Spectral knowledge of auditory organs

Recently, I have been studying the theory of 3D audio surround sound reproduction, and found that to realize the real effect of stereo surround, I must make a deeper theoretical and application of the response of human ears to audio. For example, the role of human ears and ears must be fully taken into account when creating a three-dimensional surround sound field. This is because when recording a microphone recording, the in-ear listening field effect is lost. When the picked sound is played back, it is difficult to reproduce the original 3D surround sound field. To reproduce the 3D surround sound field, it is necessary to thoroughly study the unique effect of in-ear monitoring on the audio frequency domain, and then useAlgorithmPost-processing the output PCM data. Here I will repost a small medical study on auditory organsArticle.

 

Auditory organs (auditory threshold, auditory region, outer ear, middle ear, sound transfer, cochlear implants, auditory nerve action potentials

(Key Words: Auditory Organ; hearing threshold; auditory region; outer ear; middle ear; sound transfer effect; cochlear implants; auditory nerve action potential)

 

The peripheral sensory organ of the auditory system is the ear, and the appropriate stimulation of the ear is the sound wave vibration within a certain frequency range. The ear consists of the outer ear, middle ear, and the middle ear. Air leakage occurs due to sound source vibration. The latter transmission through the outer ear canal, the eardrum membrane and the auditory bone chain, which causes the vibration of the lymph fluid and the basal membrane, and the hair cells in the cortices of the cochlear implants are excited. The Coti organ and the hair cells contained in it are real sound sensing devices. The structure of the outer ears and the middle ears is only a sound transmission device that assists the vibration wave to reach the cochlear implants. The auditory nerve fiber is distributed in the basement membrane below the hair cell; the mechanical energy of the vibration wave is converted to the nerve impulse on the auditory nerve fiber. In addition, the voice information is encoded in different frequencies and combinations of neural impulses and transmitted to the auditory structure of the cerebral cortex to produce hearing. Hearing is of great significance for animals to adapt to the environment and human beings to understand nature. In humans, sound language is mutual.

An important tool for information exchange ideas.

Therefore, the main problem solved in the study of physiological functions of the ears is: how can the sound be transmitted to the cochlear implants through sound transmission devices such as the outer ear and middle ear, and how the sensory device of the cochlear implant converts the vibration of the cochlear lymph and the basement membrane into a neural impulse.

1. human ears hearing threshold and domain

The proper stimulation of ears is the dense wave of air vibration, but the vibration frequency must be within a certain range and reach a certain intensity to be felt by the cochlear implants and cause hearing. Generally, the vibration frequency that the human ears can feel is between 16-Hz, and each of these frequencies has a minimum vibration intensity that can properly cause hearing, called the hearing threshold. When the vibration intensity increases above the threshold, the auditory perception also increases accordingly. However, when the vibration intensity increases to a certain limit, it will not only cause hearing, at the same time, it will also cause the feeling of pain in the eardrum membrane, which is called the maximum audible threshold. Since each vibration frequency has its own listening threshold and maximum or listening threshold, a coordinate chart can be drawn to indicate the feeling range of the human ears on the vibration frequency and intensity, as shown in 9-14. The lower curve indicates the hearing threshold of different frequencies, and the upper curve indicates their maximum hearing threshold. The two areas are called the listening domain. The coordinates of the frequency and intensity of the voice that a person can feel should be within the range of the listening domain. As can be seen from the listening domain diagram, the most sensitive frequency of human ears is between-Hz, while the frequency of daily language is slightly lower, and the intensity of speech is at the moderate intensity between the hearing threshold and the maximum audible threshold.

2. Sound transfer between outer ears and middle ears

(1) sound collection and resonance cavity effects of ear and external ear canal

The outer ear consists of the ear and the outer ear. The Movement Ability of the ear of human ears has degraded, but the front and side sound can directly enter the outer ear canal, and the ear shape is conducive to the aggregation of acoustic energy, resulting in a strong drum vibration; if the same sound comes from the rear of the ear, it can be blocked by the ear and the sound is weak. Therefore, you can determine the position of the audio source based on slight changes in the intensity of the two ears.

The outer ear is the acoustic signal transmission path. One end opens and one end ends with the drum film. According to the principle of physics, the inflatable pipe can produce the maximum resonance with the sound waves with a wavelength of 4 times. The length of the outer ear canal is about 2.5. Based on this calculation, as a resonant cavity, the optimal resonance frequency is around Hz. When such a sound is transmitted from the outer ear canal to the drum, its intensity can be increased by 10 times.

(2) enhancement effect of eardrum and middle ear bone chain

The middle ear includes the main structures such as Eardrum, eardrum, ossiclink, middle ear minor muscle, and throat tube. The relationship between the eardrum, ossiclink, And the egus window of the middle ear is shown in 9-15, they constitute the most effective channel for the sound to pass from the outer ear to the cochlear implants. When the acoustic waves reach the membrane, the air acts as the vibration medium, and when the membrane arrives at the window film through the auditory bone chain, the vibration medium changes to the solid state biological tissue. Because different media have different sound blocking mechanisms, theoretically, when vibration is transmitted between these media, the energy attenuation is extremely high, and it is estimated that it can reach 99% or more. However, due to the special mechanical characteristics of the transfer system from the drum film to the egular window film, a pressurization effect occurs during the transmission of the middle ear of the vibration meridian, compensating for the energy loss caused by different acoustic barrier.

The eardrum is elliptical, with an area of about 50-90mm2 square meters and a thickness of about 0.1mm. It is not a flat membrane and is in the funnel shape of a vertex toward the middle ear. The inner side is connected to the hammer bone stem, which is located between the fiber layer and the mucosal layer of the drum membrane and is located down from the top to the center of the drum membrane. The drum film is similar to the vibration film in the telephone receiver. It is a pressure bearing device with good frequency response and small distortion. Its shape is conducive to passing vibration to the hammer at the tip of the funnel. It is observed that when the frequency of sound waves below Hz acts on the drum film, the drum film can copy the frequency of the added vibration, and the drum film vibration and the sound wave vibration start at the same end, with little residual vibration.

Figure 9-15 relations between middle ear and Cochlear Implants

The point line indicates the movement of the structures related to the vibration of the drum film to the inside.

The auditory bone chain is connected by the hammer, bone and bone. The hammer is attached to the drum film, the foot Plate of the primary bone is connected to the window film of the OSS, and the middle of the primary bone is connected to the primary bone, so that the three lower bones form a fixed angle lever between the two walls. The hammer is a long arm, and the middle bone is a short arm. This mechanical rod system is characterized by a pivot point that is placed on the center of gravity of the entire auditory bone chain. Therefore, it has the smallest inertia and the highest efficiency in the process of energy transmission. When the drum film is vibrating, for example, the inner movement of the hammer bone, the inner movement of the bone length and the inner bone in the same direction as that of the hammer bone, as shown in the in 9-15.

The middle ear pressurization should mainly have the following two factors: first, because of the difference in the area of the drum film and the size of the oval window film, when the drum film vibration, the actual vibration area is about 55mm2 square meters, the area of the window film is only 2mm2. If the total pressure of the ossiclink remains unchanged, the Pressure Acting on the window film will increase by 55. 2 = 17 times; second, the ratio of the lever's long arm to the short arm is about 1.3: 1, that is, the hammer arm is longer, so the pressure on the side of the short arm will increase to 1.3 times. In this way, the pressurization effect of the entire middle ear transmission process is 17 × 1.3 = 22 times.

There are two small muscles in the middle ear, which are related to the sound transfer function of the middle ear. When the eardrum muscles contract, the hammer and the eardrum muscles can be pulled inside to increase the tension of the eardrum muscles, move the bone and foot board outward to the rear. When strong audible air flows pass through the external ear canal and when the cornea and nasal mucosa are mechanically stimulated, these two small muscles may be contracted in a reflective manner, and the result is to make the eardrum tight, the border between each small bone is more tense, resulting in a reduction in the amplitude of the Vibration Transmitted by the bone chain; the increased resistance, the overall effect is to weaken the sound transfer efficiency of the middle ear. It is believed that this reaction can prevent a strong vibration from being transmitted to the cochlear implants and play a protective role on the phonograph device; however, because the reflex contraction of the middle ear muscle caused by sound takes more than a dozen milliseconds of latency, they do not have much protection for sudden transient bursts of sound.

 

(3) Functions of the tube

The throat tube is also known as the ear and throat tube. It connects the drum chamber and the nasopharyngeal cavity, which makes the air in the drum room and the atmosphere interconnected. Therefore, the throat tube can balance the possible pressure difference between the air in the drum room and the atmospheric pressure, this is important for maintaining the normal position, shape, and vibration performance of the drum. When the drum tube is blocked, the gas in the drum room will be absorbed, so that the pressure in the drum room will decrease, causing the tube to collapse. The current internal and external pressure of the drum film is poor, which often occurs in the external ear canal. The pressure in the drum room changes first, and the pressure in the room is still in the original state, such as the sudden rise, fall, and dive of an airplane, in this case, if the external ear pressure (or atmospheric pressure) in the drum Chamber cannot be balanced through the tube, a huge pressure difference will occur on both sides of the drum membrane. It is observed that this pressure difference, such as 9.33-10.76kpa (70-80 mmHg), will cause a strong pain of the drum film; when the pressure difference is greater than 24 kPa (180 mmHg), may cause a rupture of the drum. In normal circumstances, the Throat Opening of the throat tube is usually closed. Due to the contraction of the muscle such as puguifan muscle during swallowing, yawning or sneezing, the tube mouth can be temporarily opened, which is conducive to the pressure balance.

When the sound of the bone is transmitted normally, the auditory system is triggered by the sound passing through the outer ear canal, which causes the drum film vibration and then enters the cochlear system through the auditory bone chain and the oval window film, it is called gas transmission. In addition, sound waves can also directly cause the vibration of the skull, and then cause the vibration of the cochlear lymph nodes in the temporal bone bones. This is called bone conduction. When the bone conduction is normal, it is much less sensitive than the gas conduction, and it is almost impossible to feel its existence. one of the aspects that can detect the existence of bone conduction is, directly contact the handwheel of a heartbeat, and then people will feel a slightly strange sound. When this sound gets weaker, then you can quickly move the inner to the front of the ear, and then you can hear the sound. This simple experiment demonstrates the presence of bone conduction, which also shows that gas conduction is more sensitive than bone conduction in normal cases. It can be considered that bone conduction has little effect on normal hearing. However, clinically, we often determine the locations and causes of auditory abnormalities by examining the patient's gas and bone conduction damage.

3. sensory effects of cochlear implants

The function of the cochlear system is to convert the Mechanical Vibration Transmitted to the cochlear system into the nerve impulse of the auditory nerve fiber. In this transformation process, the vibration of the base membrane of the cochlear implants is a key factor. Its vibration stimulates the hair cells located above it, causing various transitional electrical changes in the cochlear system, and finally causing the passing nerve fibers at the bottom of the hair cells to generate the action potential.

(1) Key Points of the Structure of cochlear implants

The cochlear implants are made up of a bone pipe hovering around a bone axis for 21/2-23/4 weeks. Two demarcation membranes can be seen on the cross section of the ear worm tube. One is the oblique forward membrane and the other is the horizontal basement membrane. The two membranes divide the pipe into three cavities, they are called the front level, the drum level, and the worm tube (Figure 9-16 ). The vestibular is attached to the bottom of the cochlear gland and is connected to the oval window film, and the inner is filled with external lymph nodes. The latter is also filled with external lymph nodes, and the latter is transported to the top of the cochlear implants and the external lymph nodes in the anterior order; the worm canal is a blind canal in which the inner lymph is bathed on the surface of the screw located on the basement membrane. The structure of the probe is extremely complex. On the cross-section of the worm tube, you can see a row of internal hair cells vertically arranged on the side of the worm axis; on the outer side of the worm tube, 3-5 rows of external hair cells are arranged vertically (see Figure 9-18). In addition, there are other support cells and large gaps between these cells, includes the inner and outer tunnel and nuel clearance. It should be noted that the liquids in these gaps are in the same composition as the outer lymph nodes, they do not communicate with the inner lymph nodes in the worm duct, but they can communicate with the external lymph nodes in the drum stage through the pores in the basement membrane. This structure contacts the top of the hair cells with the inner lymph nodes in the worm canal, while the outer and bottom of the hair cells are in contact with the outer lymph nodes. There are hundreds of neatly arranged listening hairs on the surface of each hair cell, some of which are longer embedded in the ice-like material covered by the film, and some are only in contact with the film. The covered membrane is in the inner side of the cochlear axis, and the lateral side is free from the inner lymph.

(2) vibration and traveling wave theory of basement membrane

When the acoustic flow vibrates to the OSS through the auditory bone chain, the pressure changes are immediately transmitted to the isolated fluid and membrane structure in the worm. If the OSS moves inside, the forward membrane and the basement membrane will also move down, the last step is the out-of-band movement of the Window Film for external lymphatic compression. On the contrary, when the window film for external movement, the whole structure of the cochlear implants moves in the opposite direction, thus forming vibration. It can be seen that in the normal gas transmission process, the window film actually acts as a buffer for pressure changes in the cochlear implants and is a necessary condition for the vibration of the structure in the cochlear implants. Someone used a direct observation method to record in detail the basement membrane vibration caused by sound stimulation, which is used to understand the form of basement membrane vibration, it also provides a reliable basis for the differences between these vibrations when the cochlear implants receive different frequencies of sound stimulation. Observations show that the basement membrane vibration is carried out in the form of traveling wave, that is, the vibration of the inner lymph is first caused by the vibration of the basement membrane near the window of the OSS, this fluctuation is then transmitted along the base membrane to the top of the cochlear implants in the form of a row wave, just as when a person shakes a silk belt, a row wave is transmitted along the silk belt to the far end. The next step also proves that the traveling waves caused by different frequencies all start from the bottom of the basement membrane, that is, close to the window membrane of the OSS, but the frequencies are different, the distance of the traveling wave propagation is the same as that of the most popular wave, as shown in 9-17. The lower the vibration frequency, the farther the traveling wave propagation is, when the largest amplitude of a row appears, the row quickly disappears and does not spread. On the contrary, the basement membrane vibration caused by high-frequency sound is, it is limited to the vicinity of the window.

Figure 9-16 cross-sectional view of the ear worm Tube

The different forms of Row-Wave Propagation of the basement membrane caused by different frequencies are mainly determined by some physical properties of the basement membrane. The basement membrane length is about 30mm, which is slightly shorter than that of the cochlear implants, but the width is only 0.04mm near the window of the oval window and gradually widened. Correspondingly, the height and weight of the ecmiter on the basement membrane also increase with the widening of the basement membrane. These factors determine the closer the basement membrane to the bottom, the higher the resonance frequency, and the closer it is to the top, the lower the resonance frequency. This reduces the resistance of the traveling wave caused by low frequency vibration when it is transmitted to the top, the traveling wave caused by high-frequency vibration is only near the bottom.

Different frequencies of sound cause different forms of basement membrane vibration, which is considered to be the basis for the cochlear implants to distinguish different sound frequencies. Experiments and clinical studies on the damage to the basal membrane of animals with different types of hearing loss have proved this conclusion, that is, when the base of the cochlear implants is subject to high-frequency hearing, when the top of the cochlear implants is damaged, low-frequency hearing is affected. It cannot be understood that since each vibration frequency has a specific traveling wave propagation range and the maximum amplitude zone on the basement membrane, the hair cells and auditory nerve fibers associated with these regions will be most stimulated, so that the nerve impulses and combinations of these nerve fibers from different regions of the basement membrane are transmitted to different parts of the auditory center, it may lead to different tones.

9-18 shows how the base membrane vibration stimulates hair cells. At the top of the hair cell, some of the hearing hairs are buried in the gel of the covering film, and some are in contact with the Covering Film. Because the covering film and the Vibrating Shaft of the base membrane are inconsistent, therefore, there is a horizontal staggered movement between the two films, causing the listening hair to bend due to a shear force (Figure 9-18, bottom ). According to research, the bending of the hair cells to the right of the hair is the first step from mechanical energy to electrical change in the cochlear implants.

Figure 9-18 force on the top of the hair cell when the base membrane and the Covering Membrane vibrate

Upper: in the case of static: When the basement membrane moves up in vibration, the shear movement between the base membrane and the Covering Membrane leads the hair to the outer of the worm canal.

 

(3) The biological phenomenon of cochlear implants

In addition to recording the Action Potentials Related to the excitation of the auditory nerve fibers, some other forms of electrical changes can be recorded in the cochlear structure. If an electrode is placed in the external lymph node of the drum stage and is grounded to keep it at zero potential, we can use another measurement electrode to measure the potential of the inner lymph node of the worm canal to about 80mV. This is called the inner lymphatic potential. If this measurement electrode is inserted into the hair cell membrane, the intrinsic potential is-70? /Font>-80mV. When the bath liquid outside the hair cell top membrane is inner lymph, the potential difference between the hair cell inside (equivalent to-80mV) and outside the membrane (equivalent to + 80mV) is 160mV; the immersion bath around the hair cells is external lymph nodes (with a potential equivalent to zero), and the difference between the potentials inside and outside the membrane is only about 80mV. This is the difference between the resting potential of the hair cells and the general cells. According to the experimental analysis, the production and maintenance of positive potential in the inner lymph node are directly related to the cell activity of the vascular structure at the lateral wall of the worm Canal (Fig. 9-16), and are very sensitive to O2 deficiency; it has been found that the membrane of vascular cells contains a large number of highly active ATP enzymes, which act as a "sodium pump". They can obtain energy by decomposing ATP and pump K + in plasma into the inner lymph, the Na + in the inner lymph is pumped into the plasma, but the transferred K + exceeds the Na + volume, which causes a large number of K + in the inner lymph, therefore, the inner lymph maintains a high positive electric position. The lack of O2 blocks the production of ATP and the activity of Na + pump. Therefore, the inner lymph is not maintained.

A special electrical fluctuation can be recorded in the cochlear implants and their nearby structures when the cochlear implants are stimulated by sound. This is an electrical change of the exchange nature. Within a certain stimulus intensity range, its frequency and amplitude are exactly the same as those of the acoustic wave (Fig 9-19 ); this phenomenon is just as if a speaker or microphone (that is, a microphone) of a telephone phone can change the sound vibration to an audio electrical signal similar to a waveform, this is the reason why this electrical change in the cochlear implants is called the potential of the microphone. In fact, if you talk to an experimental animal and the ear, while guiding its microphone potential in the cochlear implants, and amplifying the potential and connecting it to a speaker, the speaker sounds like a speech! This experiment vividly shows that the cochlear implants act like a microphone, which can convert sound waves into audio signals. Other characteristics of the microphone potential are: the latency is very short, less than 0.1 ms; there is no waiting period; it is not sensitive to O2 deficiency and deep anesthesia, and it can still appear when the auditory nerve fiber degeneration.

Figure 9-19 micro-speaker potential and acoustic nerve action potential caused by transient sound stimulation

CM: microphone potential AP: cochlear nerve action potential (including N1, N2, N3)

Comparison between A and B shows that when the sound phase changes, the potential phase of the microphone is reversed, but the neural Movement

The potential phase has not changed. C: The AP disappears and the CM still exists under the white noise.

An experiment using micro-electrodes to record the intrinsic electrical changes of cells in hair cells proves that the potential of a micro-speaker is a combination of the receptor potentials produced by multiple hair cells when receiving sound stimulation; when the cross-membrane potential of a single hair cell is recorded, it is found that there is only 0.1 of hearing hair. The angular displacement of the hair cell may cause the sensor potential, and the direction of the potential change is related to the direction of the Force on the hair, that is, the potential is depolarized; this explains why the potential fluctuation of the microphone is consistent with the frequency and amplitude of the acoustic wave vibration.

Because there is only a very short latent period between the angular displacement and the receptor potential, it is considered that the latter is generated because there is a mechanical gate channel in the top membrane of the hair cell, the slight deformation of the membrane caused by the force of hearing hair is enough to change the functional status of the channel protein, causing the movement of cross-membrane ions and the corresponding potential reaction. In hair cells, its receptor potential may change the release of transmitters (probably glutamate and barrier) at the bottom of the cell base, which in turn leads to the production of action potentials produced by the passing fibers of the nearby cochlear implants, transmits to the auditory advanced pivot to produce auditory. As for the functional differences between inner and outer hair cells, it was noted that the numbers of incoming fibers they received were significantly different. According to the calculation, the total number of hair cells in the middle of the human side is about 3500, and the outer hair cells are about 15000, however, about 32000 of the approximately 90% input nerve fibers from the spiral neuron are distributed to the bottom of the inner hair cell, which indicates that an inner hair cell accepts the distribution of Multiple Input fibers, multiple external hair cells can accept the branch of an incoming fiber. Therefore, it is generally believed that the function of the internal hair cells is to convert the sound vibration at different frequencies into a large number of nerve impulses distributed at the bottom of them, and transmit auditory information to the central center, however, the role of interest cells has been found to be somewhat special in recent years. Some people have found that when the hair cells have a microphone potential in the basement membrane vibration and hearing the force on the hair, this cell can produce a rapid change in the length of the body, hyperpolarization leads to cell elongation, depolarization leads to cell shortening, their form changes are also synchronized with the frequency and amplitude of external sound vibration. It is believed that this physical change of the outer hair cells can enhance the original vibration of the basal membrane, but also play a amplification effect on the traveling waves, this obviously makes the internal hair cells located on this part of the basement membrane more susceptible to irritation and increases the sensitivity to the vibration frequency. The mechanism of the external hair cells is unclear because of the change of the internal and external potentials of the membrane. However, this makes the basement membrane not only generate a wave of external vibration in a fixed structure, it can also "actively" enhance the vibration amplitude of the traveling wave.

Iv. Auditory Nerve Action Potential

The action potential of the auditory nerve fiber is the final electrical change in the first series of responses of the cochlear implants to sound stimulation. It is the final result of the conversion and encoding of sound stimulation by the cochlear implants, the central auditory perception can only be caused by these inputs. In Figure 9-19, N1, N2, N3 ...... It is recorded from the entire suction nerve to the compound action potential, not produced by the excitement of a single auditory nerve fiber. Use pure audios of different frequencies to stimulate the ear fly. At the same time, check the impulse distribution of different single auditory nerve fibers to check whether the traveling wave theory is correct, and to clarify some features of the Function of cochlear code. When carefully analyzing the relationship between each stochastic nerve fiber discharge farm and the sound frequency, if the sound intensity is large enough, the same fiber can often respond to a group of pure sounds with similar frequencies, however, if the sound intensity is gradually weakened, the optimal response frequency of a fiber can be found, that is, when other stimulus frequencies are too weak to cause the action potential, this frequency can still be caused. The optimal reaction frequency of each fiber depends on the distribution of the basal membrane at the end of the fiber, this part is exactly the location of the largest amplitude Traveling Wave Caused by the sound of this frequency. The preliminary conclusion drawn from these experiments is that when the sound intensity of a frequency is weak, neural signals are transmitted to the Hub by a few nerve fibers that are most sensitive to the frequency, when the sound intensity of this frequency increases, in addition to the above-mentioned fiber excited, it can also lead to more nerve fibers whose optimal response frequency is similar to this frequency, so that more fibers are involved in the frequency and intensity of the sound transfer. The result is the nerve impulse transmitted by these fibers, and the information about the frequency and intensity of the sound is transmitted to the hub. Under natural circumstances, changes in the frequency and intensity of the voice that acts on the ears are very complex, therefore, the vibration form of the basement membrane and the excitement and combination of the auditory nerve fiber caused by this are also very complicated. The human ears can distinguish different sounds, and the basis may also be this.

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