Adaptation (eye)

Under adaptation (Latin adaptare "Adjust " ) refers to the eye adapting it to the prevailing in the field luminances.

With sudden brightness differences can about the pupillary light reflex by expansion or contraction of the pupil with the iris muscles, the amount of incident light can be regulated rapidly (factor 10-20). The further adaptation to widely varying ambient brightness (up to a factor of 1012 different stimulus strengths ) - about tracks in the snow in sunlight or to see smaller stars in the moonless night sky - on the other hand comes about by a change in light sensitivity of the retina ( retina) and only after a certain delay optimal. For these retinal adaptation their differently weighted neuronal connections ( receptive fields ) are responsible in addition to various biochemical processes in the sensory cells ( photoreceptors ).

• 2.1 Chromatic Adaptation
• 2.2 Light and dark adaptation
• 2.3 Transient adaptation

Pupillary light reflex

Through the pupillary light reflex, pupillary reflex short, the stress state of the smooth muscle of the iris ( iris ) is changed - in photographic terms, the opening width of the screen - and thus the relative amount of incident light into the eye. The iris limits the Sehloch ( pupil) and has two muscles to adjust the pupil width:

• The dilator muscle of the pupil ( " Pupillenerweiterer " ) is innervated by sympathetic nerve fibers from the Centrum ciliospinale ( spinal cord segments C8 - Th3 ). The dilation of the pupil is called mydriasis.
• The sphincter pupillae ( " Pupillenverenger " ) is operated by parasympathetic fibers of the oculomotor nerve (third cranial nerve) from the Ncl. Edinger - Westphal via the ciliary ganglion innervates. It is activated at high light. A constriction of the pupil is referred to as miosis.

The reflex control of the incident light through the pupil causes a rapid adaptation to sudden change in brightness. Increase in pupil diameter at three times the increase in the opening area is on the order of a factor of 10 (101). However, since the total area of ​​Hell-/Dunkeladaptation is more than 11 orders of magnitude ( to 1012), plays the pupillary reflex in this context only a subordinate role.

Reflex chain

Afference: The information about the increased incidence of light is directed by light-sensitive photoreceptors in the retina via the optic nerve ( optic nerve ) and the optic tract in the epithalamus to the nuclei praetectales. Their efferents pass the brightness information on both sides in the Edinger - Westphal nuclei.

Efference: In the Edinger - Westphal nuclei, connecting to the parasympathetic component of the oculomotor nerve takes place. About the ciliary ganglion of the pupillary sphincter muscle becomes stimulated to contract and therefore narrows the pupil. Because on the one hand both prätektalen cores on the posterior commissure are connected and consists of each eye a link to prätektalen two cores, the reflex of both eyes is performed at the same time, even if only one eye is suddenly illuminated. Therefore, a pupillary constriction can also be triggered at a blind eye by illuminating the other healthy eye, as long as the reflex arc is intact ( consensual light reaction).

Adaptation processes of the retina

Over a wide range the light sensitive photoreceptors of the retina can change their sensitivity in response to the illuminance. Dark adaptation is a slow process, because the visual pigment must be converted to its active state; it takes about 10 minutes with pin and sticks around 30 minutes until they are fully adapted to dark conditions. Adaptation to bright light conditions is, however, already effective in split seconds and is optimal depending on the adaptation type in up to 6 minutes; they can also be understood as a protection against retinal damage by excessively strong light.

An additional adjustment procedure is the modified spatial summation, wherein the surface of the retina, from which a ganglion cell of the retina can be obtained exciting pulses ( for example, by lateral inhibition ) decreases under the influence of retinal interconnections with increasing luminance. Conversely, with decreasing luminance contribute a higher number of photoreceptors receptive of the field for the formation of action potentials that are transmitted via the axons in the optic nerve.

At low luminances can next to a slowing of eye movements or prolonged fixation duration, leading to a temporal summation, are also understood as an adjustment mode.

Retinal adaptation processes whose effect is considered limited to certain retinal areas are often referred to as local adaptation and are, for example, the Troxler effect is based. In particular characteristics, it lead to a lasting effect an after-image, which can also be observed in the so-called successive contrast - until it disappears due to retinal adaptation.

Chromatic adaptation

Since the retina is equipped with various types of light-sensitive cells that are sensitive to different spectral ranges, even the " white balance " of the eye can be done through adaptation, chromatic adaptation. If the new lighting situation a different color temperature prevails, for example through increased amount of red, then the red-sensitive cells will reduce their sensitivity in relation to the others. As a result, the viewer perceives a white surface then also again as white, although it reflects a proportionally increased amount of red light. Adaptive color shift is the difference in the perceived object color due to a change of chromatic adaptation.

Light and dark adaptation

Light and dark adaptation of vertebrate animals are bound to the Retinomotorik (movement of Pigmentepithelzellfortsätze and the outer segments of the photoreceptors ). These migration processes are likely to be detectable only in animals and not in humans. Light adaptation is the special case of Tagsehens when the entire visual system has adapted to luminance levels above 3.4 cd/m2. Dark adaptation is the special case when the visual system has adapted to luminance levels at 0,034 cd/m2. A very obvious example of (quantitative) adaptation can be observed when a person moves out into the full sun in a building. The visual environment in the building will first appear almost black. After a few minutes, the person is then able to recognize details (such as newspaper text to read). However, the view from the window is again uncomfortable, since the large luminances out now cause a lot of glare.

The dark adaptation is due primarily to the fact that both in the journal as well as the visual pigment in the rods is resynthesized. Since the reconstruction is going slower than the decay, the dark adaptation of a longer period than the required light adaptation.

Transient adaptation

Transient adaptation is a term for the special case that occurs when the eye must repeatedly switch between a high and a low light level back and forth. This is the case when the environment has very high contrast, such as when a computer monitor ( 140 .. 300 cd / m²) and a sunlit area of the window ( > 5000 cd / m²) without head rotation are visible side by side. This condition will have a speedy eye fatigue result. The Transient adaptation factor ( TAF) is an English term to refer to the relative reduction of perceived contrast by the re-adaptation between different light environments.