Vision A

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

How do the photoreceptors work? Q1

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

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Rods - rhodopsinCones – 3 types of iodopsins

rhodopsin

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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

Photoreceptor function

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The retina and the followingvisual representation areasall have clear cut layers

In contrast to other receptors, light does not depolarize, but it hyperpolarizes the receptor cell membrane

Hyperpolarization increases with light intensity

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Photoreceptor function

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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

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Photoreceptor function

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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

How does visual acuity relate to thedifferent regions of the retina? Q2

fovea centralisblind spot

vessels

With the help of a mirror you can see the retina

Visual acuity

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

The visus (visual acuity) differs in dependence on the coordinates of the image on the retina

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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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Visual acuity

D = spatial resolution (visus) = 1/α

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

How do retinal ganglion cells contribute to the perception of contrast?Q3

By analyzing sensory illusions one can better understand the mechanisms of

contrast perception

Contrast perception

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

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

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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Contrast perception

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The antagonistic receptive field organization of the retinalganglion cells is the prerequisite for the perception of contrast

Contrast perception

How are colors discriminated?Q4

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 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)

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Color vision

The human visual system discriminates about 200 wavelenghts (λ) as separate colors

Color vision

wavelenght

400 600500 700nm

Hue(wavelenght λ)

Saturation(weighting factor)

Darkness(fraction of black)

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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

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)

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)

Red, Green, Blue Cyan, Magenta, Yellow, BlacK

Color vision

Color charts (LUT, look-up tables) provide a set of defined colors

Example: Corel-Green

Color vision

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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 photoreceptorsB – for all following levels

Color vision

Color vision

Concerning A:Humans are equipped with 3 types of clones, each containing one of 3photopigments

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

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

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

The functionality of the retina can be examined byrecording an electroretinogram (ERG)

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Methods