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 kathy.mullen@mcgill.ca 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 Wavelength (nm) Violet Indigo Blue Green Yellow Orange Red 425 500 550 600 650 Wavelength (nm)

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

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. 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.

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 Long Medium Log relative sensitivity Short Wavelength (nm)

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

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

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 100 Cone 1 Cone 2 relative absorbance % 540 565 Wavelength

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 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)

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 Protanope Deuteranope Tritanope

Original Protanope Deuteranope Tritanope

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

S/(L+M) cone opponent mechanisms