HEARING

From Big Medical Encyclopedia

HEARING (auditus) — the function of an organism providing perception of a sound. Page arose in process biol. evolutions; it is most developed at mammals including at the person. Fiziol. S.'s basis is activity acoustic analyzer (see).

The audible signal comes to outside acoustical pass, in Krom there is a strengthening of a signal caused by own resonance of walls of outside acoustical pass. Further the audible signal causes fluctuations of a tympanic membrane, amplitude to-rykh can change with the broad range depending on signal strength. Thanks to system of acoustical stones of fluctuation of a tympanic membrane are transferred to liquids of an inner ear — peri-and en to a lymph. At the same time owing to distinction of the area of a tympanic membrane and the basis of a stirrup in a window of a threshold of a snail there is an increase in pressure of a sound wave. Same also the lever mechanism of acoustical stones promotes. The difference of the acoustic resistance (impedance) of two environments — air and liquids of an inner ear is as a result leveled.

At sounds of big intensity the system of a joint of acoustical stones gains protective, shock-absorbing value. It is promoted also by function in ear muscles — the stapedialny muscle and a muscle straining a tympanic membrane (see. Middle ear ).

Further mechanical processes in implementation of function C. are connected with activity of a basilar membrane (a basilar plate, T.), snails. According to the theory offered in 1863 by Helmholtz, the basilar membrane represents difficult enrollment of the resonators reacting to influence of sounds of various frequency. Resonators are located in such a way that reaction to sounds of high frequency arises in the main part of a membrane, and lower sounds are perceived by the resonators located in the wide site of a basilar membrane. Each of resonators is connected with a certain channel of an acoustical nerve, i.e. there is a transformation of mechanical oscillations into nervous impulses (see. Kortiyev body ). Helmholtz's theory was exposed to additions and review from a number of researchers. The greatest distribution and recognition was gained by the theory offered in 1960 D. Bekesha. The provision on heterogeneity of mechanical characteristics of a basilar membrane, in particular reduction of rigidity from the basis to the apical site is its cornerstone. The features of mechanical characteristics of a basilar membrane which are naturally distributed along it determine amplitude of fluctuations of its various sites at emergence of sound vibrations in fluid mediums of an inner ear. The main site of a basilar membrane has the greatest smeshchayemost. The site of a basilar membrane which underwent shift, thanks to the elasticity, aims to reach the initial position and moves a perilympha to other sites of a membrane, elasticity and inertance to-rykh are expressed less. In a result in to a basilar membrane there is a traveling wave. Location of a maximum of a traveling wave depends on the frequency of an audible signal. The maximum of amplitude of a wave is closer to those to the apical site of a basilar membrane, than the low frequency of a signal. Shift of certain sites of a basilar membrane causes irritation of the corresponding groups of phonoreceptors — the voloskovy cells connected with nerve terminations. Thus the mechanical oscillations caused by a sound incentive are transformed in an inner ear to nervous potentials, to-rye are coded further in a complex system of transfer of excitement to nerve centers.

The person is capable to perceive sound vibrations with a frequency from 16 — 20 Hz to 20 kHz (see. Sound ). Fluctuations of more low frequency are called an infrasound, higher — ultrasound. These fluctuations in usual conditions do not cause acoustical feelings. However in the conditions of bone (bone and fabric) carrying out when special bone phone directly contacts to the surface of the head or dense body tissues, acoustical perception is possible also at action of ultrasounds with a frequency up to 200 kHz. At each person of border of the perceived frequency range are defined by specific features of its acoustic analyzer and age (see. Presbyacusis ). Nek-ry researchers allow a possibility of acoustical perception of fluctuations with a frequency from 1 Hz (see. Infrasound ).

The term «ear for music» designate ability to perceive height, loudness and a timbre of musical sounds, and also their interrelation. Musical S. can be absolute and relative.

Nek-ry people possess the absolute musical S. representing knack to learn height of audibles signal without comparison them with a sound of in advance known height (e.g., during the use of a tuning fork). Relative musical S. is meant as ability to determine height of a sound by comparison it with other sound, height to-rogo is known, and also to learn musical intervals. It is accepted to allocate harmonious S. — ability of perception of many-voiced music and quality of accords. Essential value for musical activity has also so-called internal hearing giving the chance to represent music in imagination, without its execution or perception.

The important characteristic of S. is its sharpness, or acoustical sensitivity. It is defined by the indicator equal to the minimum size of the sound irritant causing an acoustical feeling. S.'s sharpness depends on the frequency of the perceived sound incentive. By the greatest sensitivity of S. of the person it is characterized in the range of 1 — 3 kHz. With increase or reduction in the frequency of stimulation acoustical sensitivity falls and consequently, the hearing threshold raises. Absolute hearing threshold — the minimum intensity of sound pressure, edge causes an acoustical feeling — makes 2*10 - 5 N/m 2 . This size is accepted in audiometriya (see) for zero level of audibility. At increase in intensity of a sound irritant emergence of unpleasant feeling is possible, and at further increase — and ear pains. The smallest size of energy of sound vibration, at a cut there are unpleasant feelings in an ear, is designated as a threshold of acoustical discomfort. It is equal on average 80 — 100 dB over the level of an absolute hearing threshold. At intensity of a sound, the equal 140 dB over an absolute hearing threshold, in an ear arises sensation of pain. This size characterizes a pain threshold. Subjective reaction to action of sounds reflects their physical characteristic. Intensity of sound influence is defined by the loudness of feeling, frequency — its height. Level of loudness can be expressed in backgrounds, i.e. in the sizes corresponding to number of decibels (sound pressure) of tone with a frequency of 1000 Hz, about the Crimea compare the loudness of the studied sound. For convenience of gradation use also unit a dream — the size corresponding to volume of sound with a frequency of 1000 Hz and intensity 40 dB over a hearing threshold. As unit of height of a sound serves chalk. Size 1000 chalk as an equivalent of a sound with a frequency of 1000 Hz and intensity 40 dB over a hearing threshold is usually used.

The essential characteristic of S. is ability to differentiate sounds of various intensity on feeling of their loudness. The minimum size of the felt distinction of sounds on their intensity is called a differential threshold of perception of sound intensity. Normal for a middle part of frequency range of sound waves this size makes apprx. 0,7 — 1,0 dB. The differential threshold of perception of sound intensity changes depending on changes of intensity and frequency of an audible signal. The similar indicator exists also for assessment of ability to distinguish sounds on feeling of their height. The threshold of differentiation of sounds of various frequency frequency range averages 3 — 5 Hz.

As S. is the means of communication of people, the special place in its assessment is allocated to ability of perception of the speech called by speech hearing unlike the ability of perception of tone signals called by tone hearing. By page it is closely connected with speech (see) that visually shows experiment of L of a barn — Barani; if during reading by the examinee aloud to apply Barani's graggers (see. Barani graggers ), he involuntarily raises the voice. This experience is sometimes used for differentiation of false deafness from true, at a cut of increase in loudness of a voice is not observed. Loss or falloff of S. at early children's age can negatively influence development of the speech. A component of speech S. is phonemic S., to-ry represents ability of perception of the minimum sound units of the speech — phonemes. Phonemic S.'s formation — a necessary condition of normal development of the speech. Speech S. is considerably connected with tone, however in its formation the most important role is played by the central mechanisms of the acoustical analysis. It is accepted to estimate speech S. by means of special tables of words on percent of legibility of the speech at various levels of their intensity, and also against the background of hindrances, in the conditions of distortion of speech signals, separate giving of a part of a signal at one, and a part — at other ear, etc. Comparison of indicators of tone and speech S. is especially important that gives an idea of a condition of various departments of the acoustic analyzer (see. Audiometriya ).

By means of S. along with the analysis of sounds on the frequency and intensity definition of provision of sources of a sound in space is carried out. This function received space S.'s name or ototopias (see). Definition of situation or movement of a source of a sound in the horizontal plane is connected with binaural hearing, i.e. with perception of an audible signal both ears that is explained by distinction on time and intensity of receipt of a sound in one and other ear. It is established that by means of S. registration of movement of a source of a sound as well in the vertical plane is possible. This ability is connected with monaural hearing, i.e. with perception by one ear, however its mechanism is finally not found out.

Methods of a research

In diagnosis of disturbances of S. the phenomenon of discrepancy between the size of intensity of the influencing sound and feeling of loudness is of great importance. This phenomenon received the name of the phenomenon of acceleration of increase of loudness (PAIL), or recruitment. Its essence comes down to the fact that at action of a sound incentive there is raised (in comparison with norm) a loudness that causes unpleasant feelings, and also complicates perception and legibility of the speech. Fiziol. the explanation of FUNG is based on a phenomenon of a parabiosis in volosko-vy cells of spiral body or in the nerve afferent terminations. Nek-ry researchers connect this phenomenon with disturbance of efferent regulation of receptor educations, with inconsistency of generating activity of outside and internal voloskovy cells and ryadokhm other factors. Emergence of a phenomenon demonstrates defeat of the receptor device of an inner ear, i.e. allows to differentiate cochlear and retro and cochlear defeats, and also in case of need to distinguish cochlear forms of relative deafness from relative deafness conductive. There are many methods of definition of FUNG. They are not equivalent on the reliability and rely on various physiological patterns that complicates their comparability. The most reasonable in definition of signs of FUNG should consider a method of balance, or alignment of loudness. However the area of its use is limited to unilateral relative deafness. In definition of FUNG measurement of size of differential thresholds of perception of sound intensity was widely adopted (see. Audiometriya ). FUNG define also by registration of answers at presentation to the examinee of repeatedly repeating short increments of intensity of an audible signal (SISI test), and also determination of size of an interval between thresholds of acoustical discomfort and audibility. The SISI test is considered positive when the examinee perceives from 80% to 100% of sound increments. Reduction of an interval between the level of acoustical discomfort and a threshold of acoustical sensitivity demonstrates disturbance of perception of loudness; at the same time, the less this interval, the is more extent of disturbance.

Also ability of perception of tones of small duration belongs to S.'s characteristic. It reflects features of accumulation in the acoustic analyzer of information on an audible signal in time, or integration of acoustic energy. With duration of a signal there is less critical size averaging 150 — 200 ms, there is an increase in a hearing threshold; the duration of an acoustic signal is shorter, the threshold is higher. As an indicator of the size of temporary summation (STS) serves the difference of hearing thresholds of a tone signal with its minimum duration and duration more critical. Measurement of hearing thresholds with gradually decreasing duration of the testing tone allows to construct the schedule «forces — duration», the configuration of curves to-rogo depends on the frequency of the testing signal. Physiologically process of integration of acoustic energy is connected by hl. obr. with the central departments of acoustical system. Therefore definition of the Air Force is used generally for identification of defeat of the central, first of all cortical, formations of the analyzer. At persons with cortical defeats during the giving of a signal in an opposite ear of the Air Force decreases due to deterioration in perception of short tones; at persons with defeats of a brain trunk of the Air Force does not undergo essential changes. Along with it the test of definition of the Air Force is used also for establishment of neurosensory frustration. Results are more contradictory here, but in general it is possible to speak about reduction of the Air Force at these disturbances. At conductive relative deafness process of integration of acoustic energy remains same, as well as is normal. A number of researchers attach special significance to discrepancy of results of integration of sound energy at different frequencies, believing that such discrepancies express the changes happening in the receptor and sound carrying out formations of an ear, napr at patients with an otosclerosis after surgical treatment. Others consider that definition of the Air Force maloinformativno because of small reliability, considerable dispersion, an illegibility of border between disturbances of sound carrying out and sound perception, labor input of carrying out researches, etc. It is supposed that it correlates with degree of a hearing loss on bone sound transmission more, than with any nozol. form.

Audibles signal higher than 10 kHz are of great importance for diagnosis of frustration of S. (the standard audiometriya has frequency range to 8 kHz). Use of the testing tones at frequencies of 12 and 16 kHz allows to reveal the early changes of S. caused by age involution of the acoustic analyzer, side effect of antibiotics and other reasons long before they are able to be registered on a standard audiogramma. To early diagnosis of disturbances of S. in a crust, time also definition of acoustical sensitivity is widely applied to ultrasounds at their bone carrying out (see. Ultrasonic diagnosis ).

The great value in a modern audiology is attached to objective electrophysiologic methods of assessment of hearing. They are based on registration of the caused acoustical potentials by means of the averaging computer facilities. Evoked potentials allow to judge electric activity of the cortical acoustical centers (dlinnolatentny potentials), the centers of a brain trunk and an acoustical nerve (korotkolatentny potentials). Besides, the method electro-kokhleografii, giving an idea of a state as acoustical nerve, and, partially, of receptor formations of an inner ear is used. Such objective (Computer) audiometriya is extremely important for identification of disturbances of S. at children of early age, and also for differential diagnosis of some forms of relative deafness and acoustical examination.

Along with a computer audiometriya for objective assessment of a condition of S. definition of acoustic is used impedance (see) a middle ear by registration of an acoustic reflex and the tympanometry — tonometry in a drum cavity (see. Manometriya ear ).

Pathology

Dysfunctions of S. (dysacusis) are very various and concern all its parameters. The most common form is decrease in sharpness of S. — a hypoacusis. It can be caused as disturbance of the mechanism of sound carrying out in structures of a middle ear, and damage of educations inner ear (see) and above the lying nerve pathways and the centers of acoustical system (see. Acoustical centers, ways ). Depending on it distinguish conductive and neurosensory forms relative deafness (see). In the first case S.'s sharpness decreases preferential on the tones which are carried out through air in the second — equally on the tones which are carried out through air and through bones of a skull. As a result at conductive relative deafness on an audiogramma the interval between curves of air and bone carrying out sounds appears, at neurosensory relative deafness this interval is absent or is insignificant in size.

More rare form of disturbance of S. is the hyperacusia consisting in abnormally raised susceptibility to sounds therefore both tone and noise signals, and the speech of usual intensity cause unpleasant and even painful acoustical feelings (odynacusis); it is sometimes observed at defeat facial nerve (see).

In nek-ry cases when the left and right ear unequally reproduce height of an audible signal, there is a symptom of doubling, or a diplacusis. At otosclerosis (see) the phenomenon of a parakuziya consisting in improvement of sharpness of S. in a noisy situation is observed. Presumably it is explained by the hyperexcitability of receptors of an inner ear arising in the conditions of noise.



Bibliography: Relative deafness, under the editorship of N. A. Preobrazhensky, page 9, M., 1978; Physiology of touch systems, under the editorship of A. S. Batuyeva, page 159, 341, L., 1976; Physiology of touch systems, under the editorship of G. V. Gershuni, etc., the p. 2, page 130, D., 1972; In yo k yo s at G. Experiments in hearing, N. Y. and. lake, 1960; Handbook of sensory physiology, ed. by H. Autrum a. o., v. 5/2, B. a. o., 1975.


B. M. Sagalovich.

Яндекс.Метрика