The brain may play a crucial role in regulating sound sensitivity within the ear and compensating for hearing loss by sending signals to structures like the cochlea. This discovery has been documented in a recent study published in Journal of Neuroscience, which could significantly impact efforts to treat complex auditory disorders such as hyperacusis, where everyday sounds seem uncomfortably loud, and tinnitus—a sensation characterized by ringing, buzzing or other sound in the ear without an external source.
The breakthrough findings are facilitated by a novel imaging tool that enables scientists to capture images of the cochlea in awake animals for the first time. The cochlea operates using sensory hair cells to detect air-borne sound waves, converting them into electrical signals that can be processed by the brain. Typically, most connections from the cochlear nerves carry information from the inner ear to the brain; however, approximately 5% of these fibers transmit signals in reverse—from the brain back to the cochlea—a phenomenon whose exact function has been enigmatic due to difficulties in measuring cochlear activity while subjects are conscious.
To address this gap, researchers at Keck School of Medicine and Baylor College of Medicine adapted an imaging technique called optical coherence tomography (OCT), commonly utilized in ophthalmology for scanning retinas. OCT employs light waves to produce three-dimensional images, similar to ultrasound’s use of sound waves. This adaptation allowed the team to capture real-time imagery of cochlear activity without causing discomfort or pain.
“What excites us is that this technique lets us observe how the brain controls the cochlea in real time,” stated Dr. John Oghalai, a professor at Keck School of Medicine and a specialist in otolaryngology-head and neck surgery.
Through their investigations with OCT, researchers discovered that in healthy mice, there was no change in cochlear activity over short periods. However, this pattern shifted in genetically hearing-impaired mice where increased cochlear function indicated an enhancement by the brain to compensate for long-term loss of hearing sensitivity.
A prevailing theory is that efferent fibers (those transmitting signals from the brain back to the inner ear) manage how quickly and intensely we perceive sounds. Using OCT, scientists also monitored changes in pupil size as a proxy for varying mental states within mice while simultaneously assessing cochlear activity. This study showed no alteration in cochlear function when mental state changed, suggesting that short-term auditory modulation is not regulated by these pathways.
In another set of experiments, researchers genetically disabled the nerves transmitting information from the inner ear to the brain (afferent fibers), triggering hearing loss. Utilizing OCT again, they observed increased activity within the cochlea as a compensatory mechanism.
“As people age and hair cells deteriorate, their ability to hear diminishes. These findings suggest that signals sent by the brain could potentially boost functionality in remaining hair cells,” explained Dr. Oghalai, adding that this discovery might pave the way for new treatments targeting efferent fibers as a method to manage hyperacusis.
Moreover, OCT’s diagnostic potential is also highlighted in these findings. With adaptations made for use on human subjects and backed by funding from the National Institutes of Health (NIH), researchers are working towards clinical trials that could revolutionize how hearing disorders are diagnosed and treated.
“Our research marks a critical step toward developing tools capable of directly examining an individual’s ear, identifying underlying issues, and delivering personalized treatment,” concluded Dr. Oghalai, indicating the transformative potential this technology holds for future auditory healthcare practices.
