The crash of a wave on the shore, a beautiful piece of music or your child’s laughter are sounds that many of us take for granted. But for the one in five Americans with varying degrees of hearing impairment, simply having a conversation in a noisy restaurant can be challenging.
The whys of hearing loss have been understood for some time. For example, being too close to a loud blast can damage the eardrum, but usually that sort of injury can heal itself.
But the cumulative low-level damage that many of us sustain on a day-to-day basis eventually gives way to what’s known as sensorineural hearing loss, which has been most prevalent in the elderly.
It comes about when the hair cells or cilia of the inner ear’s spiral-shaped cochlea become damaged from years of constant vibration. Those vibrations are translated into electric signals that are sent to the brain through the hearing nerve.
The brain then tells you that you are hearing a sound and what that sound is. It’s a complex labyrinth of finely tuned working parts. If one part doesn’t work, hearing can be impaired or completely lost. The brain compensates in many ways, but just what those ways are is still a mystery.
Tulane University assistant professor of neuroscience Hai Huang has been given a $1.8 million grant to find out.
“Let’s take tinnitus, for example, the syndrome which causes ringing in the ears,“ said Huang. “It happens after age-related hearing loss, ear injuries or even a circulatory system disorder. But when the brain doesn’t get its usual input, it tries to compensate by amplifying whatever sound is there, and this can result in an overexcited neuron giving too much feedback — causing ringing, hissing, clicking or buzzing in one’s ear.”
That’s why Huang is outfitting mice with tiny headphones: to understand just how the brain’s internal attempt to treat hearing loss.
In the lab, Huang works work with both hearing and hearing-impaired mice to see what happens in their brains when they lose their ability to hear. “Electrodes placed on their heads near their ears give us feedback,” he said.
Solving hearing loss has become more important than ever.
For centuries, the most common type of hearing loss has been geriatric, but with a current crop of baby boomers to millennials all tethered to their headphones — often at excessive volumes — hearing loss is very much on the rise.
Currently, one in 20 people has some degree of loss. By 2050, it’s expected that number will rise to one in 10. Damaged hair fibers in our inner ears don’t regenerate, and medical science can only do so much to restore hearing.
“When you lose your hearing through injury or age, you also lose your ability to understand speech,” said Ashley Brewer, coordinator of audiological services at the New Orleans Speech and Hearing Center.
“As a clinical audiologist, I test people to figure out not only what they hear but what they understand. Since hearing loss most often occurs in the higher frequencies, and consonants are primarily in those high ranges, those are the sounds people can no longer hear.”
Consonants, rather than vowels in the lower frequency range, are the sounds that give meanings to words, she said.
“It’s called a discrimination score, and those with a 70 percent score, for example, will not do much better than that in discerning speech with a hearing aid because that’s all their auditory nerve can process,” she said.
Amplifying sound with a hearing aid will allow one to hear a ringing telephone or a doorbell, but without the ability to understand speech, hearing loss can be very isolating.
Professor Huang’s goal is to better understand what’s happening in the inner workings of the brain so that future treatments will be more effective.
“We’re looking at how the large junctions between the two nerve cells in our brains, called synapses, actually release and detect neurotransmitters,” said Huang.
“This is the system which passes electrical signals and is largely responsible for the interpretation of sound. This is very important for neuroscience, in general, to find out just how neurons talk to each other. It’s applicable to so many other fields of science and medicine, and ultimately will help so many people.
“It’s a long journey to find out the specifics of the brain, but we’re making a lot of headway.”
And, with a lab full of mice busy listening to sounds through their miniature headphones, science is one step closer to solving the riddle of hearing loss.
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