wk6: ND - Dichromacy

Question 1

What is the fundamental idea behind dichromacy?

Answer 1

Only need 2 colours in a colour mixture to match all colours

Question 2

What does colour matching in a dichromat suggest about their vision?

Answer 2

Suggests they only have 2 chromatic/luminosity channels (compared to three in normal colour vision)

Question 3

In comparison to the normal luminosity peak of 555nm in normal patients, how does this peak differ in protans and duetans?

Answer 3

Protans = peak is lower
Deutans = peak is higher

Question 4

True/False: protanopes and normal subjects have similar sensitivity for long wavelength light

Answer 4

False!. Protanopes have a massive drop in sensitivity for long wavelength light. It’s DEUTERANOPES that have similar sensitivity to normals for long wavelength light

Question 5

How good is colour discrimination for protanopes at long wavelengths

Answer 5

Pretty poor. At wavelengths over 540nm, only red (650nm) is used to make a match. This demonstrates how the discrimination at these wavelengths is poor. Note: this is basically the same for deuteranopes

Question 6

How do you define “wavelength discrimination”? Use an example to explain

Answer 6

e.g. 540 vs 541 - px can’t tell difference, but 540 vs 543 px finally now can tell difference. This means their wavelength discrimination is 3nm at this wavelength

Question 7

At what wavelengths do protanopes and deuteranopes have poor/no wavelength discrimination?

Answer 7

at wavelengths over 540nm (note significant patient variability)

Question 8

What does it mean to have no wavelength discrimination?

Answer 8

colour always looks same no matter what wavelength you add

Question 9

Comparing duetreranopes and protanopes, which have slightly better wavelength discriination?

Answer 9

deuteranopes. This means dueteranopes perform slightly better at practical tasks involving colour

Question 10

At what wavelengths do protanopes and dueteranopes have their best wavelength discrimination? (2)

Answer 10

Protanopes: 490nm
Deuteranopes: 495nm

Question 11

What is the typical wavelength discrimination value for normals?

Answer 11

<2nm required to detect difference over most of the spectrum

Question 12

Define saturation

Answer 12

how far the colour is away from white

Question 13

Define saturation discrimination

Answer 13

how much of a wavelength do I need to add to white for it to look tinged with colour

Question 14

How do we measure saturation? (formula) How about saturation discrimination?

Answer 14

Lw + Llambda

deltaP = Llambda/Lw

Question 15

How does saturation discrimination in normals work? (using 570nm as a reference point)

Answer 15

At 570nm: need to add a lot of yellow spectral light for white to be tinged yellow
Either side: don’t need to add as much colour

Question 16

At what wavelength in protanopes does the saturation discrimination go to infinity? i.e when is saturation discrimination impossible and it just looks white. What is this referred to as?

Answer 16

495.5nm (492.5-497.2). This is called the “neutral point of dichromacy”, where no matter what, you’ll just see the light as monochromatic/white

Question 17

At what wavelength in deuteranopes does the saturation discrimiation go to infinity?

Answer 17

500.4nm (495-506.4)

Question 18

How do saturation discrimination thresholds for protanopes compare to normals?

Answer 18

Generally elevated thresholds across entire spectrum - so this means you have to add a lot more colour to the white to be seen

Question 19

How can we summarise basic data on dichromat performance?

Answer 19

as confusion loci on a chromaticity diagram

Question 20

Describe the orientation of the confusion loci on a protanopic CIE diagram

Answer 20

converge onto the “cone fundamental”, which is the location in colour space corresponding to the missing element in their colour vision

Question 21

How many distinct colours/confusion loci do protanopes have?

Answer 21

17

Question 22

Describe the orientation of the confusion loci on a deuteranopic CIE diagram

Answer 22

parallel confusion loci with no apparent convergence on “cone fundamental”

Question 23

How many distinct colours/confusion loci do deuteranopes have?

Answer 23

27

Question 24

As you move along the CIE diagram in protanopes and deuteranopes, what colours are perceived?

Answer 24

Saturated yellow on G-Y-R confusion locus becoming desaturated as the confusion loci move toward the NP confusion locus
Saturated blue in the B corner of the chromaticity diagram becoming desaturated for the confusion loci closer to the NP confusion locus

(NP = neutral point)

Question 25

Describe the opponent colours theory for colour perception

Answer 25

Colours have opponents:
Black - white opponency
Red-green opponency
Yellow-blue opponency

Question 26

What is the “zone theory” a combination of?

Answer 26

trichromatic theory and opponent colours theroy

Question 27

What is the “Loss” hypothesis explaining dihcromacy?

Answer 27

It is the simplest explanation for dichromacy that suggests that one of the 3 cone types is mising

Question 28

What does the “Loss” hypothesis for dichromacy predict? (3)

Answer 28

Confusion loci converging on a single point (good for protanopia but not deuteranopia)
Decreased light sensitivity over entire spectrum (less cones) (possibly ok for protanopia, but not deuteranopia)
Decreased VA

Question 29

What is the “Collapse” hypothesis for dichromats? (2)

Answer 29

A theory with 2 forms that suggests that in (some) deuteranopes:
1. cones contain a mixture of R and G pigments
OR
2. signals generated by normal R and G cones become combined or “fused”, creating a “neural short circuit” before any opponent stage operates to determine R-G balance (so signals from M and L cones going to both sides of the channel)

Question 30

Which forms of the collapse hypothesis predict parallel confusion loci

Answer 30

Both of them

Question 31

Based on non-spectral (broad-band) measures of the neutral point of dichromacy by Walls and Matthews (1952), what did they find and did this confirm or reject the collapse and loss hypothesis?

Answer 31

Used colorimetry and found 2 different NPs. However found deuteranopic confusion loci to converge.

Collapse hypothesis rejected. Loss hypothesis stands

Question 32

In 1963, what did Rushton find with his retinal densitometry? Which hypothesis does this support?

Answer 32

Found protanope has only one RG photopigment (M cone pigment). Supports Loss hypothesis.

Also found deteranope with mixture of one pigment and another, supporting collapse theory for deuteranopia

(note i simplified this a bit)

Question 33

In 1965, what did Rushton find and what did he conclude? Which hypothesis does this support?

Answer 33

Concluded D has only one photopigment (erthyroloabe). Support loss hypothesis

Question 34

Are dichromacies loss or collapse systems?

Answer 34

reduction (loss) systems. HOWEVER, spatial vision of dichromats remains normal, suggesting there is loss with replacement

Question 35

What does loss with replacemtn mean?

Answer 35

the cone that is “missing’ is filled with a different photopigment

Question 36

Do tritanopias have a region of the specturm in which co-efficients are invariant?

Answer 36

nope

Question 37

How is wavelength discrimination in tritanopes?

Answer 37

pretty good, except uncertainty around 450nm. Some tritanopes have better wavelength discrimination than some normals in some regions

Question 38

Where is the neutral point for tranopes?

Answer 38

570nm

Question 39

Describe the orientation of confusion loci in tritanopes?

Answer 39

converge to a point in the bottom left of CIE diagram (i.e. blue corner) (so good discrimination in blue corner)

Question 40

What is the presumed cause of tritanopia?

Answer 40

absence of blue (S) cone pigment

Question 41

In relation to the CIE diagram: describe the colour perception of tritanopes

Answer 41

Saturated green and red are seen with other colours distinguished by decreasing saturation to white along the neutral point confusion locus

Question 42

What mode of inheritance is tritanopia?

Answer 42

autosomal dominant (gene encoding S cone pigment is on chromosome 7)

Question 43

What mode of inheritance is protanopia and deuteranopia?

Answer 43

sex linked recessive