2 - DEFICIENCIES IN COLOUR VISION

Question 1

what causes colour blindness?

Answer 1

• 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

Question 2

type A

Answer 2

1 of 3 cones MISSING

• 3 possibilities (L, M or S missing)

Question 3

type B

Answer 3

2 of 3 cone types missing

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

Question 4

type C

Answer 4

all 3 cone types missing

• 1 possibility

Question 5

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

Answer 5

• but only 5 of the 7 have ever been observed

Question 6

dichromats

Answer 6

• 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

Question 7

protanopia (dichromat)

Answer 7

L-cones are missing

Question 8

deuteranopia (dichromat)

Answer 8

M-cones are missing

Question 9

tritanopia (dichromat)

Answer 9

S-cones are missing

Question 10

monchromats

Answer 10

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)

Question 11

rod monochromacy

Answer 11

people who have no cones and only rods

Question 12

cone monochromacy

Answer 12

people who have only s-cones and rods

Question 13

anomalous trichromats

Answer 13

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

Question 14

anomolous protanopia (protanomaly)

Answer 14

L-cones with abnormal spectral sensitivities

• red moves to green

Question 15

anomolous deutranopia (deuteranomaly)

Answer 15

M-cones with abnormal spectral sensitivities

• green moves to red
• most common form

Question 16

anomolous tritanopia (tritanomaly)

Answer 16

S-cones with abnormal spectral sensitivities

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

Question 17

colour blindness does not make someone incapable of experiencing colour

Answer 17

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’

Question 18

red-green colour blindness

Answer 18

• 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’

Question 19

what is the most common form of colour blindness?

Answer 19

anamolous trichromacy

Question 20

diagnosis of colour blindness

Answer 20

• 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

Question 21

what careers does colourblindness affect

Answer 21

technicians, interior designers and aircraft pilots

Question 22

congenital colour blindness

Answer 22

born with it and it’s due to genetics

affects cones

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

Question 23

colour blindness as a result of brain damage

Answer 23

• 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

Question 24

colourblindness

Answer 24

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

Question 25

chickens

Answer 25

``` 300-710nm wider range near infrared can discriminate between more colours tetrachromatic ```

Question 26

phenomenology

Answer 26

‘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.’

Question 27

how common is colour blindess

Answer 27

caucasian population - 8% of males - 0.5% of females

Question 28

morgan et al (1992)

Answer 28

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

Question 29

doron et al (2019)

Answer 29

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

Question 30

doron et al (2019) - 43% of dichromats - 29% of anomalous trichromats said their colour deficiency affected their occupation choice

Answer 30

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Question 31

metamers

Answer 31

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

Question 32

red light = 670nm | green light = 520nm

Answer 32

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Question 33

example working out

Answer 33

520nm > L response is 0.37 x the M response 470nm > L/M = 5.1 x 13.8 = 0.37 5. 1 = red cones absorb 5.1% 13. 8 = green cone absorbs 13.8% draw lines from the graph

Question 34

which animals are trichromats?

Answer 34

great apes, howler monkeys, honey bees and some marsupials

Question 35

trichromats = distinguish millions of colours dichromats = distinguish thousands of colours

Answer 35

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Question 36

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

Answer 36

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