Lecture #4
description
Transcript of Lecture #4
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Lecture #4
Visual pigments I2/5/13
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Atmosphere scatters short wavelengths best
Lizzie’s question
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Today’s topics
• Rod and cone visual pigmentsAbsorptionTypes in humansSeeing colorSeeing less color
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What happens with a single photo receptor?
What information can we get?
LIGHT
Electrical output
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Three layers of the eye
Webvision.med.utah.edu
1) Sclera and corneaprotect the eye
2) Iris and ciliary body in front and choroid in the back3) Retina - senses light
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Photoreceptors at the very back of the retina (face away from
light)
http://webvision.med.utah.edu/
Pigment epithelium
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Looking at an object forms an image of the object on the retina
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Human retina: rods and cones
Pigment epithelium at back of retina
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Looking directly at retina
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Fovea has only cones. Rods more prevalent outside fovea
Wolfe et al Sensory Perception fig 2.9
cones
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Photoreceptors: Rods and cones
• RodsUse at low light levels Very sensitive to lightAll rods have same
visual pigment
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Photoreceptors: Rods and cones
• Cones
Use during day Not as sensitive 3 types each with
different visual pigments which detect different parts of spectrum
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Parts of photoreceptors
• Outer segmentLots of membraneWhere light gets detected
• Inner segmentMitochondria to power cellNucleus - DNA
• SynapseSends signal to next neuron
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Inner and outer segment
Outer segment is composed of lots and lots of membrane
Either in discs or layers
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Visual pigment held in membrane
• 80% of the protein in the outer segment is the visual pigment
• Pigment absorbs light
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Visual pigments absorb light – measure their absorbance
spectra
I0
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Just like spectrometer last time:
I0 I
Measure fraction of light transmitted through cell
fT = I / I0 = exp(-ε C l)
ελ= extinction coefficient of pigmentC = pigment concentrationl = cell diameter
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Just like spectrometer last time:
I0 I
Fraction of light absorbed (neglect reflectance)
fT + fR + fA = 1
fA = 1 - fT = 1 - I / I0 = 1 - exp(-ε C l)
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Measure light from light source, I0
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Compare that to light with cone cell in beam, I
Light after cone
Calculate fT = I/I0
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Fraction of light transmitted and absorbed
fT
fA
Fraction of light transmitted, T or absorbed, A fA = 1-fT
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Measure how visual pigment in rods and cones absorb light
Absorbs
After expose to light
λmax
Fish cones
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Bowmaker and Dartnall 1980
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Human photoreceptor absorbances
Rod
Blue
Green
Red
Bowmaker and Dartnall 1980
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Summary of human rods and cones
Rod 498 nm (n=11) Green 534 nm (n=11)Blue 420 nm (n=3) Red 564 nm (n=19)
Bowmaker and Dartnall 1980
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Rod pigment - Visual purple
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Rods only have this one visual pigment
B = G > Y > R
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Rods - can only detect light and dark
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Three human cones : short, medium, and long
S M L
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Which cone types does this color stimulate?1. Short2. Medium3. Long4. Short>>medium5. Medium>>long6. Long>>medium
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Which cone types does this color stimulate?1. Short2. Medium3. Long4. Short>>medium5. Medium>>long6. Long>>medium
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Intermediate colors excite multiple cones
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How can we use multiple receptorsto create an image?
What’s the intensity of theRed light here?
The Green light?
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The retina and then your brain processes output from three cones to determine “color”
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Can take picture of mosaic of photoreceptors in a live eye
S, M, L cones all show up
Expose retina to white light– then take picture
All visual pigment excited by white light so don’t absorb any more light and look “bright”
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550 nm will only be absorbed by M and L cones
550 nm
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Can take picture of mosaic of photoreceptors in a live eye at
different λ
S, M, L cones all show up
Dark adapt, excite @ 550 nm then take picture
L + M cones excited by 550 nm so look bright S cones are dark spots
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Excitation wavelengths to preferentially excite one cone
type
470 550 650 nmM>L M=L L>M
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Cone distributions from photos of live retinas - excite at
different λS, M, L cells all show up
Dark adapt, excite @ 550 nm so S cones are dark
Excite at 470 nm = M cones so see M as bright
Bleach at 650 nm – so see L as bright
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Overlay and false colorize Human retinal mosaic in fovea
Roorda and Williams 1999
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Human retinal mosaic in foveaJW temporal nasal AN nasal
Roorda and Williams 1999
Huge variation from person to person in distribution of cones and in M/L cone ratios!
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Cone ratios
Subject
# cones
% L cones
% M cones
% S cones
L:M
JW 1462 75.8 20 4.2 3.8
AN 522 50.6 44.2 5.2 1.2
L:M ratio can vary between 0.8 and 9.7
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Psychophysical test of color matching -
mix red + green to make yellow
+ =
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Color sliders in real time!
http://www.chriscassell.net/projects/flash/color_slider.html
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Where does the light come from?
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Direct light is additive = R + G + B
Short coneMedium cone
Long cone
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Colors on computer screens
Three colors which can vary in intensity : R G and B
8 bit colors28 = 256 11111111 = 255
Each pixel varies between 0 and 255 for each of three color channels:
2563 = 16.77 million colors
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Which color is R,G,B = 255, 255, 0
1 32
5 6 7
4
8
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Which color is R,G,B = 255, 255, 255
1 32
5 6 7
4
8
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255, 0, 0
0, 255, 0
0, 0, 255
R G B
255, 255, 0
255, 0, 255
0, 255, 255
255, 255, 255
0, 0, 0
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Reflection is subtractive - wavelengths are removed from
light
Wavelengths reflected by opaque material are the same ones transmitted by a transparent material of same color
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Seeing color
Incident light
Absorbed light
Reflected light
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Seeing color
Incident light
Absorbed light
Reflected light
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Colors printed on paper are subtractive
White paper reflects all wavelengths
Print various pigments to remove some of these so see reflected color
Printers have 4 cartridges of pigments which can be laid down
CyanMagentaYellowBlack
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Transmitted light is additive - Reflected light is subtractive
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What if one of visual pigments is missing or altered
Normal λmax = 420, 535, 565 nm
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Protanope - no red cones1% males 0.01% females
λmax = 420, 535nm
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Protanope - long wavelength “colors" vary in brightness
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Protanope - long wavelength “colors" vary in brightness
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Deuteranope - no green cones1% males 0.01% females
λmax = 420, 565 nm
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Tritanope - no blue conesRare - 0.008%
λmax = 535, 565 nm
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Protanomoly - red pigment shifted towards green
λmax = 420, 535, 550 nm
1% male 0.01% female
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Deuteranomoly - green pigment shifted towards red
λmax = 420, 554, 565 nm
5% male 0.04% female
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Color “blindness”Deficiency Males Female
sProtanopia Missing
red cones1% 0.01%
Deuteranopia Missing green cones
1% 0.01%
Protanomoly Short λ red cones
1% 0.01%
Deuteranomoly
Long λ green cones
5% 0.4%
Total (red-green)
8% 0.5%
Tritanopia 0.008%
0.008%
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Series of pictures from this web site
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Color sensing - loss of one cone1% 1%
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Color sensitivity - one cone shifted1% 5%
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RGB anomaloscope
http://www.colblindor.com/rgb-anomaloscope-color-blindness-test/
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Color vision simulator
http://www.idea.org/vision-demo.php