Colour Vision IThe 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
What is colour?
What physical aspect of the world does our sense of colour inform us about?
Spectral colors
425 500 550 600 650
Violet Indigo Blue Green Yellow Orange Red
Wavelength (nm)
Reflectance curves of some common foods
Ref
lect
ance
(pe
rcen
t)
Wavelength (nm)
Lemon
Tomato
Orange
Cabbage
The colour circle
What is colour?
Colour vision allows us to distinguish between surfaces with different
spectral reflectances
How do we see colour?
White light is produced by mixing three colours
Mixing red and green lights to match yellow.
A B CA 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.
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
Spectral sensitivities of L, M & S cones
Wavelength (nm)
Log
rel
ativ
e se
nsit
ivit
y
Long
Medium
Short
A single type of photoreceptor cannot signal colour
Rel
ativ
e ab
sorb
ance
%100
50
450 550
(nm)
L1 L2
Response curve for a single receptor
Rel
ativ
e ab
sorb
ance
%
Wavelength (nm)
L1 L2
L1 = 2 (L2)
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
Response curve for a two receptor system
Cone 1 Cone 2
540 565Wavelength
rela
tive
abs
orba
nce
%
100
How is colour coded?
• Each colour produces a unique pattern of relative activities in the three cone types
The basis of colour mixing in a two receptor (dichromatic) system
M L
L1 L2L3
WL (nm)
100
50
0
M
L
L:M
L1 L2 L1+L2 L3
Rel
ativ
e ab
sorb
ancy
Rec
epto
rs
Lights
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
A dichromatic system requires 2 mixing lightsA trichromatic (three receptor) system requires 3 mixing lights (primaries)
90 55 145
50 95 145
1:1
95
95
1:1
• 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
Metameric (matched) colour pairs for colour deficient observers
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
Trichromats
• One of the three cone types is anomalous
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
Dichromats
• One of the three cone types is missing
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 0.002%
Genes for the L & M cone pigments
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
Original
TritanopeDeuteranope
Protanope
Original
TritanopeDeuteranope
Protanope
Visual scene as it appears to (a) normal and (b-d) colour deficient observers
L/M cone opponent mechanisms
The luminance mechanism
Contrast sensitivity of red/green and luminance gratings
red/green
luminance
S/(L+M) cone opponent mechanisms