Colour Vision Deficiencies (CVD)

Question 1

Describe colour vision deficiencies (CVD)?

Answer 1

• Can be genetic or acquired
• Genetically passed on CVD is present from birth
• Acquired CVD occurs secondary to eye disease – usually associated with stroke
• CVD are classified into 3 groups: monochromacy, dichromacy & anomalous trichromacy
• People with normal colour vision are called trichomats
• Monochromats are typically totally colour blind & only see things in black & white (shades of grey) & may have 1 cone pathway (Short λ cone pathway) in addition to rod pathway
• Dichromats have a cone photopigment missing so they only have 2 cone channels
• Anomalous trichromats have all 3 cone photopigments but 1 cone photopigment is anomalous having a shifted peak sensitivity

Question 2

Who are most affected by colour vision deficiencies?

Answer 2

Males are greatly affected by CVD as they lack the extra X chromosome

Question 3

Describe the classification according to photopigment deficiency: protan?

Answer 3

Protanope - missing longer wavelength cone photopigment
Protanomal - anomalous longer wavelength cone photopigment

Question 4

Describe the classification according to photopigment deficiency: deutan?

Answer 4

Deuteranope - missing middle wavelength cone photopigment
Deuteranomal - anomalous middle wavelength cone photopigment

Question 5

Describe the classification according to photopigment deficiency: tritan?

Answer 5

Tritanope - missing short wavelength cone photopigment
Tritanomal - anomalous short wavelength cone photopigment

Question 6

Describe the classifications of CVD in more detail?

Answer 6

• Dichromacy & anomalous trichromacy can be classified according to affected cone photopigment
• 3 terms that are used also used to describe CVD are protan, deutan & tritan
• A protan has longer λ cone photopigment missing or it is anomalous
• A deutan has middle λ cone photopigment missing or it is anomalous
• A tritan has shorter λ cone photopigment missing or it is anomalous –> not sex-linked as blue is on chromosome 7
• Pattern of inheritance for deutan (red-green) CVD is sex-linked recessive while tritan CVD has an autosomal dominant inheritance
• Genetics of CVD is particularly important as pxs are often keen to understand why they are CVD & whether they will pass their CVD to their children
• Tritan-like CVD are associated w/ disease, making it essential to discriminate between acquired & congenital tritan defects

Question 7

What are the children like if there is a colour-blind man and a non-colour blind woman?

Answer 7

No colour blind sons
All daughters are colour blind carriers (100%)

Question 8

What are the children like if there is a non-colour blind man and a colour-blind carrier woman?

Answer 8

50% chance of sons being colour blind
50% chance of daughters being colour blind gene carriers

Question 9

What are the children like if there is a colour-blind man and a colour-blind carrier woman?

Answer 9

50% chance of sons being colour blind
50% chance of daughters being either colour blind or a colour blind gene carrier

Question 10

What is CIE/ CIE XYZ Colour Space?

Answer 10

• CIE = International Commission on Illumination (1931)
• International authority on light, illumination, colour, & colour spaces
• Used in tests of colour vision
• CIE colour space is 2D & reflects hue & saturation
  o The 3rd dimension is its lightness

Question 11

Describe the CIE Diagrams & Diagnostic Colour Vision Tests

Answer 11

• Confusion lines are also called isochromatic lines because they show lines of the same colour (to the colourblind). A more accurate term is pseudoisochromatic lines
• CIE 1931 colour space reflects hue & saturation, which together make up chromaticity
• 3rd dimension – lightness – is not shown in diagram
  o This does not matter because confusion lines can be shown to fall on the chromaticity diagram
• Discrimination of colour by dichromats is limited due to 1 photopigment being absent
• Therefore, when it comes to colour matching, certain colours are confused with others
• Confusion lines are lines joining points on chromaticity diagram that will appear to be same in colour for dichromats
• No. of confusion lines also provide info about amount of change in λ before another colour is discriminated (when next confusion line is met)
• All confusion lines converge to a point called the co-punctal point

Question 12

What three things can be assessed by using confusion lines? And what do the terms mean?

Answer 12

Can assess Protanopia, Deuteranopia and Tritanopia using the confusion lines
Tritanope – blue deficiency
Protanope – red deficiency

Question 13

Describe confusion lines & colour tests (in practice)?

Answer 13

• Confusion lines form basis of many colour vision tests such as the Farnsworth Panel D-15 & Ishihara Pseudoisochromatic plates
• Pseudoisochromatic plate tests are also commonly used to screen for CVD
• Colours are carefully chosen based on the confusion lines
• Most commonly used pseudoisochromatic plate is Ishihara Isochromatic plates (for screening red-green colour vision deficiency) & Tritan (F-2) plate
• Pseudoisochromatic plates are designed in 4 ways:
  o Transformation plates: where a person w/ normal colour vision sees one figure & a CVD person sees another
  o Vanishing plates: where a person w/ normal colour vision sees the figure while a CVD person will not
  o Hidden-digit plates: where a person w/ normal colour vision does not see a figure while a CVD will see the figure
  o Diagnostic plates: designed to be seen by normal subjects w/ colour vision defectives seeing one number more easily than another