wk6: ND - Dichromacy Flashcards
What is the fundamental idea behind dichromacy?
Only need 2 colours in a colour mixture to match all colours
What does colour matching in a dichromat suggest about their vision?
Suggests they only have 2 chromatic/luminosity channels (compared to three in normal colour vision)
In comparison to the normal luminosity peak of 555nm in normal patients, how does this peak differ in protans and duetans?
Protans = peak is lower
Deutans = peak is higher
True/False: protanopes and normal subjects have similar sensitivity for long wavelength light
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
How good is colour discrimination for protanopes at long wavelengths
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
How do you define “wavelength discrimination”? Use an example to explain
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
At what wavelengths do protanopes and deuteranopes have poor/no wavelength discrimination?
at wavelengths over 540nm (note significant patient variability)
What does it mean to have no wavelength discrimination?
colour always looks same no matter what wavelength you add
Comparing duetreranopes and protanopes, which have slightly better wavelength discriination?
deuteranopes. This means dueteranopes perform slightly better at practical tasks involving colour
At what wavelengths do protanopes and dueteranopes have their best wavelength discrimination? (2)
Protanopes: 490nm
Deuteranopes: 495nm
What is the typical wavelength discrimination value for normals?
<2nm required to detect difference over most of the spectrum
Define saturation
how far the colour is away from white
Define saturation discrimination
how much of a wavelength do I need to add to white for it to look tinged with colour
How do we measure saturation? (formula) How about saturation discrimination?
Lw + Llambda
deltaP = Llambda/Lw
How does saturation discrimination in normals work? (using 570nm as a reference point)
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
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?
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
At what wavelength in deuteranopes does the saturation discrimiation go to infinity?
500.4nm (495-506.4)
How do saturation discrimination thresholds for protanopes compare to normals?
Generally elevated thresholds across entire spectrum - so this means you have to add a lot more colour to the white to be seen
How can we summarise basic data on dichromat performance?
as confusion loci on a chromaticity diagram
Describe the orientation of the confusion loci on a protanopic CIE diagram
converge onto the “cone fundamental”, which is the location in colour space corresponding to the missing element in their colour vision
How many distinct colours/confusion loci do protanopes have?
17
Describe the orientation of the confusion loci on a deuteranopic CIE diagram
parallel confusion loci with no apparent convergence on “cone fundamental”
How many distinct colours/confusion loci do deuteranopes have?
27
As you move along the CIE diagram in protanopes and deuteranopes, what colours are perceived?
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)
Describe the opponent colours theory for colour perception
Colours have opponents:
Black - white opponency
Red-green opponency
Yellow-blue opponency
What is the “zone theory” a combination of?
trichromatic theory and opponent colours theroy
What is the “Loss” hypothesis explaining dihcromacy?
It is the simplest explanation for dichromacy that suggests that one of the 3 cone types is mising
What does the “Loss” hypothesis for dichromacy predict? (3)
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
What is the “Collapse” hypothesis for dichromats? (2)
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)
Which forms of the collapse hypothesis predict parallel confusion loci
Both of them
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?
Used colorimetry and found 2 different NPs. However found deuteranopic confusion loci to converge.
Collapse hypothesis rejected. Loss hypothesis stands
In 1963, what did Rushton find with his retinal densitometry? Which hypothesis does this support?
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)
In 1965, what did Rushton find and what did he conclude? Which hypothesis does this support?
Concluded D has only one photopigment (erthyroloabe). Support loss hypothesis
Are dichromacies loss or collapse systems?
reduction (loss) systems. HOWEVER, spatial vision of dichromats remains normal, suggesting there is loss with replacement
What does loss with replacemtn mean?
the cone that is “missing’ is filled with a different photopigment
Do tritanopias have a region of the specturm in which co-efficients are invariant?
nope
How is wavelength discrimination in tritanopes?
pretty good, except uncertainty around 450nm. Some tritanopes have better wavelength discrimination than some normals in some regions
Where is the neutral point for tranopes?
570nm
Describe the orientation of confusion loci in tritanopes?
converge to a point in the bottom left of CIE diagram (i.e. blue corner) (so good discrimination in blue corner)
What is the presumed cause of tritanopia?
absence of blue (S) cone pigment
In relation to the CIE diagram: describe the colour perception of tritanopes
Saturated green and red are seen with other colours distinguished by decreasing saturation to white along the neutral point confusion locus
What mode of inheritance is tritanopia?
autosomal dominant (gene encoding S cone pigment is on chromosome 7)
What mode of inheritance is protanopia and deuteranopia?
sex linked recessive
