Andrew Stockman
INTRODUCTION
Advanced Colour
NEUR 3001/G001/M001 Advanced Visual Andrew Neuroscience Stockman
Light 400 - 700 nm is important for vision
How dependent are we on colour?
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No colour… Colour…
ACHROMATIC COMPONENTS
Split the image into...
But just how important is colour? CHROMATIC COMPONENTS
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CHROMATIC COMPONENTS ACHROMATIC COMPONENTS
Chromatic information by itself provides relatively Achromatic information is important for fine detail … limited information…
How do we see colours? Human photoreceptors Rods Cones An image of the world is . Achromatic . Daytime, achromatic projected by the cornea and night vision and chromatic vision lens onto the rear surface of . 1 type . 3 types the eye: the retina. Rod Long‐wavelength‐ sensitive (L) or “red” cone
Middle‐wavelength‐ sensitive (M) or “green” cone The back of the retina is carpeted by a layer of light‐ Short‐wavelength‐ sensitive photoreceptors sensitive (S) or “blue” (rods and cones). cone
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Rod and cone distribution
Central fovea is rod-free, and the very central foveola is rod- and S-cone free
0.3 mm of eccentricity is about 1 deg of visual angle
Chromophore (chromo- color, + -phore, producer) Light-catching portion of any molecule
11-cis retinal. The molecule is twisted at the 11th carbon.
Photopigment molecule (cone) From Sharpe, Stockman, Jägle & Nathans, 1999
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Chromophore Chromophore
A photon is absorbed A photon is absorbed
the energy of which initiates a conformational change to…
Chromophore Chromophore
A photon is absorbed
This process is binary: all (1) or nothing (0). the energy of which initiates a conformational change to…
What does that tell us about how wavelength all-trans retinal be encoded? (side chains omitted for simplicity).
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Colour isn’t just about physics. For example:
Is colour, as we perceive it, mainly a These two property of physics or biology? metamers of yellow
+
though physically very different, can appear identical.
There are many other such metamers or matches…
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COLOUR MIXING
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What was colour mixing able to tell us about colour vision (before we Human vision is knew about the trichromatic. underlying biology)?
Colour TV
Trichromacy is exploited in colour reproduction, since the myriad of colours perceived can be produced by mixing together small dots of three colours.
If you look closely at a colour television (or this projector output)…
Trichromacy means that 3-coloured dots 3-coloured bars colour vision is relatively simple.
It is a 3 variable system…
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The dots produced by a TV or projector are so small that they are mixed together by the eye and thus appear as uniform patches of colour
The laws of colour mixing that apply to projected lights or combinations of dots are called “additive”.
The laws of colour mixing that apply to pigments or paints But what about mixing paints? are different because they depend on what is absorbed or “subtracted” from the reflected light by the pigment.
WHITE PIGMENT Photo: Jozsef Szasz-Fabian
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WHITE PIGMENT RED PIGMENT
A white pigment reflects red, green and blue lights. A red pigment subtracts green and blue and reflects red.
GREEN PIGMENT BLUE PIGMENT
A green pigment subtracts red and blue and reflects green. A blue pigment subtracts red and green and reflects blue.
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Subtractive colour mixing (of paints or pigments)
Laws of subtractive colour mixing YELLOW PIGMENT (of paints or pigments)
MAGENTA PIGMENT CYAN PIGMENT
Subtractive colour mixing Subtractive colour mixing
YELLOW PIGMENT YELLOW PIGMENT += + = GREEN PIGMENT RED PIGMENT
CYAN PIGMENT MAGENTA PIGMENT
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Subtractive colour mixing SUBTRACTIVE COLOUR MIXING
CYAN PIGMENT + = BLUE PIGMENT
MAGENTA PIGMENT
SUBTRACTIVE ADDITIVE SUBTRACTIVE ADDITIVE COLOUR MIXING COLOUR MIXING COLOUR MIXING COLOUR MIXING
NOTE THAT THESE MIXING “LAWS” ARE BOTH But, why is normal human vision a CONSISTENT WITH HUMAN COLOUR VISION trichromatic, three variable system? BEING A TRICHROMAT, THREE VARIABLE SYSTEM.
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One reason, of course, is because just three cone But trichromacy also depends on the fact photo‐receptors underlie daytime colour vision. that the output of each photoreceptor is:
“UNIVARIANT”
What does univariant mean? Short-wavelength- Middle-wavelength- Long-wavelength- sensitive or “blue” sensitive or “green” sensitive or “red”
Use Middle‐wavelength‐sensitive (M) cones as an example…
UNIVARIANCE UNIVARIANCE Crucially, the effect of any absorbed photon is independent of its The effect of any absorbed photon is independent of its wavelength. wavelength.
M-cone M-cone
Once absorbed a photon produces the same change in So, if you monitor the cone output, you can’t tell which photoreceptor output whatever its wavelength. “colour” of photon has been absorbed.
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UNIVARIANCE UNIVARIANCE Crucially, the effect of any absorbed photon is independent of its wavelength. What does vary with wavelength is the probability that a photon will be absorbed.
M-cone
This is reflected in what is called a “spectral sensitivity function”. All the photoreceptor effectively does is to count photons.
Imagine the sensitivity to these photons…
0
-1 M-cone
-2 In order of M‐cone sensitivity: quantal sensitivity quantal 10 >>>> >> Log Changes in light intensity are confounded -3 with changes in colour (wavelength)
400 450 500 550 600 650 700 Wavelength (nm)
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UNIVARIANCE A change in photoreceptor output can be caused by a change in intensity or by a change in colour. There is no way of telling which. Univariance
If a cone is n times less sensitive to light A than to light B, then if A is set to be n times brighter than B, the two lights will appear identical whatever their wavelengths.
Each photoreceptor is therefore ‘colour blind’, and is unable to distinguish between changes in colour and changes in intensity.
If we had only one photoreceptor, we would be colour-blind…
People with normal colour vision have three univariant cones with different spectral sensitivities…
Examples: night vision, blue cone monochromats
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0
L -1 M Their colour vision is therefore three dimensional or: S
-2 TRICHROMATIC quantal sensitivity 10 Log -3
400 450 500 550 600 650 700 Wavelength (nm)
Trichromacy means our colour vision is actually limited So, if each photoreceptor is colour- blind, how do we see colour? We confuse many pairs of colours that are spectrally very different. Such pairs are known as metameric pairs.
Or to put it another way: Many of these confusions would be obvious to a being with 4 cone photoreceptors—just as the confusions of How is colour encoded at the colour deficient people are obvious to us. input to the visual system?
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Colour is encoded by the relative cone outputs
Blue light Red light Blue light Red light
If we want to be able to predict the relative responses of the three Green light Purple light Purple light Green light cones signals to lights of any spectral composition, we need to know the three cone spectral sensitivities… Yellow light White light Yellow light White light
In other words:
DETERMINING CONE SPECTRAL SENSITIVITIES We want to measure how the sensitivity of each cone type varies with wavelength.
How might we do that?
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Spectral sensitivity measurements Spectral sensitivity measurements
Flashing or flickering light Flashing or flickering light
Observer sets the threshold for Observer sets the threshold for detecting the flash or flicker as a detecting the flash or flicker as a Intensity Intensity function of the wavelength of the light. function of the wavelength of the light. Space (x) Space (x)
Spectral sensitivity measurements Spectral sensitivity measurements
Flashing or flickering light Flashing or flickering light
Observer sets the threshold for Observer sets the threshold for detecting the flash or flicker as a detecting the flash or flicker as a Intensity Intensity function of the wavelength of the light. function of the wavelength of the light. Space (x) Space (x)
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But the cone spectral sensitivities overlap throughout the spectrum.
S M L 0 M- and L-cone measurements
-1
-2 Use two special types of subjects: -3 Log sensitivity Deuteranopes -4 400 500 600 700 Protanopes Wavelength (nm)
Consequently, to measure them separately we have to use special subjects or special conditions.
Normal Protanope Normal Deuteranope
L M S L M S L M S L M S Protanopia Deuteranopia
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-8 Mean L‐cone spectral sensitivity function L-cone 0
L -9 L -1
Adjusted L- -10 quantal sensitivity cone data 10 Deuteranopes Log Macular and lens adjusted -2 -11 quantal sensitivity 10 400 450 500 550 600 650 700 Log 0.5 -3
0.0 difference 10 -0.5 400 450 500 550 600 650 700 Log 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)
Mean L‐ and M‐ cone spectral sensitivity functions 0 S-cone measurements L -1 M Two types of subjects: -2 quantal sensitivity
10 S‐cone (or blue cone) monochromats
Log Colour normals -3
400 450 500 550 600 650 700 Wavelength (nm)
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-6 Normal S‐cone monochromat
-8 S-cone data S -10
-12
-14
-16 quantal spectral sensitivity 10 -18 Normals
Log AS CF BCMs The Normal data were HJ FB -20 LS KS obtained on an intense orange TA PS adapting background that adapted (suppressed) the L- L M S L M S -22 400 450 500 550 600 and M-cones.
Wavelength (nm)
Mean spectral sensitivity functions 0 Why study spectral sensitivities?
L -1 M A knowledge of the spectral sensitivities of the cones is S important because it allows us to accurately and simply specify colours and to predict colour matches—for both -2 colour normal and colour deficient people (and to quantal sensitivity understand the variability between individuals). 10 These mean functions have enabled us to derive Log “standard” cone spectral Practical implications for colour printing, colour -3 sensitivity functions. reproduction and colour technology.
400 450 500 550 600 650 700 Wavelength (nm)
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Normal Tritanope Deuteranope
Credit: Euro Puppy Blog
Dogs are dichromats with only two cones peaking at 429 and 555 nm L M S L M S L M S Tritanopia
Normal Deuteranope
COLOUR VISION AND
MOLECULAR GENETICS Protanope
How do red-green colour vision deficiencies arise?
From Sharpe, Stockman, Jägle & Nathans, 1999
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Amino acid differences between photopigment opsins
Photopigment molecule From Sharpe, Stockman, Jägle & Nathans, 1999 From Sharpe, Stockman, Jägle & Nathans, 1999
Amino acid differences LWS all with OH between photopigment group POLAR opsins
MWS
NON-POLAR Why are the M‐ and L‐ cone opsins so similar? 180 277 285
Spectral shifts ~ 5 nm ~ 25 nm From Sharpe, Stockman, Jägle & Nathans, 1999
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Phylogenetic tree of visual pigments Mammals
About 460 – 520 nm Rh1 490 - 500 nm
About 460 – 520 nm Rh2 lost
About 420 – 480 nm SWS2 lost
About 355 – 450 nm SWS1 365 - 450 nm Basis for dichromatic colour vision
About 490 – 570 nm LWS 490 - 565 nm
Credit: Bowmaker
Humans
490 - 500 nm Gene duplication on the X-chromosome
lost Mammal lost
365 - 450 nm Human/ Old world Basis for primate trichromatic L-cone M-cone colour vision photopigment photopigment opsin gene opsin gene
Gene duplication Credit: Bowmaker
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Crossovers during meiosis are common:
Because these two genes are in a tandem array, and are very similar… Intergenic crossover
L-cone M-cone photopigment photopigment opsin gene opsin gene Intergenic crossovers produce more or less L and M- cone genes on each X chromosome
From Sharpe, Stockman, Jägle & Nathans, 1999
Intragenic crossovers produce hybrid or mixed L and M-cone genes 0 M Intergenic crossover -1
More M‐cone like -2 quantal sensitivity quantal 10
Intragenic crossover Log Hybrid (mixed) -3 L/M genes More L‐cone 400 450 500 550 600 650 700 like Wavelength (nm) Intragenic crossover The spectral sensitivities of the hybrid photo‐pigments vary between those of L the M‐ and L‐cones depending on where the crossover occurs. From Sharpe, Stockman, Jägle & Nathans, 1999
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Single-gene dichromats Multiple-gene dichromats
Protanope
With a single gene Male observers with two similar male observers must genes may also be effectively be dichromats dichromats if the two genes produce similar photopigments.
Deuteranope
Anomalous trichromats Male observers with two different genes are 1.0 1.0 “anomalous” trichromats Severe Normal Severe 0.5 Deutan 0.5 Protanomalous 0.0 0.0 350 450 550 650 750 350450550650750
Mild
1.0 1.0
Severe Mild Deuteranomalous 0.5 Protan 0.5 Protan Severe 0.0 0.0 350 450 550 650 750 350 450 550 650 750
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Main types of colour vision defects with approximate proportions of occurrence in the population. XY inheritance
percent in UK
Condition Male Female
Protanopia no L cone 1.0 0.02 Protanomaly milder form 1.0 0.03 Deuteranopia no M cone 1.5 0.01 Deuteranomaly milder form 5.0 0.4 Tritanopia no SWS cone 0.008 0.008
No red-green discrimination
The emergence of two longer wavelength (M‐ and L‐cones) is thought to have occurred relatively recently in primate evolution.
Why is it important?
Source: Hans Irtel
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Red-green discrimination
DIAGNOSING COLOUR VISION DEFICIENCIES
Source: Hans Irtel
Ishihara plates
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Tests measuring colour discrimination
Farnsworth-Munsell D-15 test
Farnsworth-Munsell D-15 Farnsworth-Munsell D-15
From: Ted Sharpe From: Ted Sharpe
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Mild Severe
Protan
D15 results
Deutan POSTRECEPTORAL COLOUR VISION Tritan
Rod monochromat
Credit: Jenny Birch
How is colour processed after the cones? The colour opponent theory of Hering
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Some ganglion cells are The colour opponent theory of Hering colour opponent
is opposed to R-G
is opposed to Y-B
RED On-centre How might this be related to visual GREEN Off-surround processing after the cones?
Red-green colour opponency Some ganglion cells are colour opponent
Four variants
GREEN On-centre RED Off-surround
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Summary Blue/yellow pathway
Trichromatic stage
Colour opponent stage
Source: David Heeger
So far, we’ve mainly been talking about the colours of isolated patches of light. But the colour of a patch depends also upon:
(i) What precedes it (in time) COLOUR AFTER-EFFECTS COLOUR AFTER-EFFECTS
(ii) What surrounds it (in space) (what precedes the patch) COLOUR CONTRAST COLOUR ASSIMILATION
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Colour after-effects
You don’t have to see things for them to produce an after-effect...
Beer & MacLeod
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Beer & MacLeod Mediafire
Lilac chaser or Pac-Man illusion
Jeremy Hinton
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Lilac chaser or Pac-Man illusion
COLOUR CONTRAST
(what surrounds the patch)
http://michaelbach.de/ot/index.html Michael Bach and Jeremy Hinton
Color contrast Color contrast
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Color contrast Color contrast
Color contrast
MacLeod
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COLOUR ASSIMILATION
MacLeod
Colour assimilation Colour assimilation
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Colour assimilation
Colour assimilation
Munker illusion
COLOUR CONSTANCY
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Credit: Gegenfurtner Colour constancy Colour constancy red green
g
blue yellow
Chromatic adaptation Colour and the illuminant and colour constancy
The change in colour appearance following adaptation is due to chromatic adaptation. Chromatic adaptation is adaptation to the colour is of the ambient illumination.
Amazing Art, Viperlib
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Colour and brightness
Monet, Claude The Regatta at Argenteuil c. 1872 19 x 29 1/2 in. (48 x 75 cm) Musee d'Orsay, Paris
S-CONE MEDIATED VISION IS UNUSUAL
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In other retinal regions, the S‐cone mosaic remains sparse. S-cones are unusual
S-cones form between 5 and 10% of the cone population.
Central area of the fovea lacks S- cones and is therefore tritanopic.
Curcio et al.
S‐cones make little or no contribution to luminance Primary S‐cone pathway is an ON pathway
S-cone ON bipolar L+M OFF bipolar
small blue-yellow bistratified ganglion cell From Rodieck (1998)
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LGN pathways S-cone vision is slow
Ganglion cell projections
Chromatic aberration
Why is S-cone vision sparse?
Base picture: Digital camera world
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Effect of chromatic blur on eye chart
COLOUR AND COGNITION
Jim Schwiegerling, U. Arizona
TEST 1 Stroop effect RED GREEN BLUE YELLOW PINK Say to yourself the colours of the ink in which the following words are written -- as fast as you can. ORANGE BLUE GREEN BROWN WHITE
GREEN YELLOW PINK RED ORANGE So, for RED, say “red”.
But for RED, say “green” BROWN RED WHITE BLUE YELLOW
Ready, steady… WHITE ORANGE GREEN BROWN RED
How long?
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TEST 2
BLUE PINK WHITE RED BROWN
BROWN RED BLUE GREEN ORANGE
YELLOW BLUE RED ORANGE WHITE
BROWN RED GREEN WHITE RED
RED PINK BLUE GREEN WHITE
How long?
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