Color Perception Combined rod + cone response yields both color and brightness perception Cell...
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Transcript of Color Perception Combined rod + cone response yields both color and brightness perception Cell...
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