How the Brain Hears Between Our Ears
University of New South Wales (UNSW) researchers have answered the enduring question of how the brain processes hearing between our ears, which is essential for localizing sound, hearing in noisy conditions and protecting us from noise damage. This study should get many hearing specialists excited.
The landmark animal study also provides new insight into hearing loss and is likely to improve cochlear implants and hearing aids. A cochlear implant is a surgically implanted electronic hearing device that helps amplify sound for a profoundly deaf person or for someone with severe hearing.
The reason for cochlear implants is that they provide hearing to nearly deaf persons who have damaged sensory hair cells in their cochleas. In these patients, the implants provide enough hearing in order to understand speech. However, the quality of sound differs from natural hearing, because the brain receives less natural sound information to interpret it. Yet the implants offer enough for the patient to hear and understand speech and environmental sounds.
UNSW Professor Gary Housley, senior author of the research paper, said his team’s primary aim was to understand the biological process behind the ‘olivocochlear’ hearing control reflex. The professor added, “The balance of hearing between the ears and how we discriminate between sounds versus noise is dependent upon this neural reflex that links the cochlea of each ear via the brain’s auditory control centre.
The key to this finding is that the researchers were able to fully understand how the olivocochlear reflex works. Professor Housley stated, “When sound intensity increases, the olivocochlear reflex turns down the ‘cochlear amplifier’ to dynamically balance the input of each ear for optimal hearing, sound localisation and to protect hearing.”
Another important aspect of this study found that the cochlear’s outer hair cells, which amplify sound vibrations, also provide the sensory signal to the brain for dynamic feedback control of sound amplification, through a small group of auditory nerve/sensory fibers that were previously misunderstood.
The researchers, using mice, discovered how the olivocochlear reflex in each ear communicates with the cochlear auditory sensory fibers, and this feedback loop allows the brain to balance sound in the environment.
Professor’s Housley’s team further speculated that hearing loss humans experience from old age may be related to the gradual breakdown of these sensory fibers connected to the outer hair cells.
“A major limitation of hearing aids and cochlear implants is their inability to work in tandem and support good hearing in noisy conditions,” Professor Housley also added. “The ultimate goal is for cochlear implants in both ears to communicate with each other so that the brain can receive the most accurate soundscape possible.” This research helps hearing specialists improve cochlear implant devices so that we can create a more natural and effective learning device for those with hearing loss.