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.

IA. A. ALTMAN