Lecture #4

Visual pigments I2/5/13

Atmosphere scatters short wavelengths best

Lizzie’s question

Today’s topics

• Rod and cone visual pigmentsAbsorptionTypes in humansSeeing colorSeeing less color

What happens with a single photo receptor?

What information can we get?

LIGHT

Electrical output

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

Photoreceptors at the very back of the retina (face away from

light)

http://webvision.med.utah.edu/

Pigment epithelium

Looking at an object forms an image of the object on the retina

Human retina: rods and cones

Pigment epithelium at back of retina

Looking directly at retina

Fovea has only cones. Rods more prevalent outside fovea

Wolfe et al Sensory Perception fig 2.9

cones

Photoreceptors: Rods and cones

• RodsUse at low light levels Very sensitive to lightAll rods have same

visual pigment

Photoreceptors: Rods and cones

• Cones

Use during day Not as sensitive 3 types each with

different visual pigments which detect different parts of spectrum

Parts of photoreceptors

• Outer segmentLots of membraneWhere light gets detected

• Inner segmentMitochondria to power cellNucleus - DNA

• SynapseSends signal to next neuron

Inner and outer segment

Outer segment is composed of lots and lots of membrane

Either in discs or layers

Visual pigment held in membrane

• 80% of the protein in the outer segment is the visual pigment

• Pigment absorbs light

Visual pigments absorb light – measure their absorbance

spectra

I0

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

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)

Measure light from light source, I0

Compare that to light with cone cell in beam, I

Light after cone

Calculate fT = I/I0

Fraction of light transmitted and absorbed

fT

fA

Fraction of light transmitted, T or absorbed, A fA = 1-fT

Measure how visual pigment in rods and cones absorb light

Absorbs

After expose to light

λmax

Fish cones

Bowmaker and Dartnall 1980

Human photoreceptor absorbances

Rod

Blue

Green

Red

Bowmaker and Dartnall 1980

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

Rod pigment - Visual purple

Rods only have this one visual pigment

B = G > Y > R

Rods - can only detect light and dark

Three human cones : short, medium, and long

S M L

Which cone types does this color stimulate?1. Short2. Medium3. Long4. Short>>medium5. Medium>>long6. Long>>medium

Which cone types does this color stimulate?1. Short2. Medium3. Long4. Short>>medium5. Medium>>long6. Long>>medium

Intermediate colors excite multiple cones

How can we use multiple receptorsto create an image?

What’s the intensity of theRed light here?

The Green light?

The retina and then your brain processes output from three cones to determine “color”

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”

550 nm will only be absorbed by M and L cones

550 nm

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

Excitation wavelengths to preferentially excite one cone

type

470 550 650 nmM>L M=L L>M

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

Overlay and false colorize Human retinal mosaic in fovea

Roorda and Williams 1999

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!

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

Psychophysical test of color matching -

mix red + green to make yellow

+ =

Color sliders in real time!

http://www.chriscassell.net/projects/flash/color_slider.html

Where does the light come from?

Direct light is additive = R + G + B

Short coneMedium cone

Long cone

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

Which color is R,G,B = 255, 255, 0

1 32

5 6 7

4

8

Which color is R,G,B = 255, 255, 255

1 32

5 6 7

4

8

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

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

Seeing color

Incident light

Absorbed light

Reflected light

Seeing color

Incident light

Absorbed light

Reflected light

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

Transmitted light is additive - Reflected light is subtractive

What if one of visual pigments is missing or altered

Normal λmax = 420, 535, 565 nm

Protanope - no red cones1% males 0.01% females

λmax = 420, 535nm

Protanope - long wavelength “colors" vary in brightness

Protanope - long wavelength “colors" vary in brightness

Deuteranope - no green cones1% males 0.01% females

λmax = 420, 565 nm

Tritanope - no blue conesRare - 0.008%

λmax = 535, 565 nm

Protanomoly - red pigment shifted towards green

λmax = 420, 535, 550 nm

1% male 0.01% female

Deuteranomoly - green pigment shifted towards red

λmax = 420, 554, 565 nm

5% male 0.04% female

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%

Series of pictures from this web site

Color sensing - loss of one cone1% 1%

Color sensitivity - one cone shifted1% 5%

RGB anomaloscope

http://www.colblindor.com/rgb-anomaloscope-color-blindness-test/

Color vision simulator

http://www.idea.org/vision-demo.php