Resesarch Topics in Neuroscience Peter H. Schiller, year 2006

Color vision and adaptation

Central questions about color vision and adaptation:

1. What are the basic facts and laws of color vision?

2. What are the major theories of color vision?

3. How is color processed in the retina and the LGN?

4. How is color processed in the cortex?

5. What is the nature of color blindness?

6. How is adaptation achieved in the visual system?

7. What are afterimages?

Color vision

Basic facts and rules of color vision

1. There are three qualities of color: hue, brightness, saturation

2. There is a clear distinction between the physical and psychological attributes of color: wavelength vs. color, luminance vs. brightness.

3. Peak sensitivity of human photoreceptors: S = 420, M = 530, L = 560, Rods = 500

4. Grassman's laws: 1. Every color has a complimentary which when mixed propery yields gray. 2. Mixture of non-complimentary colors yields intermediates.

5. Abney's law: The luminance of a mixture of differently colored lights is equal to the sum of the luminances of the components.

6. Metamers: stimuli producing different distributions of light energy that yield the same color sensations.

Newton's prism experiment

red

red orange yellow green blue

indigo violet

red

orange

yellow

green

blue

indigo

violet

AC circuits

Broadcastband

Radar

Infrared rays

Ultraviolet rays

X rays

Gamma rays

Wavelength in meters

Wavelength in Nanometers

The visible spectrum

108

106

104

102

10-2

10-4

10-6

10-8

10-10

10-12

10-14

700

600

500

400

1

Figure by MIT OCW.

The color circle

whitewhite

Green

Red

Blue

Yellow saturation

hue

black

Figure by MIT OCW.

Saturation

Y

B

RG

Hue

Major theories of color vision

Earlier Leonardo da Vinci: "Of different colors equally perfect, that will appear most excellent which is seen near its direct contrary...blue near yellow, green near red: because each color is seen, when opposed to its contrary, than to any other similar to it.

Young-Helmholtz theory

There are three types of broadly tuned color receptors. The color experienced is a product of their relative degree of activation. Problems: Fails to explain Grassman's laws.

Hering's theory

Theory of color opponency based on the observation that red and green as well as blue and yellow are mutually exclusive. The nervous system probably treats red/green and blue/yellow as antagonisgtic pairs, with the third pair being black and white.

Basic physiology of color processing

MIDGET SYSTEM PARASOL SYSTEM

or

Neuronal response profile

ON OFF ON OFF

time

Green ON and OFF ganglion cells

Red ON and OFF ganglion cells

cones

OFFOFF

Midget and blue/yellow system

bipolars

H

A

ON

OFFON

OFF ONOFF OFF

IPL, OFFIPL, ON

ON OFF ONON

Blue/yellow ganglion cell

BLUEYELLOW

Yellow/blue ganglion cell

YELLOW BLUEON OFF

?

Color selectivity in the LGN

Response to Different Wavelength Compositions in LGN Blue ON cell Yellow ON cell

90 90

30 40

135 45

0

225 315

270

50 40 60 80

135 45

180

225 315

270

Spikes per Second

20 1006010 20

20

90 90

135 45 135

010

225 315

20 30 40

45

180

225 315

Green OFF cell Red ON cell

10

maintained discharge rate

30 40 50

180 0

180 0

270 270

Major classes of midget cells in primate retina

Red ON Red OFF Green ON

Green OFF

Blue ON Yellow ON

The effects of lesions on color vision

Color discrimination

100

90

80

70

60

50

40

30

20

10

0NORMAL PLGN NORMAL MLGNV4

Perc

ent C

orre

ct

Seneca, V4, PLGN and MLGN lesions

Color Discrimination

Figure by MIT OCW.

Color discrimination with varied color saturation

Low saturation Higher saturation

Color saturation discrimination

V4 lesion MT lesion

100

Pe

rcen

t Cor

rect

9080706050403020100

Normal

V4 Lesion

1009080706050403020100

Normal

MT Lesion

1 1.5 2 2.5 3 3.5 4 4.5 5 1 1.5 2 2.5 3 3.5 4 4.5 5

Percent Color Saturation Difference

Perception at isoluminance

At isoluminance vision is compromised

DEPTH FORMTEXTURE MOTION

DEPTH FORMTEXTURE MOTION

DEPTH FORMTEXTURE MOTION

Texture, Motion and Stereo

ect

rrt C

on

Perc

e

10 20 30 40 50 60 70 80

100 90

Stereo

Texture

Motion

1.9 1.6 1.3 1.1 0.9 0.7

Red\Green Luminance Ratio

Neuronal responses at isoluminance

The response of a group of magnocellular LGN cells to color exchange

Figure by MIT OCW.

MAGNO CELLS

400

200

200

200

200

200

400

400

400

400

R/G

4.2

2.7

1.7

1.1

0.7

Num

ber o

f Spi

kes

Responses of an MT cell to luminance and chrominance differences

Chrominance

Luminance

Percent color contrast

Percent luminance contrast

Spik

es p

er se

cond

Spik

es p

er se

cond

40

20

0

40

20

0

0

0 0 0 0

0 0 01600

1600 1600 1600 1600

1600 1600 1600 ms

ms

2

2

4

4

8

8

16

16

Figure by MIT OCW.

Responses of an MT cell to luminance and chrominance differences

0 1450

50

25

00 1450 0 1450

50

25

00 1450 0 1450 0 1450 0 1450

0 1450

2 4 8 16

2 4 8 16

Chrominance

Percent color contrast

Percent luminance contrast

Luminance

Spik

es p

er se

cond

Spik

es p

er se

cond

ms

ms

Figure by MIT OCW.

Color blindness and tests for it

Color blindness

1. Incidence:8/100 in whites, 5/100 in asians, 3/100 in africans

males: females: frequency 10 times less

2. Types:lack L cones

protanopes: lack M cones deuteranopes: tritanopes: lack S cones

3. Color tests:

Ishihara plates Farnsworth-Munsell Hue T est

Dynamic computer test

Adaptation

Basic facts about adaptation

1. Range of illumination is 10 log units. But reflected light yields only a 20 fold change (expressed as percent contrast).

2. The amount of light the pupil admits into the eye varies over a range of 16 to 1. Therefore the pupil makes only a limited contribution to adaptation.

3. Most of light adaptation takes place in the photoreceptors.

4. Any increase in the rate at which quanta are delivered to the eye results in a proportional decrease in the number of pigment molecules available to absorb those quanta .

5. Retinal ganglion cells are sensitive to local contrast differences, not absolute levels of illumination.

pipigmgment epient epitthelheliiumum

IPL

amacrine

AII

OPL

rods

ON OFF bipolars ON

ON OFF

ganglion cells

to CNS

cone horizontal H

r

receptorreceptors

cones

photo-s

incoming light

ods cones

photo-

H

ON OFFganglion cells

Effective connections under light adapted conditions

pigmpigment epithelient epitheliumum

IPL

OPL

ON OFF bipolars

ON OFF

ganglion cells

to CNS

cone horizontal H

receptors

cones

photo-

incoming light

ON OFF

incoming light

Effective connections under dark adapted conditions

IPL

photo-receptors

OPL

pigment epithelium

AII

rods

ON

amacrine

ON OFF

ON OFF

to CNS incoming light

Response of a retinal ganglion cell at various background adaptation levels

400

300

200

100

0-5 -4 -3 -2 -1 0

Dis

char

ge ra

te (s

pike

s/se

c) 0-1-2-3-4-5backgroundlog cd/m2

Test flash (log cd/m2)

Figure by MIT OCW.

The after-effects of adaptation

stabilized images afterimages

PERCEPTION AND SYSTEM RESPONSE BEFORE AND AFTER ADAPTATION

Stimuli on After adaptation Post adaptation

Stimuli

light

Perception

dark

high Receptor

sensitivity to light low

high

ON cellactivity

low high

OFF cell activity

low

space

Saturation

Y

B

RG

Hue

Figure by MIT OCW.

Summary: 1. There are three qualities of color: hue, brightness, and saturation.

2. The basic rules of color vision are explained by the color circle.

3. The three cone photoreceptors are broadly tuned.

4. Color-opponent midget RGCs form two cardinal axes, red/green and blue/yellow.

5. The midget system is essential for color discrimination.

6. The parasol cells can perceive stimuli made visible by chromiance but

cannot ascertain color attributes.

7. Color is processed in many cortical areas; lesion to any single extrastriate structure fails to eliminate the processing of chrominance information.

8. Perception at isoluminance is compromised for all categories of vision.

9. The most significant aspects of luminance adaptation occur in the

photoreceptors.

10. Afterimages are a product of photoreceptor adaptation and their subsequent response to incoming light.