10. CVD - Dichromacy

Question 1

What are the fundamental ideas of dichromacy?

Answer 1

• Dichromates only need 2 primaries to match colour mixture in a bipartite field → suggest that they only have 2 chromatic/ luminosity channels

Question 2

What are the peak spectral sensitivity in photopic luminosity functions (Vλ) of normal, protanopes and deuteranopes according to Pitt?

Answer 2

Normal = 555nm
Protanopes = 540nm
Deuternanopes = 563nm

Question 3

How did protanopes match colours across the colour spectrum?

Answer 3

• With short wavelengths, protanopes match with lots of blues and the proportion of blue decreases as the wavelength increases (towards red).
• With long wavelengths, protanopes add more red primary to the other side to match
• At λ > 540nm, only red is used to match matches as protanopes cannot discriminate yellow, orange and reds.

Question 4

What is similar between protanopes and deutaneropes in colour matching experiments? What is different?

Answer 4

Protanopes and deuteranopes use similar wavelengths to match colours, therefore they have very similar wavelength matching functions. However, protanopes and deuteranopes have very dfifferent luminosity function. Protanopes have reduced sensitivity to longer wavelengths of light, such that red appears darker compared to normal or deuteranopes.

Question 5

In wavelength discrimination:
* Protanopes cannot discriminate ..., with ... variability
* Deuteranopes cannot discriminate ..., with ... variability
* Protanopes performs ... than deuteranopes in practical tasks involving ...
* Protanopes best discriminate wavelengths of ...
* Deuteranopes best discriminate wavelength of ...
* Normal require <... to detect differences over most of the spectrum and can discriminate around ... spectral hues

Answer 5

In wavelength discrimination:
* Protanopes cannot discriminate wavelengths above 540nm, with high variability
* Deuteranopes cannot discriminate wavelengths above 540nm, with lower variability
* Protanopes performs less well than deuteranopes in practical tasks involving colour
* Protanopes best discriminate wavelengths of 490nm
* Deuteranopes best discriminate wavelength of 495nm
* Normal require <2nm to detect differences over most of the spectrum and can discriminate around 150 spectral hues

Question 6

How is saturation discrimination measured ?

Answer 6

Using a white background with a bipartite field, add colour to one half of the background and measure how much colour is needed to discriminate that this half is different from white

Question 7

Where in the CIE diagram do you change wavelength/ hue and where do you change saturation?

Answer 7

As you change wavelength, you are changing along the spectral locus along the edge of the diagram As you change saturation, you change how much white it is added to a colour through the neutral white point.

Question 8

What does the asymptote in the saturation discrimination diagram represent? In which CVD does this occur?

Answer 8

Asymptote is the neutral point of dichromacy, it is the region of the spectrum whre saturation discrimination is impossible. No matter how much colour is added, it won’t look different and will look achromatic to dichromates. This occurs for protanopes.

Question 9

Which colour does normals struggle to discriminate the most in terms of wavelength?

Answer 9

570nm (yellow). Relatively more yellow is needed to be added to white in order for normals to discriminate

Question 10

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Question 11

What are the average neutral point of protanopes and deuteranopes?

Answer 11

Protanopes = 495.5nm
Deuteranopes = 500.4nm

Question 12

What does the confusion loci represent?

Answer 12

The confusion loci represent the colours confused, which shows:
* λ discrimination
* Mixture functions
* Neutral point
* Saturation discrimination

Question 13

How many colours can protanopes and deuteranopes each recognise?

Answer 13

Protanopes can recognise 17 colours and Deuteranopes can recognise around 27 colours.

Question 14

What does the confusion loci look like for protanopes?

Answer 14

Confusion loci looks like they are converging to the bottom right corner, this correlates to the missing L cone, which is where it should be located

Question 15

What does the confusion loci look like for deuteranopes?

Answer 15

Deuteranopes do not have convergence of confusion loci, they appear to be parallel and evenly spaced.

Question 16

How do protanopes and deuteranopes see the 17 to 27 colours they can discriminate?

Answer 16

As we move away from the neutral point (achromatic point), colours will appear more saturated, such that as we move from the middle (neutral point) upwards, yellow will appear increasingly saturated. At the bottom of the CIE diagram, there is increasing saturation of blue.

Question 17

How do protanopes and deuteranopes see a field of red and purple flowers with green leaves?

Answer 17

Protanopes will not be able to distinguish between the red flowers and green leaves The purple flower will appear more blue than purple. This is because of the reduced sensitivity to red. Purple appear more blue due to the confusion loci. Deuteranopes will see the red flower as yellow, therefore can discriminate from the green leaves. It is the luminous difference that allows deuteranopes to see the difference as they do not have reduced luminous sensitivity.

Question 18

Hering’s ... theory combined with the ... help understand the colour perceptions of protanopes and deuteranopes. ... and ... can still be seen without a red or green cone because of .... The 3 opponencies includes ..., ..., and .... It is thought that once we lose a cone, we also loses the ..., such that if the red or green cone is lost, the ... becomes inoperative, therefore the remaining colour perceptions are .... This also explains the ... colour experience, apart from the ..... If the blue-yellow opponent mechanism has equal inputs from blue and green, the perception is ....

Answer 18

Hering’s perceptual opponent colours theory combined with the trichromatic theory help understand the colour perceptions of protanopes and deuteranopes. White and yellow can still be seen without a red or green cone because of opponency. The 3 opponencies includes black-white opponency', red-green opponency, and yellow-blue opponency. It is thought that once we lose a cone, we also loses the opponent mechanism, such that if the red or green cone is lost, the red-green opponent mechanism becomes inoperative, therefore the remaining colour perceptions are yellow and blue. This also explains the similarity of protanopes and deuteranopes colour experience, apart from the decreased protanopes sensitivity to long wavelengths. If the blue-yellow opponent mechanism has equal inputs from blue and green, the perception is colourless = white.

Question 19

The Loss hypothesis hypothesises that .... It predicts that the confusion loci ..., which is accurate for ..., but not .... It also predicts ` … and …` due to having less cones.

Answer 19

The Loss hypothesis hypothesises that one of the 3 cone types is missing. It predicts that the confusion loci converge onto a single point, which is accurate for protanopia, but not deuteranopia. It also predicts decreased VA and decreased light sensitivity over the entire spectrum due to having less cones.

Question 20

The Collapse hypothesis is also known as the .... In some deuteranopes, cones contain a mixture of ..., therefore no differential signals between the two can be generated. Signals generated by normal red and green cones can be ..., therefore creating a ... before any opponent stage operates to determine red-green balance. Both of these predicts a ..., which explains that of ....

Answer 20

The Collapse hypothesis is also known as the fusion deuteranopia. In some deuteranopes, cones contain a mixture of red and green pigments, therefore no differential signals between the two can be generated. Signals generated by normal red and green cones can be combined or fused, therefore creating a neural short circuit before any opponent stage operates to determine red-green balance. Both of these predicts a parallel confusion loci, which explains that of deuteranopes.

Question 21

What rejected the Collapse hypothesis?

Answer 21

Walls and Matthews experiment that measure the neutral points. It clearly shows that there are two distinct neutral points. Which rejects collapse hypothesis.

Question 22

Dichromacies are ... systems, where ... is missing. This reduces the normal 3 channel chromatic systems to .... Protanopia only has ... and deuteranopia only has .... However, ... of dichromats remains normal, this therefore suggest that there is loss with ..., such that all “non-S cones” are filled with the ....

Answer 22

Dichromacies are reduction (loss) systems, where one cone photopigment is missing. This reduces the normal 3 channel chromatic systems to 2 channel system. Protanopia only has M and S cones and deuteranopia only has L and S cones. However, spatial vision of dichromats remains normal, this therefore suggest that there is loss with replacement, such that all “non-S cones” are filled with the same pigment.

Question 23

Tritanopia matches all wavelengths with .... They are good at discriminating wavelengths but are uncertain around ..., where some tritanopes can have ... than normals in some regions. Tritanopes have a neutral point around .... The confusion loci of tritanopes converges to the ....

Answer 23

Tritanopia matches all wavelengths with 650nm + 480nm. They are good at discriminating wavelengths but are uncertain around 450nm, where some tritanopes can have better wavelength discrimation than normals in some regions. Tritanopes have a neutral point around 570nm. The confusion loci of tritanopes converges to the bottom left corner = blue corner.

Question 24

What are the colour perceptions of tritanopes?

Answer 24

Tritanopes losses S cones, which therefore loses the blue-yellow opponent channel. Tritanopes can only perceive red, green and white. Saturated green and red are seen with other colours distinguished by decreasing saturation to white along the neutral point confusion locus.

Question 25

What are the modes of inheritance of CVD?

Answer 25

Protanopia and deuteranopia are X-linked recessive. The abnormal gene is on X chromosome. Tritanopia is autosomal dominant with incomplete penetrance. The gene coding for S cone pigment is on chromosome 7 and the gene is usually missing or damaged. Both males and females are equally affected and 50% of offspring’s will show genetic defect but not express it.