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Vision A
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1. Kandel ER, Schwartz JH, Jessel TM (2000) Principles of Neural Science, McGraw-Hill, Ch. xx. 2. Berne EM, Levy MN, Koeppen BM, Stanton BA (2004) Physiology, Mosby, Ch. 8.3. Schmidt RF, Lang F, Thews G (2005) Physiologie des Menschen, Springer, Ch. 18.
Recommended literature
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How do the photoreceptors work? Q1
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Photoreceptor function
Light is a form of electromagnetic irradiation
The visible light only comprises a small range of wavelenghts (400 to 700 nm).
The basic measure of light intensity is candela (cd).The light density (cd/m2) of the human environment varies from10-6 cd/m2 to 107 cd/m2.
Photons move in a straight way with a velocity of aprox. 3x105 km/s
f
I
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Rods - rhodopsinCones – 3 types of iodopsins
rhodopsin
ST
L200
4-18
-11
Photoreceptors are activated by the absorption of photons. We have two classes of photoreceptors- the rods and the cones,
each with different photopigments
Photoreceptor function
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Photoreceptor function
KS
-200
1-24
-7
The retina and the followingvisual representation areasall have clear cut layers
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In contrast to other receptors, light does not depolarize, but it hyperpolarizes the receptor cell membrane
Hyperpolarization increases with light intensity
icCaicCa
Photoreceptor function
KS
2001
-24-
9
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When photopigments absorb light, they are „boosted“ to a higher energy level, which is associated with a series of chemical changes. The photopigments are bleached. Their amount decreases with light intensity. S
TL2
004-
18-1
1
Photoreceptor function
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ST
L-20
04-1
8-16
The adjustment to decreasing light intensity is determined by measuring the intensity threshold of vision.
The dark adaption curve reflects, first of all, the recovery of the receptor pigments. In cases of vitamin A deficiency one observes night blindness due to a rhodopsin deficit.
The reciprocal of the intensity threshold is a measure os sensitivity of perception
Photoreceptor function
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How does visual acuity relate to thedifferent regions of the retina? Q2
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fovea centralisblind spot
vessels
With the help of a mirror you can see the retina
Visual acuity
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The spatial resolution (visus) reflects the ability to discriminatetwo stimulation points as being separate (= spatial threshold). The visus can also be expressed as the reciprocal of the respective spatial angle.
Visual acuity
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The visus (visual acuity) differs in dependence on the coordinates of the image on the retina
ST
L200
4-18
-21
The visual acuity is higher if an object is imaged on the fovea,but in this case the light intensity must be higher
1‘=1/60o
(~1.5 mm at a distance of 5 m)
Visual acuity
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KS
2001
-24-
14
Visual acuity
D = spatial resolution (visus) = 1/α
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One can separate 2 stimuli, if 2 receptors have another receptor in between that absorbs less light
Visual acuity
Lower receptor density, higher convergence
Higher receptor density, lower convergence
Between the fovea and the retinal periphery there are substantial differences in:- receptor density- degree of convergence
3 degrees 0.3 degreesSignificance for spatial resolution
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How do retinal ganglion cells contribute to the perception of contrast?Q3
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By analyzing sensory illusions one can better understand the mechanisms of
contrast perception
Contrast perception
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Contrast perception is based on the antagonistic organization ofthe receptive fields of retinal ganglion cells
RGCs are classified according to the response to centerstimulation
Contrast perception
On-center RGC
Off-center RGC
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The retina contains 7 classes of cells that differ in their structure, position, connectivity, transmitters and transmitter receptors
Contrast perception
Note: only ganglion cells and some amacrine cells in the ganglion cell layer have axons and generate full action potentials; all other cells generate gradual potentials
Signaling path of on-center RGCs
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The antagonistic organization of receptive fields requires inhibitory activity of horizontal cells
In the absence of this inhibition,the excitatory center expands
and the spatial threshold increases
Contrast perception
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KS
2001
-24-
11
Contrast perception
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KS
2001
-24-
12
The antagonistic receptive field organization of the retinalganglion cells is the prerequisite for the perception of contrast
Contrast perception
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How are colors discriminated?Q4
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Color vision
Color vision largely contributes to the perception of contrast and the recognition of visual objects
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Color vision
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False color coding of biological parameters
Control Pulse 3 ms, 2 Aμ
Example: ic Ca-concentration in individual inhibitory synapsesRed: Ca concentration is high (10-6 mol/l)Blue: Ca concentration is low (10-7 mol/l)
Kiri
schu
k, V
esel
ovsk
y, G
rant
yn, P
flüg.
Arc
h. (N
euro
tech
niqu
es) 1
999,
438
, 716
-724
Color vision
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The human visual system discriminates about 200 wavelenghts (λ) as separate colors
Color vision
wavelenght
400 600500 700nm
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Hue(wavelenght λ)
Saturation(weighting factor)
Darkness(fraction of black)
200
20
500
Discrimination of a total of 2x106 color values
The perception of color not only varies with hue (λ), but also with saturation and darkness
Color vision
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Any color can be produced(A) by mixing light of defined wavelenght or(B) by removing light of defined wavelenght from white light
Colored projected light:monochromatic light sources
Colorless: When all are λ are offered simultaneously(white)
Subtractive color mixtureAdditive color mixture
Mixing of colors for painting: color pigments as l-specific filters
Color vision
Colorless: When filter excludes the entire visible spectrum (black)
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Science, media and industry require defined colors,
that can be composed from defined primary colors.
Color vision
The commonly used color models for digital imaging are RGB (additive, screen) and CMYK (subtractive, printed media)
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Red, Green, Blue Cyan, Magenta, Yellow, BlacK
Color vision
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Color charts (LUT, look-up tables) provide a set of defined colors
Example: Corel-Green
Color vision
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ST
L200
4 -1
The human eye is not equally transparent for all colors; and the visual system is not equally sensitive for different wavelenghts
Note: there is an age dependency of transmission of blue
Color vision
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Two theories of color vision:A - trichromatic theory (Helmholtz, Young)B – opponent color theory (Hering)
These theories apply to particular components of the visual system:A – for level of photoreceptorsB – for all following levels
Color vision
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Color vision
Concerning A:Humans are equipped with 3 types of clones, each containing one of 3photopigments
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In a color mixing test, the color hypochromat adds more of the color that he sees worse
Anomalous trichromat: has three photopigments, but one of themwith changed absorbtion characteristics
Dichromat: one pigment is missing- Protanopy (red pigment is missing)- Deuteranopy (green pigment is missing)- Tritanopyy (red pigment is missing)
Red-green-defect:(8% of US population)More frequent in men,since responsible genes are in the X-chromosome
Color vision
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From configuration Afollows B-D
Red-on-ganglion cell
Concerning B:Green-red and blue –yellow are opponent colors. They induce an opposite responses in ganglion cells
Please note: With regard to color vision there are big differences among animal species!
Color vision
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The perception of contrast is already enhanced by dichromatic vision,because the 2 types of cones can respond to the stimuli at different wavelenghts as to stimuli of different intensity
Color vision
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The functionality of the retina can be examined byrecording an electroretinogram (ERG)
KS
2001
-24-
13
Methods