Color Perception Mechanisms and Binocular Vision

GEM1532K: Color and Vision: A look at Biophysics Seeing the light, Fig. 10.11
Thorsten Wohland
Light Vision Color, A. Valberg
Please read until next week:
Saunders and Brakel:
Retina independent color anomalies
With age the lens of humans becomes more and more yellow (same happens with cataracts).
Your brain adapts to that and you still perceive white as white etc.
However, when you paint, the colors you use will contain more yellow (Metamers).
The organization of the retina
Spatial summation
T.N. Cornsweet, Fig. 2.5
Illuminate spots on the retina of different size and determine the number of photons needed before the spot can be seen
1st spot: only few rods on average
2nd spot: smaller than summation area
3rd spot: larger than summation area
Sensitivity constant
Sensitivity decreases
Temporal summation
Adapted form T.N. Cornsweet, Fig. 2.5
How many photons have to arrive in a certain time interval so that the eye sees a flash?
0
10
20
time (ms)
Lateral Inhibition
One ganglion cell receives signal from many receptors, excitatory or inhibitory signals.
One cone/rod can contribute to some ganglion cells excitatory to others inhibitory.
STL Fig. 7.2
Afterimages
You can have negative and positive afterimages.
The effect comes from the fact that when a cone/rod is stimulated for a long time it “desensitizes”.
1) The cones perceiving the black square are not excited, the cones perceiving the white surrounding are excited and desensitize with time.
2) When looking at the white surface on the right, the desensitized cones are less excited than the rested cones in the middle and thus you see a white square.
Negative after images
Inhibition or desensitization
STL Fig. 7.12
Inhibition: If an excited cone, i.e. a cone that has absorbed light suppresses signaling, it is called inhibition. The result is a lower frequency of firing of the ganglion cell.
Desensitization: After strong excitation a cone can become less sensitive and cannot react again immediately. In this case there could be as well less firing from this cone.
Negative after images
No image
Long exposure of some cones, image is seen
The exposed cones are desensitized, give lower signal than surrounding rested cones.
Afterimages
Positive afterimages.
You can sensitize your retina by closing your eyes and resting your cones (remember when you close eyes a long time and open them you seem to be blinded first).
When you open your eyes shortly (seconds) and look at some bright object the cones get excited.
When you close your eyes again the cones will not desensitize and will stay stimulated longer and give you a positive afterimage.
See the TRY IT on page 194 of STL.
Now let’s recall what we know about the CIE system and then let’s see whether there are any facts left unexplained.
Can we perhaps resolve some of these issues with our new knowledge of the retina and its organization?
Trichromacy, Tristimulus theory
l
Sensitivity
Take one cone; shine light of constant intensity on the cone; measure the light transmitted; calculate absorption
Color mixing
l
Sensitivity
x
0
1.0
x: equal excitation of blue and green cone by 30%, no excitation of red
If P1, P2, and x have same intensity we have too much red.
Since P3 excites the red cone 4 times less P2, we can subtract 4 times P3 to get our mixture:
x = P1 + P2 – 4 P3
P1: excitation of blue cone by 30%, no excitation of green and red
P1
P2: excitation of green cone by 30%, excitation of red by 80%, no excitation of blue
P2
P3
P3: excitation only of red by 20%.
Negative values: 3 primaries are not enough to mix all colors
3 abstract colors are chosen which then can cover all visible colors with positive values.
These colors do not exist, and some of their mixtures do not give real colors either.
The normalization, the condition that x+y+z=1 allows us then to depict all colors in one graph, but only at constant intensity.
The CIE system
Complementary colors are connected by a straight line going through white.
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Mixtures of colors are easy to find.
Distance from 486 nm point is three times longer than from 545 nm point.
Therefore you need a mixture of 486:545 nm of 1:3.
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Complementarity
STL, Fig. 9.9
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Facts not explained by Trichromacy
Color naming
Experiment done by asking a person to estimate how much blue, yellow, green, and red is contained in a hue represented by a pure wavelength.
STL, Fig. 10.9
STL: Fig. 7.2
Can we connect the cones in a fashion, so that the signal at the ganglion cells will correspond to the four opponent colors red, green, blue, and yellow?
Possible combinations
Possible combinations
Blue - Yellow
So we have constructed 3 new signals from the original three cones:
Black – White
Green – Red
Blue – Yellow
STL, Fig. 10.12
W: all cones are equally excited, therefore the lines dividing the CIE in r, g, b, y regions must cross there.
W: all cones are equally excited, therefore the lines dividing the CIE in r, g, b, y regions must cross there.
Spatial Processing of Color
Double opponency
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-
-
-
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-
+
Opponency of location (inside versus outside)
Opponency of color
Red - Green
1. Individual L and M cone signals are pooled by a ganglion cell to give a Red-Green opponent signal
2. Depending on the position of the cones on your retina the Red-Green opponent signal can work as excitatory (+) some as inhibitory (-) signals.
M
L
STL, Fig. 10.16
0
0
Benham disk:
White parts excite all three cones. However, the three cones recover from activation differently. When black falls onto the excited cones, some are still stimulated (e.g. the blue one) while others (red and green) have already recovered. Thus one sees blue.
Temporal Processing
Flash of white light
Red cone de-excites fast, a blue/green (cyan) color is seen
Green cone de-excites next. Blue is seen
Return to resting state afetr blue cone has de-excited as well
Benham disk, positive afterimages