What happens when our hearing is damaged? For decades, scientists believed that it was caused by sound waves destroying the delicate hair cells, or cochlear nerve fibers, which line the interior of the inner ear. These tiny hair cells act as fine sensors, converting vibrations into electrical signals which can be interpreted by the auditory cortex in our brains. When the hair cells get damaged by especially loud sounds, it compromises our ability to hear. This mechanism of hearing loss has been well-documented and can be measured using an audiogram.
What is an audiogram?
This is a test designed to assess a person’s hearing threshold. When hearing damage occurs, it makes it more difficult for people to distinguish especially quiet sounds. As the damage becomes more pronounced, this noise threshold becomes higher. Likewise, an audiogram can be used to get approximate the degree to which a person’s hearing has been damaged.
During an audiogram, sounds are played at different pitches and frequencies in progressively lower volumes. The person conducting the test will measure the responses of each ear to determine the severity of the test subject’s hearing loss.
But what if there was another type of hearing damage that couldn’t be measured with an audiogram?
Scientists call this type of hearing loss “hidden hearing loss.”
Audiologists had longed suspected that there might be another mechanism that caused hearing loss in addition to hair cell damage. This kind of hearing loss might not directly impact a person’s volume thresholds, but could nonetheless make it difficult for them to do things like distinguish speech from background noise. In 2014, a group of researchers at working at Harvard Medical School’s Eaton-Peabody Laboratory confirmed these suspicions.
Cochlear nerve fibers are just one component of the neurons that convert sound vibrations to electronic signals for our brains. They are also the easiest part of the neuron to observe, so it makes sense that we would look for damage in these nerve fibers as indicators of hearing loss. Unfortunately, however, there’s another part of the neuron that’s much harder to observe and even more vulnerable to damage from loud noises – the synapse that connects the cochlear nerve fibers to the nerve terminal. These synapses are vital, in that they facilitate the interface between the ear and the brain. Without them, sound signals can’t reach the brain’s auditory cortex.
So why is this type of hearing loss so difficult to detect and measure?
The problem is twofold, according to Charles Liberman, director of the Eaton-Peabody Laboratory. “First, the field of auditory neuroscience didn’t appreciate until recently that you can lose up to 90 percent of your cochlear nerve fibers without a change in the ability to detect a tone in quiet,” he said in a press release. “Tone detection in quiet is the basis of the threshold audiogram – the gold standard test of hearing function. The fact that thresholds may transiently elevate and then recover within hours or days after an acoustic overexposure doesn’t mean that the inner ear has recovered.”
Alright, so to begin with, we’ve effectively been using a broken thermometer to measure hearing loss. Threshold audiograms can only give us a fleeting glimpse of the true extent of a person’s hearing loss. The second problem is that until recently, we could only observe nerve synapses using an electronic microscope. To view and count the synapses with a conventional light microscope, Liberman’s team had to strategically stain them with colored antibodies that target molecular structures within the synapse. By counting the number of missing synapses, the researchers were able to get a much more complete picture of a person’s hearing loss.
“Each missing synapse represents a cochlear nerve fiber that has been disconnected due to a retraction of the terminal segment – it will never reconnect, said Liberman. “It no longer responds to sound, and, within a few months or years, the rest of the neuron will disappear.”
How can you tell if you have hidden hearing loss?
Tinnitus, or ringing in the ears, is a common indicator of hearing damage. Even if you haven’t noticed a change in your hearing thresholds, persistent tinnitus could mean that you’ve experienced some form of hearing loss. Difficulty understanding speech is another tell-tale sign of hearing loss. If you find yourself having to read lips to follow conversations, your hearing has likely been compromised to some degree.
Even if you have suffered some hearing loss, it’s never too late to prevent further damage to your ears. By being mindful of your surroundings and wearing electronic hearing protection in loud environments, you can preserve those vital synapses and protect your hearing for the future.