Color Perception

• Combined rod + cone response yields both color and brightness perception

• Cell responses vary with illumination conditions:– low light levels: scotopic visual processing

• rods predominate, minimal color perception– medium light levels: mesopic processing– high light levels: photopic processing

• cones predominate

104

Color vision is three dimensional; any spectral color can be matched by a mixture of 3 primaries

Wavelength discrimination as a function of wavelength

Characteristics of human cone types

• Spectral sensitivity

• Mosaic

• Genetics

• Spectral tuning

0

20

40

60

80

100

120

340 380 420 460 500 540 580 620 660 700

Wavelength (nm)

Absorbance

S M L

Thomas Young

Techniques for measuring photopigment spectra:

• Derivation from CMF’s

• Microspectrophotometry (MSP)

• Electroretinography (ERG)

• Suction electrode recordings

It is possible to take a set of color matching functions and transform them into the cone spectral sensitivities.

MSP: pass a small beam of light through the outer segment and measure the amount of light transmitted as wavelength is varied.

ERG measures changes in gross electrical potential as a function of wavelength - in living eyes.

By measuring the photocurrent generated in the outer segment in response to a brief flash of light it is possible to estimate the action spectra of single cones.

VariSpecFilter

Shutter

Wedge

Lamp

Condenserlens

Achromatlens

Beamblock

Beamsplitter/combiner

Scale

~25 mm

Erfleeyepiece

Prism

70o

ERG-Flicker Photometry

Carroll et al. (2000)

0

0.5

1

1.5

2

2.5

460 500 540 580 620 660 700

Wavelength (nm)

Log Relative Spectral Sensitivity

max avg - max 559.61 0.50 558.14 0.97 559.70 0.59 558.99 0.12avg = 559.11nm 0.55

Schnapf & Schneeweis (1999)

Another illustration that individual cones are univariant. If you were monitoring photocurrent, you could not distinguish between a 550 nm flash of 103 m-2 and a 659 nm flash of 104 m-2.

Characteristics of human cone types

• Spectral sensitivity

• Mosaic

• Genetics

• Spectral tuning

L/M cone ratio is about 2:1L/M increases as you move into the peripheryS cones make up ~7%No S-cones in central 0.5 deg

50 m

Without AO With AO

*Registered sum of 6 images for each condition 1 deg. Temporal Retina, OD 20 yr, female, normal color vision

Selective Bleaching Conditions

0.00

0.25

0.50

0.75

1.00

400 450 500 550 600 650 700N

orm

aliz

ed a

bsor

ptan

ce

650 nm bleachimage bleach

470 nm bleach

0.00

0.25

0.50

1.00

400 450 500 550 600 650 700

Wavelength (nm)

imagebleach

0.75

Absorptance after 470 nm bleach

Ab

sorp

tan

ce a

fter

650

nm

ble

ach

0

0.1

0.2

0.3

0.4

0.5

0.1 0.2 0.3 0.4 0.5

MD

L cones

M cones

Absorptance angle ()

Nu

mb

er o

f co

nes

0

10

20

30

40

50

60

70

80

90

100 AP (temporal)

0

10

20

30

40

50

60

70

80

90

100

HS

YY

0

10

20

30

40

50

60

70 HS

0

10

20

30

40

50

60

70

80

90 JC

MD JP JC

YY

* A. Roorda & D. R. Williams, Nature 1999

*

*

*

HS AP nasalAN

RS JW temporal BSJW nasal

AP temporal

5 arcmin

-1

-0.5

0

0.5

1

1.5

2

2.5

460 500 540 580 620 660 700

Wavelength (nm)

Log Relative Spectral Sensitivity

100 %L

0 %L

Can use ERG to estimate L:M

Consequences of L:M Variation

• One might expect that if our color perception were constrained by the photoreceptors, large differences in the cone mosaic between individuals would lead to correspondingly large differences in color perception.

Unique Yellow

• Is thought to represent the point at which the red/green chromatic channel is in equilibrium.

• Even small differences in L:M ratio would lead to substantial differences in unique yellow.

500

520

540

560

580

600

620

640

0 10 20 30 40 50 60 70 80 90 100

Predicted Correlation

L/M Proportion (%L)

Uni

que

Yel

low

Wav

elen

gth

(nm

)

578 nm

Observed

Characteristics of human cone types

• Spectral sensitivity

• Mosaic

• Genetics

• Spectral tuning

L M

SChromosome 7

X-chromosome

Cone-Opsin Genes

0

20

40

60

80

100

120

340 380 420 460 500 540 580 620 660 700

Wavelength (nm)

Absorbance

S M L

L/M Gene Array

1) L and M genes are highly homologous,

2) Head-to-tail tandem array on the X chromosome,

3) Susceptible to unequal homologous recombination.

One Cell-Type Model

Stochastic Pigment-Gene Choice

Second Order

Neurons

???

random choicemechanism

“L vs. M

determines

gene choice

cell type”

L M

Characteristics of human cone types

• Spectral sensitivity

• Mosaic

• Genetics

• Spectral tuning

0

0.2

0.4

0.6

0.8

1

1.2

340 380 420 460 500 540 580 620 660 700

Wavelength (nm)

Ab

so

rban

ce

Mechanisms of Spectral Tuning

• Changes in the opsin protein

• Ocular filters (lens, oil droplets)

• Altering the chromophore

• Different optical density

Protonated Schiff base of 11-cis-retinal normally absorbs at 440nm in organic solvents.

However, most visual pigments have absorption maxima between 360 and 635 nm.

It turns out that small changes in the opsin (protein) component of the photopigment are responsible for determining where the pigment will absorb.

Helix 1Helix 2

Helix 3

Helix 4

Helix 5

Helix 6

Helix 7

312

180

277

285

Neitz & Neitz (1998)

Rayleigh matching reveals variation in normal pigments...

Wavelength (nm)

Fra

ctio

n of

inci

dent

ligh

t ab

sorb

ed

0.00

.025

.050

.075

.100

400 450 500 550 600 650 700

m

0

2

4

6

8

10

12

0.4 0.42 0.44 0.46 0.48 0.5 0.52 0.54 0.56 0.58

R/R + G

TestR & G Primaries

Variants of the L gene in individuals with normal color

vision

1

2

3

4

6

8

9

10

11

12

13

14

15

16

17

18

19

20

AminoAcid

I V Y M V V V A T S V

T I S L A I S I A MI

No 065 111 116 153 171 174 230 233 236178

Exon 2 Exon 3 Exon 4

65 111 116 153 171 174 230 233 236178

45

32

12

6

5

4

4

3

2

2

2

2

1

1

1

1

1

1

Total = 159

Frequency

10

1

22 1

23 1