W6 Colour Vision Flashcards
What is the nature of light and what wavelength spectrum is it within?
Light has a dual nature composed of electro-magnetic wave and Particle nature.
Wavelength is 380-750nm
What is Monochromacy and how do they differentiate between colour?
A person with only one photopigment. Cannot make wavelength based discrimination of colour and instead uses brightness.
Principle of univariance
All information about its wavelength is lost.
When a photopigment receptor absorbs light, it does not encode the wavel
What is Trichromatic Theory?
States that people have 3 cone types with 3 different peak wavelength absorptions based on the principle of univarriance. Cones are colourblind.
What are cone photospins and rod photospins peak wavelenghts absorption and how is colour discriminated?
Cones: S M L
S = 420nm - short wl
M = 530nm - middle wl
L = 560nm - long wl
Rod photospins = 500nm
Wavelenght discrimination requires the comparison of two different photoreceptors. Additionally, different wavelengths of different intensities can produce the same amount of absorption and thus generate the same signal.
What is a metameter?
When colour of a monochromatic test light can be matched by using combinations of lights or pigments.
E.g checking for Jaundice
What are the limitations of trichromatic theory
Fails to account for the four unique colours = red, green, yellow and blue.
Edward Harold noted that there are colour combinations unseen = green/red and blue/yellow
What theory was proposed instead of the trichromatic theory?
Edward Harold proposed the opponent process theory to explain the existance of after images.
Trichromatic explain the photopigments and signalling for light?
Photopigment consists of a chromophore
chemical and an opsin (protein).
The chromophore in all photopigments
is retinal, a derivative of retinol.
Photon absorption by the chromophore
triggers the chemical phototransduction
cascade which results in visual transduction.
CIE colour specification system –
key points
- Specifies relative amounts of primaries to make matches of colour samples.
- CIE primaries are imaginary.
- Any set of colour matching function can be converted to another set of real or imaginary functions.
- Dominant wavelenght = principle wavelength of colour
What are the two colour vision deficiencies?
Congenital:
- Due to absence of one or more colour photopigments
Acquired:
- Developed after birth
- Due to pathology - usually unequal btw each eye due to different pathology development.
What are the types of Congenital?
Absence of one or more photopigments:
- Absence of erythrolabe photopigment =
Protanopia
- Absence of chlorolabe photopigment =
Deuteranopia
- Absence of cyanolabe pigment =
Tritanopia
- Absence of all cone photopigments = rod
monochromasy
Due to anomalous cone photopigments
- Protanomaly, deuteranomaly, tritanomaly
What visual defects are most common?
- Deutans are most common followed by protans then tritans
- Anomalous trichromats are more common than dichromats
- Large variations exist across different ethnic groups
What are the type of inheritance of visual defects?
- Protan and Duetan colour vision
deficiencies are typically inherited as X
linked recessive so nay more males express
colour vision deficiencies than females (8%
of males have a congenital colour vision
deficiency, but only 0.5% of females) - Tritan colour vision deficiencies
are typically inherited as
autosomal dominant. Much
rarer than deutan and protan
in congenital forms.
Not sex-linked.
50% chance to be affected
Ananomolus trichromats
Anomalous Trichromats have 3
photopigments, but their colour vision is
abnormal
* The absorption spectrum of one
photopigment is displaced to an abnormal
position
General:
Can see colours, just perceived differently
* Problem with mixture of red and green
* May be unaware of any colour
deficiency
