If some of the speakers in your home audio system are broken, turning up their volume to compensate might make it last longer. It turns out that when hair cells in the ear are damaged, causing hearing loss, the brain does the same thing, which may be why tinnitus occurs.

Sensory hair cells are tiny structures in the cochlea that sway like blades of grass in the wind, but in this case it's the pressure of sound waves that sets them in motion. When they move, they generate electrical signals that are transmitted through nerve fibers to the brain, where they process the sounds you hear.

But in fact, a small number of these nerves run in the opposite direction, from the brain to the cochlea. Scientists have long puzzled over the function of these backchannels, and their activity has been difficult to study when people or animals are awake.

In the new study, scientists at the University of Southern California (USC) used an interesting imaging tool to see what exactly happens during this process. The technology, called optical coherence tomography (OCT), uses light waves to create three-dimensional images of tissue. The technology is currently used to scan the retina to diagnose conditions such as glaucoma, but the team is applying it to the ear.

John Oghalai, lead author of the study, said: "OCT allows us to follow the ear canal, through the eardrum and bone into the cochlea, and measure how the cochlea is working - non-invasively and painlessly. What's exciting is that this allows us to study in real time how the brain controls the cochlea."

Researchers genetically engineered mice to suffer from hearing loss by disabling part of the nerves in the mice's ears that carry signals to the brain. They then used OCT to monitor the cochlea's activity and found that the cochlea was working harder than usual.

Using this tool, Ogalay and his teamfound that cochlear activity in healthy mice did not change in the short term. But in mice with inherited hearing loss, cochlear function was enhanced, suggesting that the brain is increasing the sensitivity of the cochlea in response to long-term hearing loss.

"As humans age and hair cells die, we begin to lose hearing," O'Gale said. "These findings suggest that the brain can send signals to remaining hair cells, essentially telling them to turn up the volume."

One leading theory about the nerves that send signals from the brain to the cochlea (called "efferent" fibers) is that they control the cochlea's response to sound in the short term, similar to how our pupils work. Bright light causes the pupils to constrict, while stress dilates the pupils. Does the cochlea have a similar role?To explore whether the cochlea responds to short-term stimulation, the researchers used OCT to measure cochlear activity in mice. At the same time, they tracked changes in the mice's brain states by measuring changes in pupil size. Cochlear activity remained constant as brain states changed, suggesting that the inner ear does not regulate hearing in the short term.Next, the researchers altered the mice's genes to disable the nerves ("afferent" fibers) that carry information from the inner ear to the brain, causing hearing loss. Using OCT, they found that the cochlea is working overtime to compensate."As we age and hair cells die, we begin to lose our hearing."These findings suggest that the brain can send signals to remaining hair cells telling them to turn up the volume," said Ogalay, professor of biomedical engineering at the USC Viterbi School of Engineering."The next step is to conduct clinical trials to test drugs that block efferent fibers, which could reduce sound levels in people with hearing loss and may also help with tinnitus.

While this mechanism may help compensate for hearing loss, the team believes it may have unwanted side effects: it may lead to conditions such as tinnitus. The brain's regulation of the volume in the cochlea may produce the annoying ringing sound associated with tinnitus, like the hissing sound heard when the speaker volume is turned up too loud without any music playing.

On the positive side, the research team now plans to test drugs that block these backward nerve fibers as a potential treatment for tinnitus and related conditions such as hyperacusis.Now that Oghalai's team has used OCT to image the cochlea in awake mice, they are testing a version of the tool on patients in a new study funded by the National Institutes of Health.

The technology could eventually allow healthcare providers to diagnose hearing problems based on physiology, rather than just performance on a hearing test, and tailor treatment to individual needs."This is the first step toward tooling that will allow us to look at a patient's ear, identify the problem and treat it," Ogalai said.

The study was published in the Journal of Neuroscience.