What Are the Different Types of Auditory Problems?

Sound waves act on the auditory organs, causing them to sense cell excitement and cause the impulse of the auditory nerve to issue incoming information, which is caused by the analysis of the auditory centers at all levels.

Auditory theory

Various theories that explain auditory phenomena and their mechanisms. How sound waves produce hearing has always been an issue of interest. A complete hearing theory should be an elaboration of the entire hearing mechanism. However, some of the classical hearing theories in history only involve how the ear discerns the pitch, so it is only a theory of the pitch of the cochlea. With the development of modern computer technology and neuroelectrophysiology, although there is some understanding of the function of the auditory center, in general, little is known about how the auditory system processes auditory information from the periphery and how it generates hearing. .

In terms of the function of the cochlea on sound wave analysis, several different auditory theories have been proposed based on the perception of the pitch and the way it is discerned.

The position theory has two assumptions: sound spectrum is analyzed in the cochlea, and different frequencies cause excitement of neurons with certain characteristic frequencies in different parts of the basement membrane. The pitch of a sound stimulus is related to the mode of excitement produced by the stimulus. The first hypothesis has been confirmed, and particularly strong evidence comes from direct observation of basement membrane motion. The second assumption is controversial.

Auditory resonance-position theory is also called resonance theory. In 1857, H.von Helmholtz proposed that the cochlea was a row of spatially tuned analyzers for different frequencies. There are different length and length of transverse fibers on the basement membrane, which function much like a tiny resonator, and each fiber is tuned to a different frequency. Short fibers at the base of the cochlea respond to high frequencies, while long fibers at the top of the cochlea respond to low frequencies. The fibers of the basement membrane are continuously arranged from short to long, and the frequency of tuning with them is also continuously changed from high to low. When stimulated by a certain tone, the resonator in the corresponding area of the basement membrane resonates, and the nerve fibers associated with it also excite. The frequency of the tone differs depending on the resonance of the basement membrane and the corresponding neurons. Therefore, each tone has its specific location and neural representation on the basement membrane.

Since then, the discovery of new scientific facts has continued to shock Helmholtz's resonance. For example, research has found that the basement membrane is made up of interwoven fibers. Therefore, it seems impossible for each transverse fiber to act as a resonator to respond to different frequencies individually. In addition, the number of transverse fibers is far from being comparable to the number of pitches we can discern. For the tuning, selectivity and other characteristics of the resonator, Helmholtz did not give a good explanation.

Since 1928, the traveling wave theory has carried out a series of experiments. He first noticed that when any elastic body is subjected to vibration, there is always a kind of wave motion, that is, traveling wave. He further found that the transverse and longitudinal tensions of the basement membrane were almost the same. Therefore, the transverse fibers of the basement membrane cannot be resonators tuned to different frequencies. Later, he also found that the elasticity of different parts of the basement membrane is very different, and the difference between the basement and the volute is about 100 times. At the same time, the width and hardness of the basement membrane from the cochlea to the cochlea also gradually changed. These physical characteristics of the cochlear basement membrane can complete the preliminary analysis of the sound wave frequency. Becky's first on the cochlear model, then later directly observed the movement of the human cochlea basement membrane under the microscope, and found that when the pedal floor is moved, a traveling wave does occur on the basement membrane. Compared with the flexible worm movement, the amplitude of the traveling wave gradually increases, and it decreases rapidly when it reaches the maximum value. The maximum value of the envelope formed by the traveling wave peaks at each instant forms a region on the basement membrane, and the basement membrane deflection is also the largest in this region. Different areas of the basement membrane are related to different acoustic wave frequencies. The high frequencies are at the base of the cochlea, while the low frequencies are at the top of the cochlea, which is consistent with Helmholtz's earlier hypothesis.

Frequency doctrine The frequency doctrine represented by W. Rutherford holds that the basement membrane of the cochlea vibrates with the external sound wave frequency as a whole, and the pitch discrimination does not depend on the spatial analysis of the sound frequency on the basement membrane. The nerve pulse can replicate the frequency of external sound waves. The ear functions like a microphone of a telephone, and is a conversion mechanism for sound stimulation. Therefore, people often call this doctrine the telephone doctrine. Although Rutherford had noticed at the time that destroying different parts of the cochlea would have different effects on pitch discrimination, that is, hearing at different frequencies had a corresponding relationship with different parts of the basement membrane of the cochlea. Unexplainable.

Emission theory is also called resonance-emission theory. It not only recognizes that different stimulation frequencies play different roles on the basement membrane, but also affirms that the nerve impulses caused by acoustic stimulation can reflect the frequency of sound, so it is a combination of frequency and position theory.

The study of auditory physiological mechanism The study of neuroelectrophysiology proves that although the frequency of the discharge of the auditory nerve composed of thousands of nerve fibers can be synchronized with the frequency of the stimulating sound wave. However, the discharge frequency of a single auditory nerve fiber does not exceed hundreds of times per second. In order to explain this synchronized activity of the entire auditory nerve, EG Weaver proposed the emission theory in 1949. This theory holds that the synchronous firing of the entire auditory nerve to high frequencies may be the result of the coordinated activity of many nerve fibers with different excitation phases in the auditory nerve. Due to the alternate emission of nerve fibers that react to different phases, Can be synchronized with higher stimulation frequencies. However, when the sound wave frequency exceeds 5000 Hz, the auditory nerve no longer generates synchronous discharges. At this time, the resonance-position principle assumed by Helmholtz may work.

As described above, in the analysis of the frequency in the cochlea, both the position theory and the frequency theory may be correct within a certain range. As Bekathy (1960) demonstrated, for frequencies below 100 Hz, the vibration mode of the basement membrane no longer changes as a function of frequency, which indicates that the principle of position does not apply to low frequencies, however Low-frequency neurons that respond to specific phases of the signal, as described by the frequency theory, may now be active. Similarly, when the frequency of the stimulus exceeds 5000 Hz, the auditory nerve no longer has a synchronous discharge response. At this time, the traveling wave theory in the position principle may be working. For a wide range of intermediate frequencies, both theories may be valid. It can be seen that in the auditory theory, the traveling wave theory in the position theory is combined with the emission theory in the frequency principle, and the frequency analysis of the entire audible audio frequency range can be initially completed in the cochlea.

The neural mechanism for distinguishing pitch is not yet clear. From the perspective of neuroanatomy, the neural pathway from the cochlea to the auditory cortex of the brain is the most complex of all sensory pathways. Studies of neuroelectrophysiology have confirmed that the firing of a single nerve fiber mostly occurs at a specific phase of the stimulation waveform. Therefore, the firing pattern of the auditory nerve fiber contains the time information of the stimulus. In addition, different acoustic nerve fibers have their own frequency selectivity for different acoustic stimulation frequencies. And the fibers with different frequency selectivity are arranged in a certain order in the auditory nerve. The fiber that responds to high-frequency selection is on the periphery of the auditory nerve bundle, and from the periphery to the center of the nerve bundle, the selectable frequency of nerve fibers decreases from high to low in order. This shows that the principle of frequency distribution along the basement membrane is preserved in the auditory nerve. In recent years, some studies have also demonstrated that this tone-localized tissue structure is clearly present along the auditory system's conduction pathways up to the auditory cortex of the brain.

Most neurons in the high-level center of the auditory system, like neurons in the visual system, only respond to certain characteristics of the stimulus. In other words, the auditory system also has different feature detectors. These feature perceptrons provide quite complex functions for different levels of the hub. A large number of animal experiments have shown that the identification of sound frequencies does not necessarily have to be performed in the cerebral cortex. Therefore, for humans, it seems that the pitch can also be identified at a low level in the auditory center, and the function of the cerebral cortex is likely to store and analyze those stimulus factors that are more complex than pitch, such as speech and music melody. Time series etc.

bibliography

EGWever, Theory of Hearing, Wiley, New York, 1949.

G. von Békésy, Experiments in Hearing, McGraw-Hill, New York, 1960.

EC Carterette and MP Friedman, Handbook of Perception, Vol. , Hearing, Academic Press, New York, 1978.