Colour Vision 2 Metamers and Theories of Colour...

1

Colour Vision 2Metamers and Theories of

Colour Perception

PSY305 Lecture 10 JV Stone

2

Structure• Metamers: colour mixing (of light).• Three theories of colour vision

– Trichromatic Theory– Opponent process Theory– Dual process Theory

• Colour aftereffects as evidence for thesetheories.

3

Making colours

• A TV is evidence thatany colour (almost) canbe created by acombination of 3primary colours.

• But which threecolours?

• Almost any red-ish,blue-ish and green-ishlights will do …

4

Metamerism

This yellow can be made from a mixture of red and green lights ...

5

If two differentluminancespectra areperceived asthe samecolour thenthese twospectra aremetamers ofeach other.

Metamers

Different spectra,Same perceived colour

6

When viewed in isloation, if two differentlycoloured surfaces (ie with different luminancespectra) produce the same cone outputsthen they are perceived as the same colour.

Metamers

S

M

L

cones

-10mV

-20mV

-30mV Perceived Colour1

Conetripletoutputs

7

Why are there metamers?‘Metamers’ for one cone

If you had only one cone type then the output producedby a given wavelength (eg 520nm) can be reproducedexactly by a different wavelength (600nm).

The same type of behaviourensures that many differentspectra can induce thesame outputs in the threecone types.

This is the principle ofunivariance: a cone’soutput indicates how muchlight it absorbed, not thewavelength of that light.

8

Wavelength λ

L-cone responseTo test light

Test

ligh

t

M-cone responseTo test light

Metameric matching for two cone types

• If we had 2 cone types then we couldmatch any single test wavelength with acombination of two coloured torches.

9

Wavelength λ

Test

ligh

t

Gre

en to

rch

L-cone responseTo green torch

M-cone responseTo green torch

Metameric matching for two cone types

• A green torch yields a response from theM and L cone types which is less thantheir response to the test light.

10

Wavelength λ

L-cone responseTo test light

Test

ligh

t

M-cone responseTo test light

Gre

en to

rch

L-cone responseTo green torch

M-cone responseTo green torch

‘Difference in M-cone output

Difference in L-cone output

Metameric matching for two cone types

• The difference in cone outputs can bemade up by adjusting the brightness of ared torch …

11

Wavelength λ

L-cone responseTo test light

Test

ligh

t

M-cone responseTo test light

Red

torc

h

Gre

en to

rch

L-cone responseTo green torch

M-cone responseTo green torch

‘Missing’ M-cone output

‘Missing’ L-cone output

Metameric matching for two cone types

• M-cone response to test light =M-cone response to green torch + M-cone response to red torch.

• L-cone response to test light =L-cone response to green torch + L-cone response to red torch.

In practice, we get to adjust Brightness of torches too.

12

Metamers and cone outputs

S

M

L

cones

-10mV

-20mV

-30mV Perceived Colour1

Conetripletoutputs

Testlight

Luminance spectrum1reflected from white surface

13

Different illuminance spectrum

S

M

L

cones

-10mV

-20mV

-30mV Perceived colour2=perceived colour1

Luminance spectrum2=/=spectrum1reflected from white surface

Different illuminance spectrum gives different luminance spectrum, BUT same cone outputs, and therefore same perceived colour (when viewed in isolation).

Torchwavelengths

Conetripletoutputs

14

Metamer summary• Metamers exist because any single cone triplet output

can be produced by many different luminance spectra.• In terms of metameric matching, this means that any

cone triplet can be produced by many settings for threetorches used to create the luminance spectrum.

• Most importantly, because a given cone triplet producesa single perceived colour (when viewed in isloation),each perceived colour can be produced by manydifferent spectra.

15

Colour Blindness• 8 % males and < 1% females are colour

blind.• In general, being colour blind means being

unable to distinguish between certaincolours.

Engine painted by WilliamStroudley in a colour hecalled 'improved enginegreen'. This is an orangey-brown to trichromatsFrom Thompson on viperlib.

16

Theories of colour perception

17

The correspondence problem

LIGHT

INPUT OUTPUT

?

PROCESS

EXPERIENCE OFCOLOUR

PERCEPTION

In order to understandcolour perception, wemust establish whattakes place here.

A first stage in doingthis is to consider thefacts about thepsychologicalexperience of colourperception.

This was what the original theoriesof colour perception were basedon.

18

Theories of Colour Vision: 1Young-Helmholtz Trichromatic Theory

• The trichromatic theory states that there are 3 types ofcolour receptor in the human eye, with different butoverlapping responses to different wavelengths of light.

• Progressively constructed by Palmer (1777), Young (1802)and Helmholtz (1856).

• The fact that metameric matching required exactly THREElights was used to support this theory.

• Posits that different patterns of activation correspond to theperception of different colours.

• Existence of three cone types was confirmed by Dartnall,Bowmaker, and Mollon in 1983(!).

19

Strengths of Trichromatic Theory

• Accounts for the 3 (red, green, blue) dimensionsof colour space.

• Accounts for metameric matching (mixing of 3colours (red, green, blue) is sufficient to matchany other visible colour).

• Partially accounts for the three basic varieties ofcolour blindness:

Protanopes lack the long-wavelength receptorsDeutranopes lack the mid-wavelength receptorsTritanopes lack the short-wavelength receptors.

20

Doesn’t fully explain colour blindness:In reality, colour loss in colour blindnessoccurs in pairs: inability to discriminate pairsred-green or yellow-blue.Colours are never lost in isolation and pairingsother than those above are never found.

Weaknesses of Trichromatic Theory

21

Weaknesses of Trichromatic Theory

• Accounts for red/green aftereffects, but notblue/yellow aftereffects …

22

Colour Afterimages

23

Colour Afterimages

24

25

26

Take home message from these colour aftereffects:

They provide evidence for red-green andblue-yellow opponency, as the aftereffectsalways involve these particular colouropposites.

27

Theories of Colour Vision: 2Opponent process theory

• These observations led to speculation of apairwise polarity between the coloursred/green and yellow/blue.

• Mainly in response to his subjectiveexperience, Ewald Hering developed theopponent process theory of colourperception (1878-1964).

• This assumes there are 4 ‘primary’ colours(Red Green Blue Yellow).

28

Opponent process theory

Colour-opponent cells found in retinaand LGN. Here are RFs of ganglioncells sensitive to outputs of cones

ON

OFF

+

-

-

-- +

-

-

-- +

-

-

-- +

-

-

--

Eg, Output maximal if red light in centre and green in surround.

29

Opponent process theoryThese double opponent cells are found inthe blobs of primary visual cortex.Blue/yellow cells are also found there.

Output maximal if red light in centre andgreen in surround and is minimal if greenlight in centre and red in surround.

R+G-

R-G+

R-G+

R+G- D = Alignment of color blobs along L and R ocular dominance columns.Lu, H. D. et al. Cereb. Cortex 2008 18:516-533.

30

Constructing opponent channels

• This is all very well, but how to make B/Y …

rS rMrL

31

• Reconciled the debate between trichromatic theory andopponent process theory by suggesting that both occur,but at different levels in the visual system:

• Trichromatic analysis occurs first followed by anopponent process.

• This theory has subsequently been supported byneurophsyiological evidence, but remember…

• These theories were based mainly on introspection andintuitive experience, at the time the theories wereproposed scientists did not have access to the brainimaging techniques in use today.

Theories of Colour Vision: 3The Dual Process Theory

Hurvich and Jameson, 1957

32

The Dual Process Theory

• Cones yield three channels (L, M, and S) with outputsrL, rM, and rS …

Out

put

Channel outputs

33

Blue-Yellow

Red-GreenLuminance

Cha

nnel

Out

put

The Dual Process TheoryChannel outputs rL rM rS arerecombined to yield 3 newchannels:

Red-green = rL - rM,Blue-Yellow = rS -(rL + rM),Luminance = rL + rM.

34

So which theory is correct?

• The dual process theory has largely been supported byphysiological evidence.

• The dual process theory adopts ideas from both thetrichromatic theory and the opponent process theory.

• The photoreceptors in the retina are trichromatic - withpeaks at long (red), medium (green) and short (blue)wavelengths – as predicted by Helmholtz.

• Ganglion cells and cells in the LGN (both of whichreceive input from retinal photoreceptors) show red-green and yellow-blue opponent processes.

Trichromatic Stage Opponent-processStage

Retina / photoreceptors Ganglion cells / LGN

35

Summary

• There are three main theories of colour vision; thetrichromatic theory, the opponent process theory and thedual processes theory. These theories were postulatedbefore we had detailed information about the anatomy ofthe visual system.

• Neurophysiological evidence has been found whichsupports all theories, the photoreceptors are tri-chromatic, ganglion cells and cells in the LGN havecolour opponent receptive fields.

• Colour after effects provide compelling evidence ofcolour opponency (with specific colour pairings).

36

Reading

Essential• Frisby and Stone, Seeing Colour

Background• Gegenfurtner KR (2003) Cortical mechanisms of

color vision. Nature Reviews Neuroscience 4563-572.