Auditory System

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Auditory System


the aggregate of mechanical, receptive, and neural structures that enables man and animals to perceive acoustic oscillations.

In higher animals, including most mammals, the auditory system consists of the external, middle, and inner ear, the acoustic nerve, and the centers of hearing. The centers of hearing include the cochlear and superior olivary nuclei, the posterior quadrigeminal bodies, the medial geniculate body, and the auditory cortex. The superior olive is the first brain structure in which information from both ears converges; fibers from each of the cochlear nuclei run on both sides of the brain. The auditory system also includes descending (efferent) conducting pathways that run from higher- to lower-lying centers (up to the receptor cells).

The cochlear duct, a unique mechanical spectral analyzer that functions as a series of mutually misaligned filters, plays an important role in the frequency analysis of sounds. The amplitude-frequency characteristics of the cochlear duct, that is, the dependence of the amplitude of oscillations at separate points on the sound frequency, were first experimentally measured by the Hungarian-born physicist G. von Bekesy. The characteristics were later determined more accurately by using the Mössbauer effect: the fairly steep slope of the amplitude-frequency characteristics toward the high frequencies equals approximately 200 decibels (dB) per octave. The amplitude of the oscillations of the cochlear duct, according to the same data, varies from a few to several hundred angstroms, depending on the intensity of the sound.

The activity of the receptive apparatus of the cochlea is manifested by electrical reactions, one of which fairly accurately reproduces the frequency of the tone (the microphone effect of the cochlea). The frequency selectivity of individual fibers of the acoustic nerve is sometimes much higher than the amplitude-frequency characteristics of the cochlear duct. For example, the slope of these curves toward high frequencies may reach 1,000 db per octave, which demonstrates the greater frequency sensitivity of the auditory system.

Examination of the activity of auditory-system centers by recording their bioelectric potentials reveals the tonotopical organization of these centers. Nerve elements exhibiting maximum sensitivity to a particular sound frequency are arranged in an orderly fashion, which may serve as a neurophysiologic basis for the place theory. In addition to frequency, the nerve elements of the auditory system exhibit definite sensitivity to the intensity and duration of sound. The neurons of the higher centers of the auditory system also react selectively to the complex features of sound signals, for example, to the specific frequency of amplitude modulation, direction of frequency modulation, and direction of sound travel.

Experiments performed on animals whose auditory-system centers were destroyed resulted in an impaired ability to discriminate certain parameters of sound signals. For example, removal of the system’s cortical zone resulted in a rise in auditory thresholds for sounds less than 20 microseconds in duration and in an impairment of the discrimination of sound sequences and the spatial position of the sound source. In man, similar disturbances have been found in pathological lesions of the cortical centers of the auditory system.


References in periodicals archive ?
Imaging studies further show hyperactivity not only in auditory pathways of the cortex and thalamus but also in the non-auditory, limbic brain structures that regulate a number of functions including emotion.
The aim of this study was to measure increases in arousal in comatose patients using an auditory sensory stimulation program, which controlled for intact auditory pathways using BAERs.
As shown schematically in the Figure, the ascending central auditory pathways begin with the termination of the auditory nerve at the cochlear nucleus and extend through the brainstem, thalamus, and corpus callosum before terminating in the primary auditory cortex, located in the superior temporal gyrus.
Multiple lines of evidence further indicate or suggest that impairments or changes in brain function--including damage to the auditory pathways in the brain stem, compromised function in the areas of cortex normally devoted to auditory processing, reduced cortical plasticity, or cross-modal plasticity--can produce highly deleterious effects on results obtained with cochlear implants.
In addition, other approaches--such as (1) reinstatement of spontaneous-like activity in the auditory nerve [150], (2) one or more of the previously described approaches for representing FS information with implants, or (3) a closer mimicking with implants of the processing that occurs in the normal cochlea [33,52]--may also produce improvements in performance, especially for patients with good or relatively good function in the central auditory pathways and in the cortical areas that process auditory information.
Desired plastic changes may be facilitated and augmented through directed training; the optimal training procedure is likely to vary according to the age of the patient, the duration of sensory deprivation prior to the restoration of (some) function with a cochlear implant (or bilateral cochlear implants), and whether or not the patient's hearing was first lost prior to the "sensitive period" for the normal development of the auditory pathways and processing in the midbrain and cortex.
The highly deleterious effects of cross-modal plasticity or missing the sensitive period for maturation of the central auditory pathways and cortex are "moral imperatives" to screen infants for deafness or hearing impairments and to provide at least some input to the "auditory brain" if feasible and as soon as possible for cases in which severe deficits are found.