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.

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

[email protected]

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