Acquired Colour Vision 1 Flashcards
Why may colour vision vary between normal trichromats an colour deficient individuals?
This can be due to a number of different reasons:
- Genetic mutations or expression in L and/or M cone genes can cause a shift in peak sensitivity that results in differences in colour vision
- Pre-receptal filter e.g. macula pigment can explain some differences in colour matching experiments (macula pigment blocks blue light)
- Variation in the L to M cone ratio amongst people
- Variation in optical density of photoreceptors (i.e. how many photoreceptors per area).
- Variation in amplification of post receptal cone signals (i.e. not all cones may amplify the signal of see a short wavelength the same leading to a slightly diff interpretation of the colour).
What are the spectoral sensitivites of L, M and S cones?
L cones - 570nm
M cones-542nm
S cones - 442
What are the two types of Vision Information channels?
Colour Channels and Luminance Contrast Channels
How does the red-green colour channel work?
The comparison of L and M cone signals forms a chromatic channel that mediates red-green discrimination.
[I.e to detect the difference between red and green the channel literally compares the wavelength of the light it is receieving to the peak sensitivity of L and M cones]
How does the yellow-blue colour channel work?
The Yellow-blue colour channel is achieved from comparison of S cone signslas against the combined signal of L and M cones.
Describe the differences between congenital and acquired colour vision
Congenital deficiency is present at birth, stable, bilaterally symmetrical, and is thought to affect the entire field of vision.
Acquired deficiency may exhibit progression or regression, may affect one eye or both eyes asymmetrically, and may affect only a portion of the visual field.
Why do congenital colour vision deficiencies arise?
They arise from disorders in the gene coding for the photoreceptor pigment
How are congenital colour deficiencies sub-classified?
By the severity of the defect and the class/classes of cones affected
In a normal trichromat what is the seperation in peak sensitivity between an L and M cone?
28 Nanometers
(570-542)
What occurs in anomalous trichromacy?
Trichromatic colour vision is displayed however there is a shift in the peak sensitivity of a photoreceptor.
What are the different types of anomalous trichromacy?
Deutranomaly - Here the M cone sensitivity shifts towards that of the L cone making it hard to detect green correctly. This is an M cone defect.
Protanomaly - Here the L cone sensitivity shifts toward that of the M cone making it hard to detect red correctly . This is an L cone defect.
Tritanomaly (rare) -The spectral sensitivity of the S cone is shifted making it hard to detect blue correctly.
What is dichromacy?
A condition when only two types of photoreceptor are present (i.e. one photoreceptor is missing or disfucntional)
What are the different types of dichromacy and which cone is missing in each condition?
Protanopia - L cone mssing (red cone mssing) (i.e. red confused with black)
Deutranopia - M cone missing (i.e. can’t see green - thus confuse it with black)
Tritanopia - S cone missing - thus unable to percieve any blue light
What is the most common congenital colour deficiency and which is the most rare?
‘Red/green’ are the most common (whether that is an anomalous trichromacy or dichromacy), affecting ~8% of men and ~0.4% of women, ‘yellow/blue’ deficiencies are very rare.
What is monochromacy?
Where only one type of photoreceptor is present, the others are missing or disfunctional .
This is incredibly rare
What are characteristics of acquired colour deficiencies/anomalies?
- Frequently associated with other visual function loss (i.e. acuity, visual fields,…)
- Onset after birth (colour vision previously normal)
- Type and severity changes with time
- Monocular differences in severity frequently occur
- Monocular testing required
- Equal prevalence in males and females
- May affect both red/green and yellow/blue colour channels
Name the different types of Colour Vision tests and give examples of each
What are the purposes of colour vision tests?
Colour vision tests perform different functions:
- Screening tests; which identify people with normal RG and YB colour vision
- Classify type of colour vision defect; protan‐, deutan‐ or tritan‐type
- Quantify or grade the severity of the colour vision loss
- Diagnose dichromacy/anomalous trichromacy
- Assess Occupational suitability
What is needed for an efficient colour vision assessment?
The ensurance that a true isolation of colour signals is obtained
How may acquired colour vision deficiencies be classified?
Acquired colour vision deficiency may be classified by its mechanism or the site of pathology or by the type of colour deficiency present.
What is the link between acquired colour vision and aging?
There is an arguement that as you age you develope a colour deficiency due to pre-receptal filtering i.e. the natural yellowing of our lens overtime.
and generally due to neural aging (just normal aging anyway).
Is there a link between colour vision and diabetes?
Yes - studies have shown a correlation between weaker detection of colours and diabetes
Is there a link between colour vision loss and AMD?
AMD patients can display a large loss of both RG and YB chromatic sensitivity.
Px with AMD tend to lose more YB colour vision than RG colour Vision
