Vision A
Recommended literature
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.
Q1
How do the photoreceptors work?
Photoreceptor function
Light is a form of
electromagnetic irradiation
f
I
Photons move in a straight way with a velocity
of aprox. 3x105 km/s
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.
Photoreceptors are activated by the absorption of photons. We
have two classes of photoreceptors- the rods and the cones,
each with different photopigments
Rods - rhodopsin
Cones – 3 types of iodopsins
rhodopsin
STL2004-18-11
Photoreceptor function
KS-2001-24-7
Photoreceptor function
The retina and the following
visual representation areas
all have clear cut layers
Photoreceptor function
KS2001-24-9
In contrast to other receptors, light does not depolarize,
but it hyperpolarizes the receptor cell membrane
icCa
icCa
Hyperpolarization increases with light intensity
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.
STL2004-18-11
Photoreceptor function
The adjustment to decreasing light intensity is determined
by measuring the intensity threshold of vision.
The reciprocal of the intensity threshold is a measure os sensitivity of perception
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.
STL-2004-18-16
Photoreceptor function
Q2
How does visual acuity relate to the
different regions of the retina?
Visual acuity
With the help of a mirror you can see the retina
vessels
fovea centralis
blind spot
Visual acuity
The spatial resolution (visus) reflects the ability to discriminate
two stimulation points as being separate (= spatial threshold).
The visus can also be expressed as the reciprocal of the respective spatial angle.
Visual acuity
STL2004-18-21
The visus (visual acuity) differs in dependence on
the coordinates of the image on the retina
1‘=1/60o
(~1.5 mm at a distance of 5 m)
The visual acuity is higher if an object is imaged on the fovea,
but in this case the light intensity must be higher
KS2001 -24-14
Visual acuity
D = spatial resolution (visus) = 1/α
Visual acuity
One can separate 2 stimuli, if 2 receptors have
another receptor in between that absorbs less light
Between the fovea and the retinal
periphery there are substantial
differences in:
- receptor density
- degree of convergence
Significance for spatial resolution
Lower receptor density,
higher convergence
Higher receptor density,
lower convergence
3 degrees
0.3 degrees
Q3
How do retinal ganglion cells contribute
to the perception of contrast?
Contrast perception
By analyzing sensory illusions one can
better understand the mechanisms of
contrast perception
Contrast perception
Contrast perception is based on the antagonistic organization of
the receptive fields of retinal ganglion cells
On-center RGC
Off-center RGC
RGCs are classified according
to the response to center
stimulation
Contrast perception
The retina contains 7 classes of cells
that differ in their structure, position, connectivity,
transmitters and transmitter receptors
Signaling path of
on-center RGCs
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
Contrast perception
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
KS2001 -24-11
Contrast perception
The antagonistic receptive field organization of the retinal
ganglion cells is the prerequisite for the perception of contrast
KS2001 -24-12
Contrast perception
Q4
How are colors discriminated?
Color vision
Color vision largely contributes to the perception of contrast and the recognition of visual objects
Color vision
False color coding of biological parameters
Control
Pulse 3 ms, 2 μ A
Example: ic Ca-concentration in individual inhibitory synapses
Red: Ca concentration is high (10-6 mol/l)
Blue: Ca concentration is low (10-7 mol/l)
Kirischuk, Veselovsky, Grantyn, Pflüg. Arch. (Neurotechniques) 1999, 438, 716-724
Color vision
Color vision
400
500
600
wavelenght
The human visual system discriminates about 200 wavelenghts (λ)
as separate colors
700nm
Color vision
The perception of color not only varies with hue (λ),
but also with saturation and darkness
200
Hue
(wavelenght λ)
20
Saturation
(weighting factor)
500
Darkness
(fraction of black)
Discrimination of a total of 2x106 color values
Color vision
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
Additive color mixture
Colorless: When all are λ
are offered simultaneously
(white)
Mixing of colors for painting:
color pigments as l-specific filters
Subtractive color mixture
Colorless: When filter excludes the entire
visible spectrum (black)
Color vision
Science, media and industry require
defined colors,
that can be composed from defined primary colors.
The commonly used color models for digital imaging
are RGB (additive, screen) and CMYK (subtractive, printed media)
Color vision
Red, Green, Blue
Cyan, Magenta, Yellow, BlacK
Color vision
Color charts (LUT, look-up tables) provide a set of defined colors
Example: Corel-Green
STL2004 -1
Color vision
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
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 photoreceptors
B – for all following levels
Color vision
Concerning A:
Humans are equipped with 3 types of clones, each containing one of 3
photopigments
Color vision
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 them
with 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
Concerning B:
Green-red and blue –yellow are opponent colors.
They induce an opposite responses in ganglion cells
Red-on-ganglion cell
From configuration A
follows B-D
Please note: With regard to color vision there are big differences among animal species!
Color vision
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
The functionality of the retina can be examined by
recording an electroretinogram (ERG)
KS2001 -24-13
Methods