Differential sensitivity, or sensitivity to distinction, also is in inverse relationship to distinction threshold size: the threshold of distinction is more, the less differential sensitivity.
Sensitivity of the visual and acoustic analyzer is very high. A human eye as showed S. I. Vavilov's experiments, is capable to see light at hit on a retina of only 2 - 8 quanta of radiant energy. It means that we would be capable to see in full darkness the burning candle at distance to 27 kilometers. At the same time in order that we felt a touch, it is necessary in 100 – 10 000 000 times more for energy, than at visual or acoustical feelings.
The analyzer consists of three parts: peripheral department (a receptor, being the special transformer of external energy in nervous process; afferent (centripetal) and efferent (centrifugal) nerves - the carrying-out ways connecting peripheral department of the analyzer with central; subcrustal and cortical departments (the brain end) of the analyzer where there is a processing of the nervous impulses coming from peripheral departments.
From the point of view of data of modern science the accepted division of feelings on external and internal is not enough. Some types of feelings can be considered externally - internal. Treat them temperature and painful, flavoring and vibration, musculoarticulate and statiko-dynamic.
The phenomenon of adaptation can be explained with those peripheral changes which happen in functioning of a receptor at long impact of an irritant on it. So, it is known that under the influence of light the visual purple which is in eye retina sticks decays. In the dark, on the contrary, the visual purple is restored that leads to sensitivity increase. The phenomenon of adaptation is explained also by the processes proceeding in the central departments of analyzers. At long irritation the cerebral cortex answers with the internal guarding braking reducing sensitivity. Development of braking causes the strengthened excitement of other centers that promotes increase of sensitivity in new conditions.
Absolute sensitivity of the analyzer is limited not only the lower, but also top threshold of feeling. The maximum force of an irritant with which else there is a feeling adequate to the operating irritant is called as the top absolute threshold of sensitivity. The further increase in force of the irritants operating on our receptors causes in them only a sensation of pain (for example, very loud sound, the blinding brightness).
Sense organs are received, selected, accumulate information and transfer it to the brain which is every second receiving and processing this huge and inexhaustible stream. Adequate reflection of world around and a condition of the organism results.
The visual feeling possesses some inertia and disappears not how the irritant which caused it ceases to work. On inertia of sight, the principle of a cinema is based on preservation of visual impression during some time.
Adaptation as a total disappearance of feeling in the course of long action of an irritant. In case of action of constant irritants the feeling tends to fading. For example, the easy freight which is based upon skin soon ceases to be felt. The usual fact is also distinct disappearance of olfactory feelings shortly after we get to the atmosphere with an unpleasant smell. Intensity of flavoring feeling weakens if for some time to hold the corresponding substance in a mouth and, at last, the feeling can die away absolutely.
Adaptation call also other phenomenon close to described which is expressed in obtusion of feeling under the influence of action of a strong irritant. For example, at immersion of a hand in cold water intensity of the feeling caused by a temperature irritant decreases. When we from the dark room get to brightly lit space, at first we are blinded and are not capable to distinguish any details around. After a while sensitivity of the visual analyzer sharply decreases, and we start seeing normally. This decrease of sensitivity of an eye at intensive light irritation is called light adaptation.