Andrew Stockman
McCollough effect adapting pattern
Colour Vision
MSc Neuroscience course
Andrew Stockman LONG-TERM “CONTINGENT” ADAPTATION Akiyoshi Kitaoka
Light 400 - 700 nm is important for vision
INTRODUCTION
Lecture notes at http://www.cvrl.org
Click on “MSc Neuroscience” in left menu.
Colour Vision 1 Andrew Stockman
No colour…
How dependent are we on colour?
Colour…
But just how important is colour?
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ACHROMATIC COMPONENTS CHROMATIC COMPONENTS
Split the image into...
CHROMATIC COMPONENTS
By itself chromatic information provides relatively limited information…
ACHROMATIC COMPONENTS How do we see colours?
An image of the world is projected by the cornea and lens onto the rear surface of the eye: the retina.
The back of the retina is carpeted by a layer of light-sensitive photoreceptors (rods and cones). Achromatic information important for fine detail …
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Rods and Human photoreceptors cones Rods Cones . Achromatic . Daytime, achromatic night vision and chromatic vision . 1 type . 3 types
Rod Long‐wavelength‐ sensitive (L) or “red” cone
Middle‐wavelength‐ sensitive (M) or “green” cone
Short‐wavelength‐ sensitive (S) or “blue” cone Webvision
Rod and cone systems are optimized for different light levels
Moonlight Sunlight Typical ambient light levels Starlight Indoor lighting
Photopic retinal illuminance -4.3 -2.4 -0.5 1.1 2.7 4.5 6.5 8.5 (log phot td)
Why do we have rods and cones? Scotopic retinal illuminance -3.9 -2.0 -0.1 1.5 3.1 4.9 6.9 8.9 (log scot td)
SCOTOPIC MESOPIC PHOTOPIC Visual function
Absolute rod Cone Rod saturation Damage threshold threshold begins possible
Rod system Cone system
A range of c. 106 A range of > 108
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Rod and cone distribution Moonlight Sunlight Typical ambient light levels Starlight Indoor lighting
Photopic retinal illuminance -4.3 -2.4 -0.5 1.1 2.7 4.5 6.5 8.5 (log phot td)
Scotopic retinal illuminance -3.9 -2.0 -0.1 1.5 3.1 4.9 6.9 8.9 (log scot td)
SCOTOPIC MESOPIC PHOTOPIC Visual function
Absolute rod Cone Rod saturation Damage threshold threshold begins possible
Scotopic levels Mesopic levels Photopic levels (below cone threshold) where rod and cone (above rod saturation) where rod vision vision function where cone vision functions alone. together. functions alone. 0.3 mm of eccentricity is A range of c. 103 A range of c. 103 A range of > 106 about 1 deg of visual angle
Rod density peaks at about 20 deg eccentricity During the day, you have to look at Cones peak at the centre things directly to see them in detail of vision at 0 deg
At night, you have to look away from things to see them in more detail
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Original photograph The human visual system is a foveating system
Simulation of what we see when we fixate with cone vision.
Central fovea is rod-free, and the very central foveola is rod- and S-cone free
Credit: Stuart Anstis, UCSD
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 How can this process A photon is absorbed encode wavelength?
the energy of which initiates a conformational change to… 11-cis retinal.
all-trans retinal all-trans retinal. (side chains omitted for simplicity). Can it?
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Four human photoreceptors with different spectral sensitivities 441 500 541 566 max (nm, corneal, quantal)
0 Vision at the photoreceptor stage is Note logarithmic scale relatively simple because the output -1
of each photoreceptor is: -2
UNIVARIANT -3 Rods quantal sensitivity quantal L 10 -4
Log S -5 M
400 450 500 550 600 650 700 We’ll come back to this… Wavelength (nm)
Colour… Colour isn’t just about physics. For example:
Two metamers of yellow
+
Is it mainly a property of physics or biology? though physically very different, can appear identical.
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There are many other such metamers or matches…
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Colour mixing
What can colour mixing tell us about colour vision?
Trichromacy means that colour vision is relatively simple.
Human vision is It is a 3 variable system… trichromatic
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Colour TV The dots produced by a TV or projector are so small that they are mixed together by the eye and Trichromacy is exploited in colour reproduction, since thus appear as uniform patches of colour 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)…
3‐coloured dots 3‐coloured bars
Why is human vision trichromatic? The main reason is because just three cone photoreceptors are responsible for daytime colour vision.
Short‐wavelength‐ Middle‐wavelength‐ Long‐wavelength‐ sensitive or “blue” sensitive or “green” sensitive or “red”
But it is also depends upon the fact that...
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UNIVARIANCE Crucially, the effect of any absorbed photon is independent The output of each photoreceptor is: of its wavelength.
“UNIVARIANT”
M-cone What does univariant mean?
Once absorbed a photon produces the same change Use Middle‐wavelength‐sensitive (M) cones as an example… in photoreceptor output whatever its wavelength.
UNIVARIANCE UNIVARIANCE Crucially, the effect of any absorbed photon is independent Crucially, the effect of any absorbed photon is independent of its wavelength. of its wavelength.
M-cone M-cone
So, if you monitor the cone output, you can’t tell All the photoreceptor effectively does is to count which “colour” of photon has been absorbed. photons.
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M-cone spectral sensitivity function UNIVARIANCE High sensitivity (less photons needed to have an effect) 0 What does vary with wavelength is the probability that a photon will be absorbed. -1
-2 quantal sensitivity quantal
10 Low sensitivity (more photons needed Log to have an effect) -3 This is reflected in what is called a “spectral sensitivity function”. 400 450 500 550 600 650 700 Wavelength (nm)
Imagine the sensitivity to these photons…
0 0
-1 -1 If you have four lights of the same intensity (indicated here by their heights) -2 In order of M‐cone sensitivity: -2 quantal sensitivity quantal quantal sensitivity quantal
10 The green will look brightest, then 10 >>>> >> yellow, then blue and lastly the Log Log red will be the dimmest -3 -3
400 450 500 550 600 650 700 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)
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0
-1 M-cone We can adjust the intensities to compensate for the sensitivity differences. -2 quantal sensitivity quantal
10 When this has been done, the four lights will look completely Log identical. Changes in light intensity are confounded with -3 changes in colour (wavelength)
400 450 500 550 600 650 700 Wavelength (nm)
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.
Colour or intensity Univariance in suction electrode recordings change??
Each photoreceptor is therefore ‘colour blind’, and is unable to distinguish between changes in colour and changes in intensity.
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If we had only one photoreceptor, we would be colour‐blind…
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.
Examples: night vision, blue cone monochromats
With three cone photoreceptors, our colour vision is trichromatic…
So, if each photoreceptor is colour-blind, how do we see colour?
Or to put it another way: How is colour encoded?
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Colour is encoded by the relative cone outputs Trichromacy actually means our colour vision is limited Blue light
0 We confuse many pairs of colours that are spectrally L very different. Such pairs are known as metameric -1 M pairs. S uantal sensitivity
q -2
Many of these confusions would be obvious to a 10
being with 4 cone photoreceptors—just as the Log
confusions of colour deficient people are obvious to -3 us. 400 450 500 550 600 650 700 Wavelength (nm)
Colour is encoded by the relative cone outputs Colour is encoded by the relative cone outputs
Blue light Blue light
0 0 L L Green light -1 M -1 M S S uantal sensitivity uantal sensitivity Red light q q -2 -2 10 10 Log Log
-3 -3 Red light
400 450 500 550 600 650 700 400 450 500 550 600 650 700 Wavelength (nm) Wavelength (nm)
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Colour is encoded by the relative cone outputs
Blue light Red light
DETERMINING CONE SPECTRAL SENSITIVITIES Green light Purple light
Yellow light White light
In other words…
The cone spectral sensitivities overlap S M L extensively throughout 0 the spectrum. -1
-2 Consequently, we have How can we measure how the sensitivity of each cone -3 to use special subjects Log sensitivity type varies with wavelength (or spectral colour)? or special conditions to -4 be able isolate the the 400 500 600 700 response of a single cone type. Wavelength (nm)
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Normal Protanope
M- and L-cone measurements
Use two special types of subjects:
Deuteranopes Protanopes
L M S L M S Protanopia
Normal Deuteranope -8 L-cone
L -9
(Adjusted) L‐cone -10 quantal sensitivity
data 10 Log Macular and lens adjusted -11
400 450 500 550 600 650 700
0.5
0.0 difference 10 L M S L M S -0.5 Log Deuteranopia 400 450 500 550 600 650 700 Wavelength (nm)
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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 Colour normals Log -3
400 450 500 550 600 650 700 Wavelength (nm)
-6 Normal S‐cone monochromat
-8 S-cone data S -10
-12
-14
-16 The Normal data were quantal spectral sensitivity
10 obtained on an intense orange -18 Normals adapting background that was Log AS there to suppress the L- and CF BCMs HJ FB M-cone sensitivities. -20 LS KS TA PS
L M S L M S -22 400 450 500 550 600
Wavelength (nm)
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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
10 These mean functions understand the variability between individuals). 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)
Normal Tritanope Deuteranope
Credit: Euro Puppy Blog
Dogs are dichromats with only two cones peaking at L M S L M S 429 and 555 nm L M S Tritanopia
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But what happens next (i.e., how is colour encoded after the photoreceptors)?
POSTRECEPTORAL COLOUR VISION
Colour phenomenology The colour opponent theory of Hering Can provide clues about how colours are processed after the photoreceptors…
Which pairs of colours coexist in a single, uniform patch of colour? Which pairs never coexist?
Reddish‐greens? Reddish‐yellows? Bluish‐reds? Bluish‐yellow? ?? Reds can get bluer or yellower but not greener
WHY?
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The colour opponent theory of Hering The colour opponent theory of Hering
Yellows can get greener or redder but not bluer Greens can get bluer or yellower but not redder
The colour opponent theory of Hering
The colour opponent theory of Hering
Blues can get greener or redder but not yellower
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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
Imagine that this is the region of space that the cell “sees” in How might this be related to visual the external world processing after the cones?
Some ganglion cells are Some ganglion cells are colour opponent colour opponent
A red light falling on the central area A green light falling on the surround area excites the cell (makes it fire faster) inhibits the cell (makes it fire slower)
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Red-green colour opponency Blue/yellow pathway
Four variants
Source: David Heeger
LGN cell responses
LESS COMMON
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Summary So that’s colour (chromatic) ACHROMATIC COMPONENTS encoding, but what about “luminance” (achromatic) encoding? Trichromatic stage
CHROMATIC COMPONENTS Colour opponent stage
In addition to neural pathways that signal colour there are also pathways that signal intensity or luminance:
Chromatic and achromatic pathways
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Luminance is encoded by summing the L‐ and M‐cone signals:
Blue light Red light
L+M + + Colour is in many ways secondary Green light Purple light to luminance L+M + +
Yellow light White light
L+M + +
+ +
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+ +
+ +
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Boynton illusion
Rob van Lier, Mark Vergeer & Stuart Anstis
Boynton Boynton illusion illusion
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Boynton Fading dot illusion
Watercolour effect
What are the postreceptoral neural substrates of the chromatic and luminance pathways?
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Magnocellular
Luminance pathways, which produce Diffuse bipolar diffuse cells sum across achromatic (colourless) percepts, have bipolar M‐ and L‐cones been linked to the magnocellular stream.
parasol ganglion cells From Rodieck (1998)
Koniocellular
Chromatic pathways, which produce diffuse bipolar diffuse chromatic percepts, have been linked to the S-cone bipolar koniocellular stream for S‐(L+M)… bipolar
blue-yellow bistratified ganglion cell From Rodieck (1998)
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Parvocellular
Chromatic pathways, which produce chromatic percepts, have been linked to the koniocellular stream for S‐(L+M)... In the fovea, midget bipolar cells have single M‐ and L‐cone midget bipolars inputs.
And also to the parvocellular retinal stream for L‐M.
midget ganglion cells From Rodieck (1998)
[Based on Boynton (1979)]
Although luminance pathways have been linked to the magnocellular stream, they must also depend on the parvocellular stream.
Parvocellular Koniocellular Magnocellular pathway pathway pathway
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Austin Roorda, 2004 The parvocellular pathway must be double‐duty supporting finely detailed luminance vision as well as much more coarse colour vision.
To be able to resolve this E at the resolution limit, there must be enough samples.
The parvocellular pathway, with its one‐to‐one cone to bipolar connections, provides enough samples.
The magnocellular pathway, with diffuse bipolar cells, does
not. Austin Roorda, 2004
Magnocellular pathway: Parvocellular pathway: High temporal frequencies (motion) High spatial frequencies (detail) Low spatial frequencies Low temporal frequencies Achromatic Chromatic Multiplexing colour and Higher contrast sensitivity Lower contrast sensitivity luminance information
From Rodieck (1998)
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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
Colour after-effects
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You don’t have to see things for them to produce an after‐effect...
Beer & MacLeod
Beer & MacLeod Mediafire
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Lilac chaser or Pac-Man illusion
Jeremy Hinton
Color contrast
COLOUR CONTRAST
(what surrounds the patch)
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COLOUR ASSIMILATION
Colour assimilation
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Munker illusion
COLOUR CONSTANCY
Credit: Gegenfurtner Colour constancy Colour constancy red green
g
blue yellow
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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
Colour and brightness
Monet, Claude COLOUR AND The Regatta at Argenteuil c. 1872 19 x 29 1/2 in. (48 x 75 cm) COGNITION Musee d'Orsay, Paris
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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?
TEST 2 McCollough effect test pattern
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? Version by Akiyoshi Kitaoka
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McCollough effect adapting pattern McCollough effect test pattern
LONG-TERM “CONTINGENT” ADAPTATION Version by Akiyoshi Kitaoka Akiyoshi Kitaoka
Tritanope
COLOUR VISION, COLOUR DEFICIENCIES AND MOLECULAR GENETICS
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Appearance of visible spectrum
Colour deficiency simulations From Sharpe, Stockman, Jägle & Nathans, 1999
Normal Deuteranope
COLOUR VISION AND MOLECULAR GENETICS Tritanope Protanope
From Sharpe, Stockman, Jägle & Nathans, 1999
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The spectral sensitivity of the photopigment depends on the energy required to flip the chromophore from it 11‐ cis form to its all‐trans form.
But the same chromophore is used in all four human photo‐ pigments.
The energy is modified by changes in the amino acids in the surrounding opsin molecule.
Photopigment molecule Photopigment molecule (cone) From Sharpe, Stockman, Jägle & Nathans, 1999 From Sharpe, Stockman, Jägle & Nathans, 1999
Amino acid differences between photopigment opsins LWS all with OH group POLAR
MWS NON-POLAR
180 277 285
From Sharpe, Stockman, Jägle & Nathans, 1999
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MWS LWS Amino acid Tuning Site differences between 180 alanine serine OH- ~ 5 nm photopigment opsins
277 phenylalanine tyrosine OH-
~ 25 nm Why are the M‐ and L‐ cone opsins so similar? 285 alanine threonine OH-
From Sharpe, Stockman, Jägle & Nathans, 1999
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
mya (million years ago) mya (million years ago) Credit: Bowmaker
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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
Duplication mya (million years ago) Credit: Bowmaker
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 numbers of L and M-cone genes on each X chromosome
From Sharpe, Stockman, Jägle & Nathans, 1999
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Intragenic crossovers Intragenic crossovers produce hybrid or mixed L and M-cone genes 0 M Intergenic crossover -1
-2 quantal sensitivity quantal 10
Intragenic crossover Log Hybrid (mixed) -3 L/M genes 400 450 500 550 600 650 700 Wavelength (nm) Intragenic crossover The spectral sensitivities of those hybrid L photopigments vary between those of the M- and L-cones depending on where the
From Sharpe, Stockman, Jägle & Nathans, 1999 crossover occurs.
M Single-gene dichromats
Protanope More M-cone like With just one gene a male (with only one X- chromosome) must be a dichromat More L-cone like Deuteranope L
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Deuteranope Protanope
Males with two different Multiple-gene dichromats Anomalous trichromats genes are anomalous trichromats
Severe Protanomalous Males with two very similar genes may also be effectively (or behaviourally) dichromats, simply Mild because the spectral sensitivities of the two cone types are so similar. Deuteranomalous
Severe
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Main types of colour vision defects with approximate 1.0 1.0 proportions of appearance in the population. Severe Normal 0.5 Deutan 0.5 percent in UK
0.0 0.0 350 450 550 650 750 350450550650750 Condition Male Female
Protanopia no L cone 1.0 0.02 Protanomaly milder form 1.0 0.03 1.0 1.0 Deuteranopia no M cone 1.5 0.01 Deuteranomaly milder form 5.0 0.4 Severe Mild 0.5 Protan 0.5 Protan Tritanopia no SWS cone 0.008 0.008
0.0 0.0 350 450 550 650 750 350450550650750
XY inheritance The emergence of two longer wave‐ length (M‐ and L‐cones) is thought to have occurred relatively recently in primate evolution.
Why is it important?
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No red-green discrimination Red-green discrimination
Source: Hans Irtel Source: Hans Irtel
Ishihara plates
DIAGNOSING COLOUR VISION DEFICIENCIES
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S-cones are unusual
S-CONE MEDIATED VISION IS UNUSUAL
Central area of the fovea lacks S‐ cones and is therefore tritanopic.
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In other retinal regions, the S-cone mosaic remains sparse. S‐cone pathway ON pathway
S-cones form between 5 and 10% of the cone population.
Curcio et al.
Chromatic aberration
Why is S-cone vision sparse?
Base picture: Digital camera world
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Effect of chromatic blur on eye chart McCollough effect test pattern
Version by Akiyoshi Kitaoka Jim Schwiegerling, U. Arizona
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