by Amrita Anand
Ludwig van Beethoven’s symphonies are celebrated masterpieces of Western classical music. But, did you know that he composed several of them while suffering from severe hearing impairment? Beethoven’s sad predicament is by no means an isolated case: many acclaimed modern day musicians, including Eric Clapton and Pete Townshend, suffer from hearing loss. And this is not just about rock music: violist Betty Hauck a classical musician has spoken publicly about her hearing loss. Constant exposure to high sound levels has been implicated as the cause of auditory impairment in many of the above cases. World Health Organisation’s statistics indicate about 360 million people, accounting for 5% of the world’s population, suffer from disabling hearing loss due to various causes. It is anticipated that this number will escalate to a whopping 900 million in the next three decades. In addition, about 1.1 billion youngsters could be at a risk of hearing loss owing to unsafe listening practices. Now that I have your attention, let’s try to understand how we hear sounds before we examine the loss of this precious ability, the ways to prevent hearing loss, current prosthetic technologies and how we could cure hearing loss in the future.
To date, science has helped us understand the physical basis of sound and the mechanism of hearing. Sound is a wave that travels through a medium like air or water. It can also travel through solid substances, which is why you can overhear conversations by pressing your ear to a wall, door or window. By vibrating in concert with the source of the sound, the molecules of the medium aid in sound transmission. Most of the day to day sounds we hear are all propagated through air. Each sound wave has its own pitch and loudness. Humans can sense a range of sound waves whose particles can be vibrating as low as 20 times per second (frequency) to as high as 20,000 times per second.
The ability to hear sounds across this wide range of frequencies is something that human babies are born with. In fact, studies have shown that a developing human foetus, inside the mother’s womb, can respond to sounds as early as 30 weeks after conception. This innate ability to hear sounds allows us to experience music.
Wikipedia’s crowd sourced definition of music is “an art form and cultural activity whose medium is sound organized in time”. But, this definition lacks any hint of the enormous effect that music can have on people. Music has an undeniable ability to influence our moods and emotions, across the entire spectrum of mental states that we can experience. Sadness, joy, introspection, depression, motivation… music can help us tap into all these emotions and states of mental activity. Is it any surprise that music is an under-acknowledged yet constant companion in our lives? As background music in movies, as workout music in the gym, as ringtones in mobile phones, so on and so forth. One only has to name a human activity and there will usually be some form of music to go with it. But, whatever genre or form music might take, it is still composed of sounds.
Our ability to experience music is rooted in our ability to hear sounds, by the coordinated function of our ears and brain. When sound waves enter the ear, they pass through to the inner ear, which consists of cells that act as sound sensors. These cells are arranged inside the surface of a snail-shaped structure called the cochlea. These cells are called “hair cells,” as they contain projections on their surface that move in response to the incoming sound.
The process of sensing sound is comparable to a stadium full of people performing a ‘Mexican wave’ when their favorite players walk out. Here, for the sake of the analogy, each player is a unique sound wave, each spectator is a hair cell, and the waving activity influenced by the popularity of the player, is the hair movement. Hair cells enable the conversion of all kinds of sound waves from their mechanical form (as vibrations of air particles), to electrical impulses, which are sent to the brain for sound perception.
These auditory hair cells, which are just 3000 in number, have long been known to undergo damage when subject to various kinds of trauma, resulting in temporary as well as permanent forms of hearing loss. When the trauma is due to loud noises, the resulting auditory impairment is called noise-induced hearing loss. This produces a greater degree and earlier manifestation of hearing loss than is normal due to ageing. In humans, the ability to hear dwindles naturally with increasing age and almost 50% of individuals above the age of 75 years suffer from hearing loss. The first signs of hearing loss in adults is the inability to hear high pitched sounds, followed by decreased sensitivity to sounds at low volume. But noise-induced hearing loss can affect people at any age, so long as the trauma causes sufficient damage to hair cells. Preventing noise-induced hearing loss is possible by taking certain precautionary measures. These include reducing exposure to loud sounds in recreational and professional settings, with the proper use of ear-plugs, noise-cancelling earphones or by voluntarily moving away from the offending sound source.
But what if the damage is done and hearing loss is an undeniable part of someone’s life? The psychological impact of having to live with disabling hearing loss, which prevents one from clearly communicating with other people, is devastating. Not to mention the practical difficulties in both professional and personal situations as well as being unable to enjoy one’s own and other people’s music.
In this situation, is there any hope to regain this lost ability? Can music never again be a part of their life? For the most part, hearing loss is not yet a curable medical problem. Instead, current strategies for managing hearing loss are based on the use of electronic prostheses like hearing aids or cochlear implants, and counselling. How effective are these technologies at restoring hearing in auditorily challenged people? Hearing aids amplify incoming sound waves and help the ear get a better coverage of the sound, but they only produce minimal improvements in hearing. Cochlear implants have a sound sensor placed outside the ear which converts sound waves to digital codes. The actual implant is placed inside the inner ear and attempts to perform the function of hair cells by converting incoming digital sound information into electrical impulses for the brain. Unfortunately, owing to their simple design, these prostheses are nowhere near the efficiency of the structurally complex inner ear.
Just how large is the gap between normal, undamaged hearing and the assisted hearing enabled by technological aids? Particularly, how does that affect a person’s experience of listening to music? Dr. Charles Limb, an ear surgeon at the University of California San Francisco, and a jazz musician, studied how patients with hearing loss (outfitted with cochlear implants) perceived various characteristics of music. He observed that these individuals had heightened brain activity while perceiving speech but not music. They misidentified musical tones, which meant they would never be able to recognise a specific musical instrument by its unique sound. These patients were also unable to differentiate sounds of varying pitch and quality. As much as two to three octaves were distorted in the music they heard, which meant they were no longer hearing the intended melody — it was just noise.
Are there any new ideas and technologies that can improve this situation? The increasingly popular hearing loop is one example of technology that can heighten the sensitivity and function of hearing aids in specific situations. Essentially, a hearing loop consists of a fine copper wire installed in a room which selectively transmits the required sound waves (for example, the music at a concert) and cuts out background noise. These waves are decoded by an inbuilt receiver in the hearing aids and cochlear implants. Scientists are also working on improving the design of cochlear implants to try and simulate the functions of the inner ear with greater fidelity.
While these incremental advances provide tangible benefits to people suffering from hearing loss, they do not solve the fundamental problem. Besides finding ways to manage and reduce the impact of hearing loss, could it be possible in the future to initiate a repair mechanism in humans that regenerates auditory hair cells that have been damaged and lost?
In the 1980s, two research groups led by Drs. Rubel and Cotanche at the Universities of Washington and Honolulu respectively, made a remarkable discovery: auditory hair cells in two bird species, chicken and quail, regenerated naturally within few weeks after they were specifically damaged under laboratory conditions.
In addition to birds, other organisms like fishes, amphibians and reptiles also regenerate hair cells throughout their lives. But this regeneration process seems to have been lost during the evolution of mammals, which leaves a large set of animals, including human beings, unable to naturally reverse their hearing loss.
But, humans have always sought to improve what nature provides. Inspired by what is possible in the avian ear, scientists all over the world are trying to induce hair cell regeneration in the mammalian ear through various kinds of controlled genetic manipulation. Considerable success has been achieved in the quest to regenerate auditory hair cells in young and adult individuals of mammalian species such as mice and guinea pigs, respectively. Scientists have also discovered that specific genetic interventions in embryonic stem cells (which possess the ability to potentially develop into any type of cell in the body) can make them turn into something very similar to an auditory hair cell. However, there is a need for rigorous experimentation to establish safety and feasibility, extend these findings to older animals, and weigh the attendant ethical considerations before therapeutic techniques for hearing loss can be established in humans.
Humanity has already made great progress in understanding hearing loss and creating technological aids to partially restore hearing in patients. One could argue that it is good enough to regain the ability to hear speech with the available devices. But, musicians with hearing loss will certainly have something to say about that! Speech, after all, is only a small part of the acoustically rich world we inhabit. Without even realizing it, we are influenced by many of the sounds we hear. For instance, did you know that ambient music can influence how a glass of wine tastes to you ?
Trying to solve hearing loss is an endeavour that seems hard to justify in the face of fatal health issues such as cancer, malaria or other such diseases. But, what is life worth when we cannot experience the joy that our senses can provide us? Can only an increased lifespan justify the deteriorating sense of well-being, the decreased ability to communicate with other people and the diminished enjoyment of our shared culture that hearing loss produces?
Amrita is a graduate student at the Baylor College of Medicine in Houston, Texas. She works in a hearing lab in the Department of Molecular and Human Genetics.