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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.
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