Colour vision Flashcards
In the dark adapted state, do the different wavelength colours appear the same brightness or variably?
Variably; in the dark adapted state, some colours appear more brighter than others
In the dark adapted state which colours appear the brightest?
Blue-green colours
In the dark adapted state, at what wavelength are the blue-green colours the brightest?
500nm
In the light adapted state, what colours appear the brightest?
yellow-green
In the light adapted state, at what wavelength do yellow-green colours appear the brightest?
555nm
What is the purkinje shift?
The phenomenon of shorter wavelength colours appearing brighter as luminance is reduced
How can the Purkinje shift be experimentally evaluated?
Flicker ERG, because the flicker is set above 25 cycles/s which the rod photoreceptors cannot detect but cone photoreceptors can
What is the photochromatic interval?
The measure of the difference in “brightness” between the a photoreceptor that has evoked a visual response and the brightness of when a colour is registered
What is the ‘trichromatic theory’ of colour vision?
To perceive colour, the electrical responses from the three different wavelength cones needs to be summised
In the fovea, which wavelength of cone is infrequent?
Blue wavelength cone
What is meant by ‘colour constancy’?
Despite different lighting conditions (and therefore wavelengths of light reflected off an object), the colour of the object remains the same i.e. a letterbox appears red in dark and light luminance
What area in the visual system determines colour constancy?
Cortex (higher visual processing)
What is rod monochromatism?
Patient who have faulty cone photoreceptors in the retina, i.e the cones are present but they do not function. Hence, they see everything in various shades of grey
What is the frequency of rod monochromatism in the population?
1 in 30,000
What are the features of rod monochromatism?
- Grey colour
- Poor visual acuity (0.1-0.3)
- high intensity lights are uncomfortable
- nystagmus
- macular dystrophy
- morphologically normal cones but with non-functioning
What is cone monochromatism?
Both rods and all three types of cones are present therefore, the defect may be in the cortex (area V4, involved in cortical processing) therefore patients are unable to distinguish hues
What is the frequency of cone monochromatism?
1 in 100,000
What are the features of cone monochromatism?
- Normal visual acuity
- Unable to distinguish hues of colour
- Defect in V4 of cortex
What is dichromatism?
Defect in one of the cone photoreceptors, such that the retina can only respond to 2/3 of the cone photoreceptors, therefore the colour that appears is a mixing of two colours (restricted range of secondary colours)
What are the three types of dichromatism?
- Protanopes
- Deuteranopes
- Tritanopes
What is a protanope?
Missing red wavelength cone
What is a deuteranope?
Missing green wavelength cone
What is a tritanope?
Missing blue wavelength cone
What is anomalous trichomatism?
Patient have full trichromatic colour, but do not make the same colour mathces as the rest of the population
This is because they use different proportions of the three primary cones to make colour
What are the three types of anomalous trichromacy?
- Protrans
- Deutans
- Tritans
What are protans?
Type of anomalous trichromacy that uses more of the red colour
What are deutans?
Type of anomalous trichromacy that uses more green colour
What are tritans?
Type of anomalous trichromacy that uses more blue colour
What is cerebral achromatopsia?
colour blindness due to defect in cerebral cortex V4
