1. Colour Vision I The retinal basis of colour vision and the inherited colour vision deficiencies
Prof. Kathy T. Mullen
McGill Vision Research (H4.14)
Dept. of Ophthalmology
8th Sept 2005

2. What is colour?
What physical aspect of the world does our sense of colour inform us about?

3. Spectral colors 425 500 550 600 650 Wavelength (nm)
Violet Indigo Blue Green Yellow Orange Red
Wavelength (nm)

4. Reflectance curves of some common foods
Reflectance (percent)
Orange
Lemon
Tomato
Cabbage
Wavelength (nm)

5. The colour circle

6. What is colour?
Colour vision allows us to distinguish between surfaces with different spectral reflectances

7. How do we see colour?

8. White light is produced by mixing three colours

9. Mixing red and green lights to match yellow.B C
A and B. Green and red lights on the top are mixed by the subject to match the yellow light presented on the bottom.
C. The red-green mixture perfectly matches the yellow.
The same match as it appears to a deuteranomalous observer.

10. Principle of Trichromacy
Mixing together three coloured lights in suitable proportions enables us to make an exact match to any other colour
The 3 mixing lights are called ‘primaries’
The match is called ‘metameric’ - meaning that identical colour sensations are produced even though the stimuli are physically different
3 mixing lights
test light to be matched
L1 + L2 + L3 L4

11. Spectral sensitivities of L, M & S cones
Long
Medium
Log relative sensitivity
Short
Wavelength (nm)

12. A single type of photoreceptor cannot signal colour
100
Relative absorbance % 50 L1 L2
450
550
(nm)

13. Response curve for a single receptor
Relative absorbance %
L1 = 2 (L2) L1 L2
Wavelength (nm)

14. Principle of Univariance
The response of a photoreceptor to any wavelength can be matched to any other wavelength simply by adjusting the relative intensities of the two stimuli
Therefore: any single receptor type is colour blind

15. Response curve for a two receptor system
100
Cone 1
Cone 2
relative absorbance %
Wavelength

16. How is colour coded?
Each colour produces a unique pattern of relative activities in the three cone types

17. The basis of colour mixing in a two receptor (dichromatic) system
100 M L
The mixture of red and green light looks the same as the yellow light because the red-green mixture and the yellow produce the same quantal absorptions in the L and M cones
Relative absorbancy 50 L1 L2 L3
WL (nm)
Lights L1 L2
L1+L2 L3 M 90 55
145 95
Receptors L 50 95
145 95
L:M
1:1
1:1
A dichromatic system requires 2 mixing lights
A trichromatic (three receptor) system requires 3 mixing lights (primaries)

18. Colours with different wavelength distributions will look identical if they produce the same ratio of quantum catches in the L, M and S cone types

19. Metameric (matched) colour pairs for colour deficient observers

20. Inherited color vision deficiencies
Systematic and predictable losses
Both eyes affected
Male - sex linked for L & M (red-green) deficiencies
Genetic S cone deficiencies are autosomal and rare - many are undetected
Color vision tests may not detect achromats

21. Trichromats
One of the three cone types is anomalous

22. Trichromats Three colours are required to match any other
See a full range of colours, but with poorer discrimination in some regions
Types
Protanomalous = anomalous L cones 1% (m)
Deuteranomalous = anomalous M cones 5%(m)
‘Tritanomalous’ = incidence unknown

23. Dichromats
One of the three cone types is missing

24. Dichromats Only need two colours to match any other
Sees a much reduced range of colours
Types
Protanope = lacks L cones 1% (male)
Deuteranope = lacks M cones 1% (male)
Tritanope = lacks S cones %

25. Genes for the L & M cone pigments

26. Monochromats No colour vision: any colour matched with any other
Rod monochromat (0.003%)
All cones are functionally absent
Blue cone monochromat (atypical monochromat)
Only S cones are present (0.001%)
Difficult to differentiate the two types
May use colour names effectively
May perform OK on some standard colour tests

28. Original
Protanope
Deuteranope
Tritanope

29. Original
Protanope
Deuteranope
Tritanope

30. Visual scene as it appears to (a) normal and (b-d) colour deficient observers

31. L/M cone opponent mechanisms

32. The luminance mechanism

33. Contrast sensitivity of red/green and luminance gratings

34. S/(L+M) cone opponent mechanisms