There are two main points to this question – this matter has aspects that relate both to the people of guidance who are attached to God, and to those who are not.

It is the duty of the people of guidance to communicate to other people the truth as they believe, the truth they have found and had a taste of; the truth they are satisfied with or a view with which they mature with, and to guide them to peace. First and foremost, this is a debt of conscience we owe to humankind. A person who knows these truths, the content of which has been bestowed to our consciences as a divine favor, should communicate them to those who do not know and open for them the doors leading to God.

Now, let’s have look at how God has commanded about this matter: “The believers, both men and women, they are guardians, confidants and helpers of one another. They enjoin and promote what is right and good and forbid and try to prevent the evil” (Qur’an 9:71). Another verse reads: “O you who believe! There must be among you a community calling to good, and enjoining and promoting what is right and good and forbidding and trying to prevent evil (in appropriate ways). They are those who are the prosperous.” (3:104). God has always referred to those who promote what is right and good as an honorable community: “You are the best community ever brought forth for (the good of) humankind, enjoining and promoting what is right and good forbidding and trying to prevent the evil, and (this you do because) you believe in God” (Qur’an 3:110).

These divine lights considerably clarify this obligation towards humanity. This matter is clarified by many other verses and hadiths and it is so open and clear that it does not require any further interpretation or explanation. Therefore, we can gather from these verses that we are responsible for communicating the divine truths. This is such a compelling responsibility that if there was no one left on earth one day in future, then we will entertain the possibility that there might be people living in a colony near Sirius or some distant planet around some unknown star in the Hercules constellation and somewhere in the Milky Galaxy, and with an urge that is unique to human species to reach out to them, we will build colonies and cities as interim stations on our route toward them, and we will communicate our heart-felt inspirations to the people there (if any). We will intermingle and sympathize with them and we will show them the roads that will lead to God. We will say, “Say, ‘There is no god but God,’ and be saved and be prosperous,” as the Prophet, peace and blessings be upon him, put, and this will always be our main motto. This is our duty. On the other hand, their duty is to lend an ear to this call of ours that invites them to peace and to search for the straight path wherever they are and everywhere they go.

If we do not fulfill our duty, we will fail to comply with our obligation towards God. Then, Almighty God will ask questioningly: “You knew the truth, but you failed to communicate to other people? You knew God, but you did not introduce Him to other people? You could see the straight path, but why didn’t you show it to other people? You were aware of the light, but why didn’t you turn other people to look toward that direction?”

In the light of divine revelations, I can say that on that day, Almighty God will say to them: “Although you satisfied all of your animal drives to the full, why didn’t you search for the way which would provide you with full gratification? Even during your most troubled and distressed times, you did not think for an instant to experiment this way? In other words, you have resorted to all sorts of recipes for your worldly welfare, but why didn’t you consider making the truth which you somewhat heard about even for once?”

In the final analysis, Almighty God will hold every individual against his or her own sins. Since we are responsible only for communicating the truth to them, we will be questioned only for our faults in fulfilling this duty of ours. On the other hand, they will be questioned for failing to search for truth or lend an ear in a sincere manner to the truth, if they have already heard about it. May God save us from falling tumbling down by failing to enjoin and promote what is right and good, and forbid and try to prevent the evil while having capability to do so!

He has granted you from all that you ask Him. Were you to attempt to count God’s blessings, you could not compute them. But for sure, humankind is much prone to wrongdoing (sins and errors of judgment) and to ingratitude. (Ibrahim 14:34)

Today a large part of modern science focuses on understanding the human body. Researchers working on life sciences hope that one day the secrets of every single detail that make us human will be revealed. Every year billions of dollars are spent by scientific institutions on learning more about us. Actually this fact by itself is enough to suggest how little control we have over things happening in our bodies, and we know even less about the mechanisms of moving, touching, speaking, seeing, or hearing, and so on.

As a scientist, I really cannot guess whether life scientists will ever be able to learn enough to solve the puzzles of the human body, but I feel a lack of satisfaction when the knowledge we have gained from scientific discoveries is compared with what is unknown. In my opinion this is why one of the most intelligent physicists in history, the Nobel laureate Richard Feynman, once said, “I was born not knowing, and have only had a little time to change that here and there” [1]. My understanding is that such a conclusion must be inevitable if the primitive knowledge given to us by modern sciences is not interpreted in the light of a far superior logic that is meant to explain the whole creation. In that sense, I believe that we have to consider every single detail in creation as a vital part of the whole in order not to feel lost before the grand picture of this masterpiece.

Last year, in a seminar at Osaka University Graduate School for Frontier BioSciences, I was thrilled to hear Professor Keichi Namba say, “Japan’s fastest supercomputer dissipates more than billion times the power dissipated by a fly’s brain, yet it is not able to simulate the brain of such a tiny animal.” This worked as a wakening call or a reminder for me to think again about the magnificent arts of the Creator. In particular, I wanted to revise my research on a hearing-related protein from a new perspective, rather than using the mechanical attitude that is followed most of the time.

This article is an attempt to explain an amazing mechanism in our ears that enables us to hear the faintest whispers. A mechanism that is switched off at loud cries to protect us from disturbing noises, yet amplified to make the softest sounds audible. Before starting to explain the basic anatomy of the human ear, I should mention that today the ear’s active amplification mechanism is still being investigated in research centers by biologists and physicists together.

How do we hear? What is happening in the inner ear?

Findings from the last century have shown that our ears are not just simple receivers as we had imagined. In 1979, David Kemp of University College, London discovered that mammalian ears can also emit sound vibrations. By placing a very sensitive microphone close to the eardrum he could detect whistles, implying that there is a source of vibration within the ear [2]. However, before trying to explain the cause of vibrations in the ears, we have to go over the mechanism of hearing briefly: The delicate design of the outer ear, the tympanic membrane (eardrum), and the tiny bones (malleus, incus and stapes) enables to collect sound waves traveling in the medium and transfer them to the inner ear (Figure 1a). In the inner ear the sound waves are sorted according to their frequency and amplitudes and then converted into electrical signals which can be transported to the brain via nerves. At the onset of this process the sound waves are transformed into standing waves on the basilar membrane which is laid along the organ resembling a snail, the cochlea (Figure 1b). The frequency of the incoming sound wave determines the positions of the distortions along the cochlea: High pitches create vibrations at the basal end of the cochlea (i.e. adjacent to the middle ear) whereas low frequencies vibrate closer to the apical end (Figure 1c) where the cochlea gets narrower. This geometry helps our ears to act as a frequency analyzer.

The efferent and afferent nerves that connect the ear to the central nervous system are attached to the organ of Corti, which is situated right next to the basilar membrane, extending over the cochlea. In other words, Corti is the sense organ of hearing, converting the motion of the basilar membrane into electrical signals that are conducted to the brain via neuronal cells [3]. The organ of Corti is also lined with multiple rows of sensory hair cells.

The hair cells of the organ of Corti

Corti is decorated with two different sets of sensory cells: single row of inner hair cells (IHCs) accompanied with 3–4 rows of outer hair cells (OHCs), both spanning the whole cochlear tube (Figure 2a). They are called “hair cells” because both IHCs and OHCs have typical bundles of stereocilia that contain mechanosensitive ion channels (Figure 2b).

The major function of IHCs is to detect the sound waves and then convert them into equivalent electrical signals that are to be interpreted by the brain. When the basilar membrane is perturbed by the incoming sound waves, the IHCs found in that region sense this activity by the movement of their hair bundles (bundles of stereocilia). The hair bundles of IHCs deflect and re-align as the basilar membrane moves up and down (Figure 3). We should note that this is an amazingly sensitive process such that deflections of the stereocilia on the order of a few nanometers (one millionth of a millimeter) can be detected and converted into nerve signals by the IHCs [4].

However, this by itself is not sufficient for hearing; no matter how effective IHCs work, the fluid that fills the cochlear tube is a threat to the sound waves traveling in the inner ear. In 1948, a young astrophysicist named Thomas Gold was the first person who has pointed out that the fluidic nature of the cochlea would dampen the sound vibrations and make them too weak to be detected by IHCs. He has concluded that an inherent vibration amplification mechanism is necessary in order to overcome such a problem [5]. Unfortunately, Gold’s statements were overlooked by the physiologists of his time who had performed their hearing related experiments on dead cochleas.

Gold’s predictions were justified around ten years later by William Rhode, a physiologist from University of Wisconsin, who has shown that the vibrations of the basilar membrane in live tissue samples are stronger than anticipated [6]. In the present day the existence of an amplification mechanism within a live cochlea is a well accepted fact. The only disagreement among scientists is about how the mechanism of the amplification works. Several scientific laboratories have reported different experiments performed on the organ of Corti and they have proposed different models. At the center of one of these models is prestin, a membrane protein which is not found in any cell but OHCs in the human body.

Electro-motile outer hair cells and prestin

In 1985, the distinctive properties of OHCs were first discovered by William Brownell, a University of Geneva neuroscientist, who has shown that these cells can convert electrical signals into motion: A phenomenon called electromotility. Electromotile OHCs can elongate or shrink in response to electrical charge density changes in their membranes. About a decade ago Peter Dallos and co-workers from Northwestern University in Chicago discovered a membrane protein, unique to OHCs, that can respond to electrical signals [3]. The Dallos group coined the name “prestin” for this protein in an analogy with the musical term “presto” (quickly) due to its rapid response to electrical signals. Various kinds of mammalian cells genetically engineered to produce prestin at their membranes displayed the electromotile responses that are very similar to OHCs.

According to Peter Dallos prestin protein works as a tiny machine which is a crucial element for cochlear amplification [7]. His theory is verified by recent studies which show that cochlear sensitivity in mice decreases hundredfold when prestin activity is disrupted by genetic means [8]. As the sound waves reach the inner ear, prestin-driven electromotility enables the OHCs to move like pistons. The piston movement in phase with the basilar membrane motion amplifies the vibrations and makes them stronger for IHC detection (Figure 4a,b). The prestin-driven vibrations were what Thomas Gold proposed and David Kemp had detected so many years ago. However, scientists are still searching and learning new things about this nanometer scale machine. One of the discoveries showed that prestin can adjust itself according to the amplitude of the incoming sound waves: Basically, the amplification is stronger when the sound waves are hard to hear but gets weaker as the volume increases.

Up to this point, we have briefly explained how the amplification mechanism of hearing in mammals works. Unfortunately, even though it took decades of research for scientists to discover and define the active nature of the mammalian ear, this explanation highlights only a minuscule part of the whole picture. That is why we are still incapable of curing most hearing problems. For example, hearing loss due to slightly disturbed hair cells with damaged stereocilia turns out to be chronic (Figure 5). The medical treatments we have to hand are too primitive to mend such delicate structures. Moreover, hearing aids made by today’s technology are not nearly as effective and functional as needed.

On the other hand, the delicacy of the hair cells and the limited control scientists have over them are not the only lessons we have learned from research on the inner ear. We cannot overlook the other messages attached to the research on the grounds that the time given to us is just too short to comprehend. It is an undeniable fact that the sense of hearing is designed in the best way to serve human beings. The different characteristics of hearing amplification at different sound levels make life much easier for us: Prestin-driven hearing is most effective when the sound waves are weak and harder to hear. This way the incoming sound waves are amplified enabling us to hear the faintest whispers. However, as the sound strength increases, the prestin-driven amplification gradually gets weaker and finally diminishes after a point to make sure that loud noises are less disturbing and hazardous for us. In my opinion, this amazing quality of a tiny protein found in our ears is one of the pieces of evidence that remind us of the necessity of pondering the favors of our Creator. Qur’anic verses such as Ibrahim 34 at the beginning of this article give us clues about how to interpret scientific findings that reveal the amazing qualities of our bodily organs. May the Creator of our ears allow us to reflect more on His favors and live accordingly.

Hamdi Sener is a biophysicist living in Boston. He can be contacted at hamdisener@gmail.com.

References

1. Gleick, J., Genius: The Life and Science of Richard Feynman. Reprint ed. 1993: Vintage. 560.
2. Kemp, D.T., The evoked cochlear mechanical response and the auditory microstructure- evidence for a new element in cochlear mechanics. Scand Audiol Suppl., 1979. 9: p. 35–47.
3. Zheng, J., et al., Prestin is the motor protein of cochlear outer hair cells. Nature, 2000. 405(6783): p. 149–55.
4. Robles, L. and M.A. Ruggero, Mechanics of the mammalian cochlea. Physiol Rev., 2001. 81(3): p. 1305–52.
5. Gold, T., Hearing II. The physical basis of the action of the cochlea. Proc. Roy. Soc. B., 1948. 135: p. 492–498.
6. Rhode, W.S., Observations of the vibration of the basilar membrane in squirrel monkeys using the Mossbauer technique. J. Acoust. Soc. Am. , 1971. 49: p. 1218–1231.
7. Cho, A., What’s Shakin’ in the ear? Science, 2000. 288: p. 1954-1955.
8. Liberman, M.C., et al., Prestin is required for electromotility of the outer hair cell and for the cochlear amplifier. Nature, 2003. 419: p. 300-304.
9. Fettiplace, R. and C.M. Hackney, The sensory and motor roles of auditory hair cells. Nat Rev Neurosci., 2006. 7(1): p. 19-29.
10. Dallos, P. and B. Fakler, Prestin, a new type of motor protein. Nat Rev Mol Cell Biol, 2002. 3(2): p. 104-11.

Life without me is only silence

Every artist wishes to present his work to admiring eyes. In the whole universe, from atoms to star systems, God shows all the details of His art to you, a conscious being; and among His works, He has installed the most splendid ones in your body. He has given you reason and knowledge so that you can easily see and understand them. With knowledge, you can appreciate the meaning and different qualities of existence. However, you need another tool, your five senses, through which you will look at and learn about the material world around you and then turn this knowledge into an appreciation of the meanings behind God’s creation.

If you were not able to perceive light and color (by your eyes), your knowledge of material existence would be insufficient. Similarly, if God had not placed me in your skull, you would not be able to hear and know the songs of birds, the rustling of trees, the babbling of water, or the whistling of wind, which are each a note in the divine musical harmony throughout the universe. Indeed, everything speaks in its own tongue in order to introduce God to people. You use your eyes to perceive the things that speak with the wavelengths of light. You use me to perceive other wavelengths called “sound,” which is caused by the vibration of molecules.

The wavelength of the sounds that I can perceive ranges between 20 and 20,000 Hertz. I am unable to sense frequencies of sound that are above or below those limits. Indeed, it would be better to call this an advantage given by God rather than an “inability.” If the Creator of everything in the universe had not created me with this limited capacity, you would be facing unbearable pain in your head. If He had made me work with a wider range of hearing, you would be disturbed by the footsteps of a little ant, the moaning of an insect laying eggs, the buzzing of beehives, and the sound of the fluttering birds. Therefore, the fact that I have sufficient sensitivity for you to meet your needs is an advantage and an indication of God’s mercy. After all, my Creator gives everybody exactly what they need in a most suitable way and in the best measurements; He never does anything absurd. Do not ever want to have an ear like that of a bat. I am the best one for you.

Do not ever think that my outer, visible part is too simple. My outer ear, which sometimes turns red when you are nervous, is placed in the best position according to the shape of your head so that it can receive sounds in a most efficient way. Because it is made up of elastic cartilage, my outer ear (A) is very flexible, and it won’t break when you lie on it. The curves on me (known as the helix) and the hairs inside my channel are not made without a reason, either. My cartilages have the perfect shape to channel the sound down towards my middle ear according to the intensity of the sound and the direction it comes from. Because this special shape is formed according to the genetic code of a person, it is different in every person. The hairs in the canal serve to protect me from foreign objects like insects or dust. The canal that connects my outer part to my middle part is pretty wide, but if too much fatty wax accumulates here, I might experience temporary hearing loss.

My outer part is followed by my middle ear, which begins with the ear drum (tympanic membrane) (C). Attached to this thin ear drum are three bones: the malleus (D), the incus (E), and the stapes (F), which are all placed in order. These little bones are jointed to each other at an angle of 105 degrees. With an action like a piston, they amplify even the smallest sound vibration coming from the ear drum and transmit it to the middle ear. My middle ear space is connected to your pharynx by a very thin canal called the Eustachian tube (G). In order to protect my ear drum from rupture, I recommend that you open your mouth during an explosion or an intense sound. In that way, the sound waves that enter through your mouth will balance with the sound waves in my canals so that my ear drum is protected.

My inner part, followed by my middle part, is the most vital and sensitive area. Therefore, it is surrounded and protected by the bones of your skull. This inner part, which is an amazing piece of art and technology, comprises two wonderful receptor components. Those two little parts are placed in the same narrow area inside the temporal bone, but they perform different tasks. One of them is the cochlea (H), which is involved with hearing. The other part is the balance (vestibular) canals, which consist of the semicircular canals (I), the saccule (J), and the utricle (K). This balance organ enables you to stand straight and walk, run, or move without bumping or falling.

Like carved marble or forged metal, those parts are crafted out of bones that form a beautiful and intricate whole. My cochlea is divided widthwise by a bony tube. The upper compartment above the tube is connected to an oval window, which is an outlet to the middle ear. The lower compartment below the tube is connected to a round window. My inner part is a labyrinth of fluid-filled tubes. The fluid in the bony labyrinth, between the bone and the membranes, is called perilymph, and the other fluid within the membranous structure is called endolymph.

Situated on the basilar membrane (L) of my cochlea is a very small and special organ that you call the organ of Corti. The organ of Corti contains the hearing cells (or hair cells), the receptors (M) that are sensitive to sound waves, and other supporting cells. Because the length of the cells in the organ of Corti varies, different parts of my cochlea are sensitive to sounds of different wavelengths.

The sound waves travel via the malleus, the incus, and the stapes and through my oval window, agitating the perilymph of my cochlea. After that, the sound waves cause Reissner’s membrane (N) in my cochlea to vibrate, which then results in a wave movement in the endolymph. The wave movement continues along this membrane until it reaches my organ of Corti. The special receptor cells (or hair cells) of the organ of Corti are the ultimate vibration receptors. Their surfaces consist of very small strands (cilia). Those little strands bend and twist when the sound waves are received. Right at this point, a very important event occurs: it is the movement of these strands which converts the mechanical energy (that is produced by the vibrations of the sound waves) into electrical impulses. Those electrical impulses are then sent to your brain via the auditory nerve (nervus cochlearis) of the brain, where they are perceived as “sound.” The same sound waves continue their way to the perilymph and pass into the round window, the section between the middle ear and the inner ear. The round window pushes out to dissipate the sound vibrations in the perilymph and thus lessens their pressure.

The speed of the hearing depends on the speed of the sound that travels through my membrane and little bones. However, once the sound waves begin to pass to your brain as an electrical impulse along the auditory nerve, the hearing process increases its speed. Then your brain immediately interprets and reacts to the sound waves. You are not aware of all these rapid activities which are done perfectly in fractions of a second. You only say that you can hear something ordinarily. Have you ever thought before about how hearing takes place? Do you think you would have a clue about the sounds and music in the universe if God had not created me as your hearing organ?

Think about it, Peter! God knows exactly what you need for your life and equips your body accordingly. If there were no God, would such a complicated organ as your ear form by itself in your skull? Can it be a simple “coincidence” where some biological mechanisms take place successfully and in order without any plan or project and they produce such a splendid organ as me with all my sections? Like every reasonable and thoughtful person, you now understand that I cannot be the result of simple coincidence but only a creation of our God Almighty, don’t you?

Maintaining your balance

So far, what I have told you about is my duty to hear. Now I must also tell you about my duty of balance, so that you can better understand how miraculous I am.

Have you ever seen an acrobat walking on a rope or a mountain climber in action? Or shall I give a better example that might be more familiar to you? Remember what you do on your bicycle to keep from falling off. At the slightest mistake, the acrobat might topple from the rope, the climber might slip off the cliff face, and you might fall off your bicycle. While you are making unconscious (reflex) movements to keep your balance, have you ever thought about what busy operations are going on in my system? I have been equipped with very sensitive receptors which help you stay stable during your continual, different movements. Those receptors immediately recognize the changes occurring as a result of your slightest motion; they warn your body to adjust to your new position by sending out information to the spinal cord and to the brain about the new situation.

You may wonder how these two processes, hearing and balance, can take place in such a small area of the body, the inner ear. It is our Creator, God, who puts microscopic cells in a narrow place and runs the most sensitive and important operations via those little cells.

How do you feel the sensation of balance and how do you react with the right reflex action? To find an answer to that, you need to re-examine my anatomical structures mentioned before. At the base of my semicircular canals is a bulb-like enlargement which opens to the saccule and the utricle. My three semicircular canals are situated at 90-degree angles to each other in three-dimensional space.

My semicircular canals contain few sensory hair cells but there are plenty of them in the bulb-like enlargement. The strands of these cells, which are placed delicately, have enough elasticity to twist and bend during a movement. The receptors for balance in the saccule and the utricle are covered by a thin membrane which contains a gelatinous layer and tiny calcite crystals (cupula terminalis). Depending on its density, the endolymph fluid in my semicircular canals moves against the direction that your head and body move in. Similar to the uncontrolled movement of passengers in an accelerating or moving vehicle, depending on the speed and the direction, the movement and the speed of the endolymph differs from the general movement of your body. For example, when a car turns right, the passengers move to the left with the turning acceleration, and when a fast-moving car brakes suddenly, the passengers are thrown forward. Similarly, depending on its acceleration and momentum, every change in your movement causes the fluid in my semicircular canals to move. Triggered by the movement of the endolymph fluid, the gelatinous mass with the calcite pieces is displaced, causing the strands of the receptors to twist. Every movement of your head warns the cells of different parts, and via the vestibular nerve (nervus vestibularis) the nervous system is notified of changes occurring in your balance.

Thankfulness and contemplation

You have now seen what amazing works my two compartments, the balance and the hearing organs produce. All through your life, the former serves you by maintaining your balance without missing any of your movements, while the latter enables you to learn about the thousands of types of sounds in the world. Once you consider all of your movements in your life, you will see that my two organs perform their duties perfectly without ever getting tired, giving up, or complaining. We do not ask for any fee from you in return for those benefits, either. In fact, when God Almighty created you, placed us in your skull and set up our connection with the related center in your brain, He did not ask for any fee from you. All He wants you to do is to think about those blessings and be thankful to Him.

If you visited a hospital, you might see a lot of scenes which would lead you to think about God’s blessings on you and thank Him. Serious ear illnesses include middle ear infection (otitis media), which is frequently seen in children; otosclerosis, which is the limited ability of the stapes to transmit sound waves because its base becomes fixed to the oval window; and several hearing disorders which might be present at birth or occur later in life, depending on the level of damage to the auditory nerve. Witnessing the effects of those illnesses, you would understand how important it is to be able to hear and stand straight and balanced, and so see how blessed you are. At every step you take, when you are lying down or standing up, or every time you hear the twittering of birds, a nice melody, or the sweet voice of your parents, you will now appreciate the greatness and the mercy of our Lord God Almighty, who has engraved the meanings of all those sounds in your mind.

Peter! Until now, you have used me to listen to others, but today it was my turn to be listened to while I told you about myself. However, I must admit that I have only been able to explain to you the details of about one-hundredth of the beauties displayed in me and my delicate anatomical structure. If I attempted to present you with all the details about me discovered by developing technology and science and the meanings attached to them, there would not be enough pages in the magazine that you are holding now. Indeed, you do not need that much information either. My main aim here is to draw your attention to me, and thus let you know our God and bring you closer to Him. I hope I am successful in that. From now on, you will hear my ringing occasionally and remember me so that you will be saved from your heedlessness once again.

Irfan Yilmaz is a professor of biology at Dokuz Eylul University, Izmir, Turkey.

The human ear and hearing

The human ear can be divided into several functional sections: the outer ear, the middle ear, the inner ear, and the auditory nerve. Sound goes through a series of changes as it travels through these sections until reaching the brain. The outer ear picks up sound pressure waves, amplifies them and then converts them into mechanical vibrations on the ear drum, which is connected to a series of small bones in the middle ear. These small bones further amplify or diminish the mechanical vibrations in the ear drum and transfer them to the cochlea, a snail-shaped cavity filled with fluid which is located in the inner ear. Change in fluid pressure caused by vibrations within the cochlea lead to changes in the flexible membrane, called the basilar membrane. These changes contain information about the frequency and strength of the sound that has entered the ear. Attached to the basilar membrane are mechanical receptor cells, called hair cells, which are bent according to the deflections of the basilar membrane.The hair cells have hair-like structures. The bending of these hairs assists the release of an electrochemical substance that causes neurons to send electrical signals to the brainstem through the auditory nerve. These signals are in the form of a message (or a code) that the brain understands.

Cochlear Implant (CI) devices

If there is a broken link in any part of the auditory pathway, the brain does not receive any coded signals, and hearing impairment occurs. If a large number of hair cells or auditory neurons in the cochlea have been damaged, then the person is diagnosed as profoundly deaf. The hair cells can be damaged by certain diseases (e.g., meningitis, Meniere’s disease), by congenital disorders, by certain drug treatments, or by other causes. One negative outcome of damaged hair cells is that they can subsequently lead to the degeneration of adjacent auditory neurons. Research has indicated that the most common cause of deafness is the loss of hair cells (>95%) rather than the loss of auditory neurons. This has encouraged scientists to try implanting a device inside the iner ear or cochlea, bypassing the normal hearing mechanism of the ear, to stimulate the remaining auditory neurons directly through electrical signals. These are called Cochlear Implant (CI) devices, which can restore partial hearing in profoundly deaf people . A standard CI system, shown in Figure 1, composes of and performs the following functions: a microphone picks up sound pressure waves and converts these into electrical signals. The signals are sent to the speech processor that is worn by the patient. The speech processor analyzes and encodes these sound signals, sending them back to the external pick-up coil . After passing through a wireless radio link that lies between the external and implanted coils and an implanted electronic devise, coded signals are sent to the implanted array of electrodes in the cochlea to electrically stimulate the remaining auditory neurons , and the brain receives what it interprets to be sound.

Electrical stimulation of the ear, or CI research, can be traced back to the 1800s. The Italian scientist Alessandro Volta used a battery as a research instrument to demonstrate that electric stimulation could result in a number of human sensations . After connecting a 50- volt battery to his ears, he noted that “…at the moment when the circuit was completed, I received a shock in the head, and some moments after I began to hear a sound, or rather noise in the ears, which I cannot well define: it was a kind of crackling with shocks, as if some paste or tenacious matter had been boiling…”. That electric stimulation of the auditory nerve provides hearing sensation in deaf people was reported more than 100 years after Volta . Electric stimulation in two deaf patients resulting in hearing was reported in 1957. These successes resulted in intensive research into helping deaf people hear in the 1960s and 1970s. One of the early successful single-channel CI devices was developed in the early 1970’s (3MCorp/House) and became the first commercially available CI device approved in the United States in 1984. The University of Utah developed a six electrode implant called the Ineraid or the Symbion device in the early 1990s. It was followed by other devices in Europe, the United States, and Australia.

The present status of Cochlear Implants

Today, around 10% of the population in developed countries suffers from hearing impairment. At present, the number of CI users has reached more than 100,000 worldwide, and is still growing rapidly. Functionally, CI has evolved from the single-electrode device that was used as an aid for lip-reading and

sound awareness to a modern, multielectrode device that can allow an average user to talk on the telephone. Even though significant technological progress has been achieved in the last 50 years, there are still many mysteries about the human hearing process and the parts of the ear. Here, we will compare some aspects of the healthy human ear and CI devices, looking to the future. The human ear operates over a range of sound pressures (its dynamic range) which is greater than one million to one (120dB), with as many as 200 discrete steps in the range. In contrast, today’s CI devices typically provide a dynamic range of three to one (10dB) to ten to one (20dB) with 20 discrete steps. This major difference is mainly due to the fact that the human ear is very adaptive in noisy environments, and is able to suppress noisy background, while picking up and processing appropriate sound signals for better perception. CIs do not differentiate between sounds, but amplify all sounds, which results in poor sound perception. Today, a typical multi-channel CI system uses 16 to 24 electrodes implanted in the cochlea with 8 to 22 signal processing channels. A potential shortcoming of having so many electrodes and channels in current CI technology is the electrical interference of electrodes during simultaneous electrode stimulation. These electrical interactions can disrupt the stimulus waveform prior to neural activity and degrade sound perception. The normal ear contains roughly 3,500 inner hair cells in the cochlea that are tuned to different frequencies from 20 to 20,000 Hz. They are connected to about 35,000 auditory nerves. Hair cells work as signal processing channels, yet each of the inner hair cells has also been wired in a sophisticated and little-understood fashion to 10-20 auditory nerve fibers that carry information to the central nervous system. Since they work in the chemical domain, they do not have the gross interference issues of CI electrodes. While good speech understanding has been achieved by users of modern multi-electrode CIs operating in quiet environments with 70–80% sentence recognition, allowing users to talk on the telephone, the CI devices do not discriminate between noise and the meaningful signals, only achieving speech understanding at between 70% and 80%, which falls to 10% or lower in noisy environments. It is a great challenge for CI users to appreciate music. Some CI listeners reported that they can enjoy music and are able to recognize melodies, but most described musicas sounding unpleasant and noisy, and performance could not be increased with current CI technology. CI users have difficulty in identifying differences in frequencies. Typically, they cannot discriminate any frequency difference for frequencies higher than 500 Hz, while the normal ear can hear up to 20,000 Hz with frequency discrimination between 2 to 3Hz at best. This gross difference is related to the issues surrounding signal processing strategies and electrodes of current CI systems. Predicting post-surgical performance based on presurgical conditions and tests of a CI candidate is still a problem for the physician. The cost of surgery is still high; in the United States, for example, a typical cost is between $40,000 and $75,000. Beyond these issues, the moral, cultural and ethical issues related to CIs are very complex. They are still debated, and are an important part of CI development in the world today. The hair cells in the human ear naturally deteriorate and die as we grow older. This process is typically sped up with exposure to loud noise. In common with all mammals, new hair cell generation in human ears stops right after the birth. However, in fish and amphibians, very similar cells are present and reproduce throughout life. Recently, it was found that hair cells of birds are repaired after being damaged by exposure to noise or ototoxic agents. It was also discovered that hair cells in the mammalian vestibular (balance) organ, very similar to those in the hearing system, can regenerate. These findings, along with other advancements in medical fields, lead to long-term research into different aids for hearing- impaired people. Despite the fact that hearing loss is usually permanent, scientists are optimistic that it may eventually be possible to reverse the damage in the ear by repairing or regenerating the sensory hair cells through gene therapy, stem cell transplantation, or ultimately by replacing the human cochlea with an artificial one. Today, Auditory Brainstem Implants are also being tried on humans for direct brainstem stimulation, bypassing the ears and the auditory nerves. Human beings and most animals on earth are born and equipped with a pair of ears for a good reason: having two ears enhances hearing and sound localization. Scientists are examining whether this is also true for deaf children who receive not one, but two CIs.

Conclusion

The sense of hearing is a gift for human beings which they hold dear and are grateful for, as much as for any of the other senses with which they have been equipped. It is important to strive to find cures for all kind of diseases, yet, more important than the cure is prevention of harm to our body and its amazing senses. Here, we have tried to open a small window onto human hearing, to examine how related impairments are being dealt with through cochlear implant (CI) devices, as well as looking at the issues related to these devices and the future directions of research for restoring hearing to deaf people. It is obvious that we have learned much about human hearing and ear in the past century; yet, this may well be just the tip of the iceberg.

References

1. S.U. Ay, F.-G. Zeng, B.J. Sheu, “ Hearing with bionic ear,” IEEE Circuits & Devices Magazine, Vol. 13, No. 3, pp.18-23, May 1997.

2. F.-G. Zeng, “Trends in cochlear implants,” Trends in Amplification, Vol. 8(1), pp.1-34, 2004.

3. A. Volta, “On the electricity excited by mere contact of conducting substances of different kinds,” Royal Soc. Philos.Trans., vol. 90, pp.403 431, 1800.

4. A.M. Andreev, G.V. Gersuni, A.A.Volokhov, “On the electrical excitability of the human ear: On the effect of alternating currents on the affected auditory apparatus,” Journal of Physiology USSR, Vol. 18, pp.250-265, 1935.

Yes, illness utters this warning to us: “Your body is not composed of stone and iron, but of various materials that are always disposed to parting. Leave your pride, understand your impotence, recognize your Owner, know your duties, learn why you came into this world!” It declares this secretly in the heart’s ear.

Moreover, since the pleasures and enjoyment of this world do not continue, particularly if they are illicit, they are fleeting, full of pain, and sinful. Do not weep on the pretext of illness because you have lost those pleasures. On the contrary, think of the aspects of worship and reward in the Hereafter to be found in illness, and try to receive pleasure from those.