(r), and s(b) cones combine their output in colour opponent parvocellular retinal ganglion cells - the colour opponent stage.

45
Q

What kind of receptive fields do parvocellular retinal ganglion cells have?

A

Chromatically opponent (excitatory centre and inhibitory surround). Also cells which respond to a certain wavelength anywhere in the receptive field.

46
Q

Describe an example receptive field of a colour opponent ganglion cell.

A

RED On-centre, GREEN Off-surround (centre stimulated by red light, surround stimulated by green light). Either red/green or blue/yellow.

47
Q

What are the chromatic pathways?

A

‘M-L’ and ‘S-(M+L)’.

48
Q

What is achromatic information important for?

A

Fine detail.

49
Q

How is luminance encoded differently to colour?

A

Colour is opposed (encodes differences), luminance input is summed.

50
Q

What is the achromatic pathway?

A

‘M+L’.

51
Q

Give an example of how achromatic and chromatic pathways work together.

A

White light stimulates all cone types equally, and therefore produces no net response in chromatic pathways. However achromatic pathways sum their input, so they respond well.

52
Q

What different retinal streams are the chromatic and achromatic pathways linked with?

A

Chromatic = parvocellular, achromatic = magnocellular, but also parvocellular.

53
Q

Where do the colour opponency and trichromacy stages occur?

A

Colour opponency occurs in the retinal ganglion cells, trichromacy at the receptors.

54
Q

How are the layers of the lateral geniculate nucleus linked to chromatic opponency?

A

Layers 1 and 2 of the LGN receive their input from M RGCs (achromatic luminance channel)
Layers 3-6 receive their input from P RGCs (chromatic pathways)

55
Q

What kind of stimuli do nearly all cells at the level of the LGN prefer?

A

Stimuli that are moderated along the cardinal directions or axes of colour space.

56
Q

What are the axes of colour space?

A
  • Red-green (0-180 hue angle)

- Blue/yellow (90-270 hue angle)

57
Q

What do cortical cells (in the visual cortex) show preference for?

A

Cortical cells show preferences for a wide range of hue angles, unlike the LGN.

58
Q

The fact that tuning width remains fairly constant across cortical areas V1, V2 and V3 is used as evidence for what?

A

The synthesis of multiple chromatic mechanisms at cortex level.

59
Q

Describe the double opponent receptive fields which a proportion of cortical cells have.

A

The centre is excited by long wavelengths and inhibited by middle wavelengths, whilst the surround is excited by middle wavelengths and inhibited by long wavelengths.

60
Q

What is the common definition of colour selectivity?

A
  1. Cells that add L and M cone inputs are called luminance cells
  2. Cells which subtract L, M or S cone inputs are called colour cells
61
Q

Under the common definition of colour selectivity, about what proportion of cells are colour sensitive?

A

50% of cells in V1, similar proportions in V2, V3 and V4.

62
Q

Define colour constancy.

A

The ability to assign a fixed colour to an object despite the fact that the wavelength composition entering the eye changes under different luminance conditions .

63
Q

Define chromatic induction.

A

The change in perceived colour in different luminance conditions, despite the colours being the same.

64
Q

What makes different colours look the same in different luminance conditions?

A

The chromatic difference between the colour and the background - if this is the same, they appear to be the same colour.

65
Q

What cells contribute to colour constancy and induction?

A

Double opponent colour cells in the cortex - they signal chromatic contrast differences.

66
Q

What can neurons with larger receptive fields, such as V4, do in terms of colour constancy and induction?

A

Compute an average colour across an extended region of space.

67
Q

Are defects of colour vision congenital, or acquired?

A

Can be either.

68
Q

How do individuals with congenital colour deficiencies’ visual systems differ?

A

Although they have normal cone numbers, they perform on colour tasks as though they have fewer photopigments available (e.g. dichromats instead of trichromats).

69
Q

What cone types do congenital defects mostly affect?

A

M or L rather than S.

70
Q

What is confused when M and L cones are missing?

A

Middle and long wavelengths.

71
Q

What is the lack of M or L cone types commonly referred to as?

A

Red-green dichromacy/deficiency.

72
Q

How is discrimination affected when S cones are missing?

A

Discrimination is poor at short wavelengths but normal at middle and long.

73
Q

What channel is unaffected in people with missing S cones, and why?

A

The achromatic channel, because the achromatic pathway is M+L.

74
Q

What is the lack of S cones commonly referred to as?

A

Blue/yellow dichromacy (rare)

75
Q

What is anomalous trichomacy?

A

Abnormal absorption properties of a photopigment type.

76
Q

How does the prevalence of all types of colour deficiency vary among populations?

A
  • 8% Caucasian males
  • 5% Asiatic males
  • 3% African males
  • Incidence a lot lower in females
77
Q

Why is incidence of colour deficiency much lower in females?

A

Sex-linked inheritance - the defect is located on a particular portion of the X-chromosome.

78
Q

What is a neutral point?

A

The point at which a particular wavelength of light cannot be distinguished from sunlight (containing roughly equal amounts of all wavelengths).

79
Q

What is the existence of a single neutral point indicative of?

A

A two photopigment/dichromatic system.

80
Q

What pigment is missing in protanopia?

A

L.

81
Q

What pigment is missing in deutranopia?

A

M

82
Q

What pigment is missing in Tritanopia?

A

S

83
Q

What is the most common test of colour blindness?

A

Ishihara Isochromatic Plates

84
Q

How does the Ishihara Isochromatic Plates test work?

A
  • Each dot differs in its hue and not its luminance
  • Unless an individual can distinguish between the colours (hues) of the dots defining the number and those of the surround then the number will appear invisible.

C82COG - Perception (9 decks)

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