From Big Medical Encyclopedia

ADAPTATION ACOUSTICAL — adaptation of an acoustic organ to changes of intensity of sound intensity. Directly after the beginning of sound irritation acoustical sensitivity goes down and after the termination of a sound is recovered to initial level. Gradual recovery of sensitivity is observed also after a certain period of influence of a continuous sound of average intensity. At adaptation to silence acoustical sensitivity increases a little.

In the mechanism of Ampere-second. imputations of excitability of both peripheral, and cortical department of the acoustic analyzer play a role. At action of a sound the frequency of categories in fibers of an eighth cranial nerve decreases; at action of sounds of high frequency and big intensity of Ampere-second. it is caused, probably, by changes both in spiral (kortiyevy) body, and in a cerebral cortex.

For measurement of Ampere-second. use three methods: 1) poststimulyatorny, 2) perstimulyatoriy and 3) method of disappearance of threshold tone.

The Poststimulyatorny method consists in definition of shifts in auditories threshold after cancellation of superthreshold tones intensity in 70 — 80 dB and duration of return of hearing to initial level (the return adaptation). Lengthening of the return adaptation is considered as an indicator of braking of the acoustical centers. Some insignificant fluctuations in the size of Ampere-second. take place and at people with normal hearing. Therefore only considerable aberrations are important for diagnosis. After irritation a sound in 70 — 80 dB within 3 min. auditories threshold at normal hearing usually increase by 20 — 30 dB. To initial level they are returned in 20 — 30 sec. At disturbance of sound carrying out Ampere-second. it is normal, at the sound perceiving relative deafness — it is raised.

At a perstimulyatorny method check shifts of thresholds at the irritation tone and time of their recovery. Along with true tone give a white noise (the noise consisting of a wide range of frequencies — see Audiometriya) in a tensivnost in 80 dB. Against the background of noise define an auditory threshold of this tone. Then noise is switched off, and tone continues to be given that raises an auditory threshold on tone. At inclusion again of noise tone is not perceived any more. It becomes heard only at lowering of the level of noise. Number of decibels, on a cut it is necessary to lower noise level to keep perception of tone, determines the size of Ampere-second. It fluctuates from 10 to 30 dB in 1 min.

The method of disappearance of threshold tone was entered into clinic by Karkhart (R. Carhart, 1957). By this method measure extent of adaptation at irritation by tone of threshold intensity. Perception of threshold tone at normal hearing remains within a minute and more. Give to drink some diseases of an ear tone quickly stops being heard (in 5 — 6 sec.), appears again only at increase in its intensity by 5 dB, then again vanishes. To keep perception within a minute, it is necessary to increase gradually tone, sometimes on 30 — 40 dB and more over a threshold. This method measures Ampere-second. regardless of exhaustion since it is enough to switch off tone for an instant, and the patient hears it again.

The expressed disturbances of Ampere-second. often happen at defeat of voloskovy cells of spiral body, at Menyer's disease, more rare at neuritis of an acoustical nerve. At diseases of a middle ear and an otosclerosis of Ampere-second. usually it is not broken.

See also Hearing .

Bibliography: Temkina I. Ya. Acoustical adaptation at defeat of hearing, Vestn. otorinolar., No. 5, page 24, 1965; Undrits V. F., Temkin Ya. S. and Neumann L. V. Guide to a clinical audiology, M., 1962, bibliogr.

I. Ya. Temkina.