ND - Colour Vision Disorders II - Week 6 Flashcards
What was an early proposition on the cause of CVD?
The vitreous was blue tinted, absorbing all red light
What is the fundamental concept behind dichromacy regarding colour matching?
Only need two colours in a mixture to match all colours compared to three in normals
What is the peak spectral sensitivity in protanopes, deuteranopes, and normals (what wavelength)? Is the change from normals trivial?
P - 540nm
D - 563nm
Normal - 555nm
Reduction of sensitivity is not trivial
When doing dichromatic colour matching, do deuteranopes and protanopes perform similarly or very differently?
Very similar
Consider a long wavelength presented to a deuteranope and a normal. How do they see it in terms of luminosity?
They see it dimmer than the other two
Are protanopes or deuteranopes more able to discriminate colour?
Deuteranopes generally perform better than protanopes
What nanometre length is required to detect a difference over most of the spectrum? How many discriminally different spectral hues does this result in?
<2nm
Estimated number of hues is 150
Consider a CIE diagram. Movement in what direction relative to C (the centre white) will cause a change in hue and saturation?
Hue - Movement orthogonal to C, towards one of three ends of the diagram
Saturation - movement towards C decreases saturation
Consider the amount of colour needed to add to white before a change is noticed. How do normals compare to portanopes?
Generally elevated thresholds across the entire spectrum
What is the neutral point of dichromacy? What is it for protans and deuterans?
A region in the spectrum where it is impossible to saturate in dichromats
P - 495.5nm
D - 500.4nm
How are confusion loci constructed using the spectral neutral point?
The sectral neutral point is marked on the CIE diagram (edge)
The R+B mix that matches the neutral point (from colour matching) is plotted on the bottom straight line of the diagram between red and blue
The two points are connected, giving one confusion line
Wavelength steps (in the smallest increment they are able to discriminate) are plotted in either direction of the spectral neutral point, giving confusion lines for the rest of the spectrum
Using the spectral neutral point to construct confusion loci, how many colours are protans and deuterans estimated to be able to discriminate?
P - 17
D - 27
Consider protan and deuteran confusion loci. Which appear to show convergence on cone fundamental and which do not? Which have parallel loci and which do not?
P - appears to converge on cone fundamental (in the red area)
D - parallel confusion loci and no convergence
Consider a protanopic CIE diagram. What colours appear on this diagram for protans and how does saturation change as confusion loci move towards to spectral neutral point?
The top-right section (which is originally green and red) is saturated yellow, and becomes more saturated as the loci move to the neutral point
The blue bottom left corner is still saturated blue, but becomes only a less saturated blue as the loci move towards the neutral point
At the neutral point itself, it is white across that entire locus
Is there any viable theory of a pre-retinal filter for CVD?
No
The loss of one cone results in what exactly in protans and deuterans?
The loss of one of the opponent mechanisms
Consider the opponent system. List the two of them and explain why losing any of red or green will result in seeing only blue, white, and yellow.
Blue-yellow and red-green opponency
If either red or green cones are lost, the red-green opponent mechanism is inoperative
The remaining colour perceptions will therefore rely on blue and yellow
Equal input of blue and green will result in white light
What is the loss hypothesis? What three things does it predict and are these predictions in line with what we see in protans and deuterans?
One of the 3 cones is missing
Predicts confusion loci converge on a single point
-good for protans, not deutans
Predicts decreased light sensitivity scross the entire spectrum
-good for protans, not deutans
Predicts decreased VA
-possible for both
What is the collapse hypothesis (2) and what is it an alternative to?
Cones contain instead a mixture of R and G pigments, so no differential signal can be generated or R and G signals become fused creating a neural short circuit before any opponent stage
An alternative to the loss hypothesis
What do both forms of the collapse hypothesis predict concerning confusion loci? What does it rely on and what is an issue with this regarding protans and deuterans and individuality? How is this solved?
Parallel confusion loci
Relies on estimates of neutral points
This is an issue as deuterans and protans arent able to be clearly grouped into two classes, with a lot of noise influenced by factors like ocular pigmentation
Solved by using broad band, non-spectral measures of the neutral point
Based on using non-spectral measures for the neutral point, what happens to the deuteranopic confusion loci? Which hypothesis must be rejected and which still stands?
The loci converge (at a point off the CIE diagram in the bottom right corner)
Which hypothesis did retinal densitometry support?
Loss hypothesis
Does spatial vision remain in dichromats? What does this suggest?
It remains normal, suggesting there is loss with replacement
-no empty cones in dichromacy
-all non-S cones are filled with the same pigment
What is the neutral point of tritanopia?
570nm
Consider the matching functions for dichromacy. What is the variability like amongst the three of them among individuals with the same CVD?
Protans and deutans had high variability
Tritans did not
What is the wavelength discrimination like amongst tritans compared to other CVD and to normal (2)?
Other CVD - quite good
Some tritans have better discrimination than some normals in some regions
They have uncertainty around 450nm
Consider the loss of the S cone. What opponent system is lost and what colours would the individual see the world?
Blue-yellow opponency is lost, so the world is seen in red, green, and white only
Describe what the CIE diagram of a tritan would look like including what colours are there.
The confusion loci convergence is at the bottom left corner
The left side is green, becoming more desaturated moving towards the neutral point
The right side is red, becoming more desaturated moving towards the neutral point
What mode of inheritance does tritanopia have (2)?
Autosomal dominant with incomplete penetrance
-some individuals fail to express the trait, even though they carry the allele
Are males and females affected equally for tritanopia?
Yes
