How brain balances hearing between the ears

Researchers have answered the long-standing question of how the brain balances hearing between the ears. The balance is essential for localising sound, hearing in noisy conditions and for protection from noise damage.

A team from the University of New South Wales (UNSW) sought to understand the biological process behind the 'olivocochlear' hearing control reflex.

"The balance of hearing between the ears is dependent upon this neural reflex that links the cochlea of each ear via the brain's auditory control centre," explained senior researcher Gary Housley.

Our hearing is so sensitive that we can hear a pin drop and that is because of the "cochlear amplifier" in our inner ear. This stems from outer hair cells in the cochlea which amplify sound vibrations.

In animal experiments, the team found that the cochlear's outer hair cells, which amplify sound vibrations, also provide the sensory signal to the brain via a small group of auditory nerve fibres of previously unknown function.

In mice lacking the sensory fibre connection to the cochlear outer hair cells, loud sound presented to one ear had no effect on hearing sensitivity in the other ear.

In normal mice, this produced an almost instant suppression of hearing. The researchers speculate that some of the hearing loss that humans experience as they age may be related to the gradual breakdown of this sensory fibre connection to the outer hair cells.

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 will help us move closer to that goal," the authors concluded in the paper published in the journal Nature Communications.