Understanding how we experience sound and what that means for hearing loss and hearing restoration is Professor David Ryugo’s speciality. David explains the brain mechanisms of hearing.
Have you ever wondered about the sound of complete silence? It is not something we normally encounter. In fact, sound is always around us as a result of vibrations in the air or any elastic medium including water. There is the sound of traffic, of rustling leaves, of wind, car alarms, and ringing phones; there is conversation, dogs barking, and music.
From an evolutionary perspective, sound provided information about the environment from a safe distance when the foliage was too dense or light too dim to see. Smell is too diffuse and is only effective when one is downwind of the odour. Touch is too late to alert us about a sabre-toothed tiger. Animals, including early humans, needed to know if a sound indicated a predator, the next meal, a competitor, or a potential mate. In the modern world, sound provides warnings of danger, alerts us to visitors, soothes us with music, and facilitates our humanity through spoken communication.
Understanding sound and hearing Loss starts with understanding how we hear
There are three physical parameters to sound: frequency, loudness, and time.
is the number of oscillations (vibrations or cycles) that a sound makes in a second, and humans hear a range from 20 cycles per second up to 20,000 cycles per second (also known as Hertz, Hz). The note middle C is 256 Hz; the highest note on a piano is 4186 Hz. Most electronic devices emit a high-frequency hum when turned on but we don’t hear it because it is beyond the range of normal human hearing. Vowel sounds involve low-frequency sounds, whereas consonants involve primarily high-frequency sounds.
is related to the pressure of the airwaves conveying the sound. This pressure is measured in decibels and we are sensitive to sounds that can vary over 7 orders of magnitude, from a near-silent whisper to the roar (and pain) of a jet engine. Loud noise is damaging to the ears in the same way that ultraviolet light exposure is damaging to the skin and radiation is damaging to genes; a little won’t hurt, but over time it all adds up.
Prolonged exposure to noise will result in hearing loss; sports fans who attend events attempting to break the Guinness World Record for Loudest Crowd Roar, are giving themselves noise trauma that will damage hearing.
is the third component to sound: there is onset, offset, and duration. It is used to define a melody, to represent letters in the international Morse Code, and forms the cadence or prosody for speech. Neural activity and features of sound are linked in time and that is how the brain learns to associate specific acoustic events to auditory experience. This link is why we find it annoying if conversations in a movie do not precisely synchronise to mouth movements. And why we find speech with a “foreign” accent difficult to follow.
Our auditory system is a most remarkable biological entity that receives these vibrations in the air by way of the ear and converts them to neural signals that are processed by the brain. The processing occurs over neural circuits that analyze the components of frequency, loudness, and time to produce our precept of the sound—a conversation, a melody, or the ocean roar. We don’t always appreciate our hearing until we lose it.
What’s in store?
In Australia, the incidence of hearing loss increases with age. Estimates put just over 20% of people over 18 years of age with hearing loss. And that this number increases to 50% for those over 65 years. Three-quarters of those over 70 years suffer from severe hearing loss according to the Listen Hear! Access Economics Report 2006 (1). As life expectancy rises, it means that a majority of us will spend the final decade of life with communication impairment.
The World Health Organisation reported in 2013 that approximately 360 million people worldwide are affected by disabling hearing loss and it emerges as one of the largest public health concerns for the future. It is not “life-threatening” but has an insidious adverse impact on our quality of life.
The cost of hearing loss
In children, hearing loss undermines speech and language development, as well as socialization skills, which in turn can affect long-term academic achievement. Poor academic performance limits further educational opportunities such as university programs that will influence employment options.
Success in school influences one’s sense of self-worth, identity, and financial independence (2). In adults, hearing loss has a negative impact on employment opportunities because sound is an important aspect of safety and communication in the work environment. Hearing loss impairs your ability to communicate, enjoy radio or television, or be alert to important warning signals (e.g., smoke alarm, doorbell, and telephone rings).
The difficulty in communicating when there is competing noise is a major complaint by those with hearing loss. It prevents the enjoyment of dining out in restaurants, attending parties, and even hosting family holiday dinners. Fundamentally, people find it embarrassing to constantly say “what?” and still not understand after several repetitions. Withdrawal from these kinds of social events leads to social isolation, which can in turn bring on depression and early cognitive decline, especially in the elderly (3).
The financial expense to society resulting from lost productivity and disability is estimated to be nearly $12 billion per year in Australia alone1. One cannot, however, put a price tag on the personal cost of lost quality of life.
So, what allows us to hear
The external ear
The external ear, where you park a pencil or hang jewellery, funnels sound into your ear canal. The sound vibrates the eardrum, and these vibrations are delivered to the coiled inner ear by 3 tiny bones. The vibrations in the inner ear stimulate the hearing receptor cells, which in turn cause neural signals to be sent to the brain for processing. The inner ear, smaller than the size of a pea, is responsible for our hearing.
The inner ear
Our inner ear is an extremely delicate structure. Disease, head trauma, drugs, and loud noise can cause sensory receptor cells to die. When these cells die our hearing diminishes. Hearing loss reduces the resolution of our sound environment. When we are young, we have high-fidelity sound input to the brain. It is analogous to having high definition television for sight. Living in a noisy, industrialized society causes sensory receptor cells corresponding to high frequencies to die over time. We might notice this loss when consonant sounds of speech, such as ‘s’, ‘f’ and ‘th’, become difficult to distinguish. As hearing loss increases, our sound environment “pixilates”. The sounds can be heard but the details are indistinct. Softer sounds are not heard and sounds that are heard seem to run together.
Amplification alone cannot treat hearing loss
Many people wrongly assume that hearing loss can be treated by simple amplification. In fact, hearing loss has many adverse symptoms: speech comprehension in a noisy background is impaired; tinnitus or ringing of the ears typically emerges, and sometimes there is severe loudness distortion. Hearing loss is not just about volume. Increasing the loudness often does not improve understanding; amplifying a fuzzy signal just gives you a loud fuzzy signal.
Hearing loss superficially resembles presbyopia—the need for reading glasses as one gets older. Presbyopia is a natural consequence of aging due to a stiffening of the lens that focuses the external world onto our retina. The retina is the sensory receptive part of the eye; visual problems caused by loss of lens flexibility, however, can be corrected by glasses that compensate for the lens. There is no neural tissue involved in this condition. Hearing loss, in contrast, results from damage or loss of the actual hearing receptors. The receptors are small cells in the inner ear that convert mechanical vibrations of sound into neural activity in auditory nerve fibres. When these cells are damaged or die, they are gone forever.
Hearing aid amplification can’t stimulate damaged or missing cells. So, part of the sound environment is lost. The added energy of amplification causes a spread of vibrations along with the inner ear so that adjacent healthy receptors are stimulated, but these receptors represent a different frequency. It would be as if a pinprick on your skin felt like pressure from your index finger—the sensation of touch and its location would be experienced, but not with precise accuracy.
Loud sound and hearing loss – keep it down!
The most common cause of hearing loss is a result of exposure to loud sounds, particularly over an extended period of time. We live in a complex and noisy environment. Consider the volume of personal listening devices, environmental, industrial, and recreational noise such as sirens, trucks, stadium cheers, dance clubs, motorcycles, and even domestic devices such as vacuum cleaners, power tools, coffee grinders, and blenders.
The effects of loud noise are cumulative
Small exposures are acceptable but over time, the summed amount of excess noise energy causes irreparable damage to the inner ear, where the sensory receptors are the most vulnerable. The insidious nature of noise damage means that it normally takes years for us to become aware of it. A loud concert can produce temporary hearing loss and ringing in the ears but these symptoms tend to go away after a few days, and hearing seems normal again. It’s not. The problem is that the process of hearing loss has been initiated and we don’t even know it.
Studies have reported that noise exposure produces transient threshold elevation and no loss or damage to the receptor cells. Nevertheless, it causes the death of up to 50% of the sensory neurons that conduct information from the sensory cells to the brain (4). The death of these cochlear neurons is diffuse and slow and continues for months and years after exposure.
The damage has been done but is not detected by threshold sensitivity
The sound detection threshold is so basic and unchallenging that it is not a useful measure. It is analogous to setting off a photographic flashbulb in front of the eye as a test of visual acuity. The audiology test needs to measure hearing ability and brain responses at different sound levels and under challenging conditions.
It is common for an animal with hearing loss to exhibit normal thresholds but no growth in the response when presented with louder sounds. It is this “stunted” auditory response that is a problem because it demonstrates that the system has but one small response for all volumes. The system has lost the ability to make loudness distinctions.
What is especially worrying is that it isn’t only loud sounds that cause cochlear neuron death. A new study examined the effect of moderate noise exposure: 84 dB for a week (5). This level of noise exposure is within the American guidelines for 8 hr/day exposure for life (http://www.osha.gov) for which there was no measurable hearing loss. Examining the inner ears using a microscope and special stains found loss in up to 20% of the functional connections (synapses) between cochlear neurons and sensory receptors.
National noise exposure standards were established with the assumption that there was full “recovery” of auditory thresholds following noise trauma. Now we can see that such standards are out-of-date, generally incorrect, and therefore quite dangerous.
Our brains pick up the slack
Because there is this relatively long delay between noise exposure and hearing loss, it is generally difficult to appreciate the consequences of noise damage. Our brains are remarkably accommodating.
With diminished signals from the ear, the brain is good at making sense of what it receives or guessing at what it receives when it can’t really hear it. The brain does this by using “context” cues where the conversation subject matter helps limit the possible words it needs to guess.
A discussion of football will use substantially different words compared to a discussion of a dinner menu or a work topic. By narrowing the range of words and guessing verbs, nouns, and adjectives when necessary, communication can be achieved. Body language is also important, lip-reading can come into play, and logic allows one to fill in the missing words.
Hearing tests are not a routine part of a visit to your doctor. The estimated average of 10 years will pass before someone actually gets their hearing tested after first noticing some hearing loss. At the audiologist’s office, your hearing sensitivity test will be over a range of typically 100 to 6,000 cycles per second, or if you’re lucky, 100-8,000 cycles per second (CPS or Hz).
The problem is that our normal audible hearing range is from 20-20,000 Hz. In other words, the tests don’t cover the entire range of hearing or the high-frequency range where hearing loss usually occurs. Ironically, this is because the equipment to measure hearing is not designed to test higher frequencies. So, by the time the audiologist can detect a hearing loss, you’ve lost 60% of your hearing range.
The rationale for incomplete test is that regulatory agencies decided human speech signals can be understood using frequencies below 4,000 Hz (www.safework.sa.gov.au). This standard seems based on the old black Bakelite telephones: the frequency response for the original telephone earpiece was reportedly under 3,000 Hz.
Speech understanding was reasonable so why would we need anything better? The human voice, however, contains frequency components that exceed 30,000 Hz. Perhaps a range up to 4000-8000 Hz is adequate for basic communication without background noise but it represents an impoverished signal that is lacking many of the subtleties of the spoken word – the richness of voice, emotional tone such as excitement or sadness, and localization cues. There is also an old brain function maxim: “use it or lose it.” It means that if we let brain circuits lie dormant, like a neglected piece of equipment, they will eventually disintegrate.
In the brain, congenital deafness and hearing loss cause clear pathology in the structure and function of auditory circuits (6). If we start losing high-frequency hearing and do nothing about it, the 10 years of neglect will undoubtedly cause additional loss. The area damaged will get worse and the healthy areas might become damaged.
In this instance, hearing loss that accompanies aging could be a side effect of other dynamics in play. First, acoustic trauma experienced as a young person initiates processes of cell death that continue indefinitely. Add to this the continued effect of noise exposure. Third, other variables such as antibiotics and anticancer agents contribute to hearing damage. There is a link between aspirin and the onset of tinnitus and hearing loss. As people age, the relative amount of prescribed medication increases. And all the time, there is denial about hearing loss and perhaps an inadequate treatment by hearing aids.
Many people buy hearing aids but don’t use them. Why is this? Hearing aids are not cheap, so purchasing them and not using them is a problem. The common explanation is that hearing aids don’t do what the owner wants. Remember, the idea is that all conversations occur within a 20-4,000 Hz frequency range, although hearing aids can operate in a frequency range up to 10,000 Hz. But the highest frequencies tend not to be amplified because of a practical problem: When patients with hearing loss receive amplification of high-frequency with their hearing aids, they tend to react negatively to the sound. The high frequencies are distressing. Herein lies a conundrum where the needed frequencies, when amplified, are annoying.
There is also a mismatch in expectations. A primary reason for abandoning hearing aids is that the device isn’t nearly as good as Mother Nature’s original ear and the user has unrealistic expectations. The audiologists, perhaps, need to prepare the potential buyer for the realities of life with hearing loss: hearing aids work best in quiet and in one-on-one situations, and the most difficult situation is the most difficult to correct—understanding speech in the presence of other speech. And to get the maximum effectiveness from a hearing aid, it might take from six to eight visits for your audiologist to fit the aid properly.
Hearing aids are not “plug and play”. They need to fit your hearing needs and comfort exactly. And then you’ll need to learn how to use them effectively. Lastly, there will be situations with noise where hearing aids won’t be effective.
Sound and hearing loss – the last word
- Protect your hearing from noise. There are free apps for your smartphone that will measure ambient noise levels (RTA lite, which is basically a sound level meter; and Max dB Exposure Time, which sets time limits based on the measured ambient noise level). It is easier and more economical to protect your hearing rather than try to repair an injured system.
- If you think you have hearing loss, go and get a test. If you’re not hearing, you’re not using that part of your brain; you’ll lose the neurons and circuits you’re not using. Perhaps analogous to muscle atrophy from time in a plaster cast while a fractured bone heals. A hearing aid should boost the signals in that part of your impoverished auditory system and help preserve what is remaining.
- If you purchase a hearing aid, have realistic expectations of what it can do.
- Finally, it is promising that hearing aids are generally improving every year.
- Listen Hear! The economic impact and cost of hearing loss in Australia. 17-May-06. Report from Access Economics.
- Gouskova, E. and E. Stafford (2005). Trends in household wealth dynamics, 2001–2003. Ann Arbor, MI: Institute for Social Research, University of Michigan.
- Lin F.R., K. Yaffe, J Xia J, Q.L. Xue, T.B. Harris, E. Purchase-Helzner, S. Satterfield, H.N. Ayonayon, L. Ferrucci, and E.M. Simonsick (2013). Hearing Loss and Cognitive Decline in Older Adults. JAMA Intern Med. 173(4):293-299.
- Kujawa, S.G. and M.C. Liberman (2009). Adding insult to injury: Nerve degeneration after
“temporary” noise-induced hearing loss. J. Neurosci. 29(45)14077-14085.
- Maison, S.F., H. Usubuchi, and M.C. Liberman 2013 Efferent feedback minimizes cochlear neuropathy from moderate noise exposure. J. Neurosci. 33(13)5542-5552.
- Ryugo, D.K., E.A. Krezmer, and J.K. Niparko (2005) Restoration of auditory nerve synapses by cochlear implants. Science 310:1490-1492.
- Kral, A., J. Tilein, S. Heid, R. Hartmann, and R. Klinke (2004). Postnatal cortical development in congenital auditory deprivation. Cerebral Cortex 15(5):552-562.
- Tirko, N.N., and D.K. Ryugo (2012) Synaptic plasticity in the medial superior olive of hearing, deaf, and cochlear-implanted cats. J. Comp. Neurol. 520:2202-2217.
- Ryugo, D.K., B.T. Rosenbaum, P.J. Kim, J.K. Niparko, and A.A. Saada (1998) Single unit
recordings in the auditory nerve of congenitally deaf white cats: Morphological correlates in the cochlea and cochlear nucleus. J. Comp. Neurol. 397:532-548.
About the author:
David Ryugo earned a bachelor’s degree in psychology from Yale University and a PhD in biological sciences from the University of California. He spent 9 years on the faculty at Harvard Medical School and 23 years at Johns Hopkins University School of Medicine before retiring as Professor Emeritus. He joined Australia’s Garvan Institute as Head of Hearing Research. David’s research focuses on understanding brain mechanisms of hearing and studying the structure-function relationships in neuronal circuits of the auditory system.
This sound and hearing loss article originally appeared in Hearing HQ Magazine Dec 14.