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PPA > 2 - DEFICIENCIES IN COLOUR VISION > Flashcards

2 - DEFICIENCIES IN COLOUR VISION Flashcards

1
Q

what causes colour blindness?

A
  • problem with the cone receptors (almost always)
  • ‘one or more of the three types may be missing from the retina’
  • or they could have different sensitivities than the ‘normal’ cones
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2
Q

type A

A

1 of 3 cones MISSING

  • 3 possibilities (L, M or S missing)
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3
Q

type B

A

2 of 3 cone types missing

  • 3 possibilities (only L, M or S present)
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4
Q

type C

A

all 3 cone types missing

  • 1 possibility
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5
Q

so overall 7 potential types of colour blindness due to missing cones

A
  • but only 5 of the 7 have ever been observed
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6
Q

dichromats

A
  • people who have 2 cone types
  • 2% of male population (less for females)
  • common in other mammals (blue and green)
  • primates are usually trichromatic
  • ‘can still see thousands of shades of all the different colours’
  • as only require two primaries
  • ‘no implication that red or blue would be absent from their experience. this is because colours are properties of conscious experience, not properties of the world’
  • ‘you need two or more different photoreceptors in order to distinguish wavelengths’
  • ‘the wavelengths would causes different sensitivities in the cones and therefore a different response pattern’
  • this pattern doesn’t change with intensity - it’s about the ratio
  • cannot be distinguished if the wavelengths produce the same/very similar ratio responses
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7
Q

protanopia (dichromat)

A

L-cones are missing

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8
Q

deuteranopia (dichromat)

A

M-cones are missing

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9
Q

tritanopia (dichromat)

A

S-cones are missing

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10
Q

monchromats

A

only have one cone type or none

  • truly colourblind
  • ‘congenital achromatopsia’
  • cannot make any discriminations between visual stimuli based on wavelength
  • principle of univariance means one single cone type is not sufficient for making discriminations between stimuli based on wavelength
  • cannot know what colour they see in
  • cone monochromats - may be able to see more than one colour (maybe blues, yellows and greys) but cannot know for sure
  • less than 1 person in a million in european populations
  • photophobic in daylight illumination
  • ‘can only see at light levels when the rods are active (scotopic and mesopic conditions)’
  • ‘normal people do not see colour in scotopic conditions’
  • ‘cone monochromats will have their blue cones active in mesopic conditions and some report seeing shades of blue and yellow (but we have no idea what they actually experience)’
  • only very few animals (seals, owl monkeys, raccoons)
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11
Q

rod monochromacy

A

people who have no cones and only rods

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12
Q

cone monochromacy

A

people who have only s-cones and rods

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13
Q

anomalous trichromats

A

colour blindness due to abnormal insensitivity

  • theoretically 7 types as well but only 3 have been observed
  • they have the three cone types but one of them differs in sensitivity
  • the cones work normally, but the sensitivities are too close together for colours to be discriminated
  • most common form of colour blindness
  • 6% of males
  • less than 1% of females
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14
Q

anomolous protanopia (protanomaly)

A

L-cones with abnormal spectral sensitivities

  • red moves to green
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15
Q

anomolous deutranopia (deuteranomaly)

A

M-cones with abnormal spectral sensitivities

  • green moves to red
  • most common form
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16
Q

anomolous tritanopia (tritanomaly)

A

S-cones with abnormal spectral sensitivities

  • blue moves to green? i think
  • the rarest form
17
Q

colour blindness does not make someone incapable of experiencing colour

A

it does make ‘it impossible for you to distinguish visual stimuli based on the wavelengths of light they contain that colour-normal people can distinguish on this basis’

‘it limits a persons ability to distinguish between stimuli’

‘many colourblind individuals have no idea that they have any kind of deficiency’

18
Q

red-green colour blindness

A
  • the most common
  • dichromats lack either l-cones (protanopia) or m-cones (deuteranopia)
  • difficulty distinguishing red and green
  • ‘impairs ability to discriminate between stimuli based on wavelength’
  • ‘it doesn’t limit ability to experience different colours’
19
Q

what is the most common form of colour blindness?

A

anamolous trichromacy

20
Q

diagnosis of colour blindness

A
  • developed by Dr Shinobu Ishihara at the University of Tokyo in 1917
  • very easy to use and very effective in diagnosis
  • the circles with coloured circles representing numbers in different colours
  • a red-green colourblind person can usually tell there are different colours on the plate, but either cannot or has difficulty identifying the figure

Ishihara test plates

21
Q

what careers does colourblindness affect

A

technicians, interior designers and aircraft pilots

22
Q

congenital colour blindness

A

born with it and it’s due to genetics

affects cones

  • it is possible to have damage from disease or chemical poisoning
23
Q

colour blindness as a result of brain damage

A
  • damage to the visual pathway (thalamus and visual cortex)

achromatopsia:

  • absence of colour vision
  • similar to that of monochromats

thalamic achromatopsia:

  • due to damage to the thalamus
  • LGN

cerebral achromatopsia:

  • cortical damage (V1,V2,V4)
  • occipital lobe (V4)
  • typically see in grey/bluish grey

dyschromatopsia:

  • ‘ability to make discriminations based on wavelengths is impaired’
  • colour looks weaker and paler
  • residual colour vision
24
Q

colourblindness

A

= ‘limited ability to use the spectra of images to obtain information about surfaces and light sources’ and discriminate wavelengths

25
Q

chickens

A 
300-710nm 
wider range 
near infrared 
can discriminate between more colours 
tetrachromatic
26
Q

phenomenology

A

‘study of structures of consciousness as experienced from the first-person point of view. The central structure of an experience is its intentionality, its being directed toward something, as it is an experience of or about some object.’

27
Q

how common is colour blindess

A

caucasian population

  • 8% of males
  • 0.5% of females
28
Q

morgan et al (1992)

A

dichromats performed better at colour camouflage tasks than trichromats (colour normal)

suggests that dichromats had this advantage as colour can interfere with differentiation based on the texture of an area, therefore dichromats were less susceptible to this interference

29
Q

doron et al (2019)

A

anomalous trichromats have ‘superior basic visual functions’

  • visual acuity (VA)
  • contrast sensitivity (CS)
  • stereo acuity
  • ‘better ability to detect objects camouflaged in natural grey scale figures’

BETTER SPATIAL VISION

  • may explain the high prevalence of this inherited disorder
30
Q

doron et al (2019)

  • 43% of dichromats
  • 29% of anomalous trichromats

said their colour deficiency affected their occupation choice

A

-

31
Q

metamers

A

two wavelengths that produce the same response pattern and cannot be distinguished but appear physically different

32
Q

red light = 670nm

green light = 520nm

A

-

33
Q

example working out

A

520nm > L response is 0.37 x the M response

470nm > L/M = 5.1 x 13.8 = 0.37

  1. 1 = red cones absorb 5.1%
  2. 8 = green cone absorbs 13.8%

draw lines from the graph

34
Q

which animals are trichromats?

A

great apes, howler monkeys, honey bees and some marsupials

35
Q

trichromats = distinguish millions of colours

dichromats = distinguish thousands of colours

A

-

36
Q

preponderance of light = ‘most of it is at the end of the spectrum’

A

-

PPA (19 decks)

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