SOUND
The 'olivocochlear' hearing control reflex links the cochlea of each ear via the brain's auditory control centre to help discriminate between noise and soundREUTERS

A landmark study that unveils the biological process of how the brain balances the hearing between two ears to localise sound and hear in noisy conditions could help improvise cochlear implants and hearing aids.

University of New South Wales researchers have discovered the crucial role played by a group of auditory nerve fibres in the hearing process.

The "olivocochlear" hearing control reflex links the cochlea of each ear via the brain's auditory control centre to help discriminate between noise and sound.

When sound intensity increases, the olivocochlear reflex turns down the "cochlear amplifier" to balance the input of each ear for optimal hearing and to protect hearing.

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

The study in animals found that the cochlear's outer hair cells also provide the sensory signal to the brain for dynamic feedback control of sound amplification, 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.

Similarly, the olivocochlear reflex normally causes a rapid reduction in hearing in the ear receiving an increase in sound. This hearing adaptation was also absent in the mice lacking the sensory fibre connection.

Age-related hearing loss

Some of the age-related hearing loss in humans may be related to the gradual breakdown of this sensory fibre connection to the outer hair cells.

Cochlear implants that can communicate between the two ears to enable the most accurate sound to the brain as shown in the study will be the next step.

Around 1.1 billion teenagers and young adults are at risk of hearing loss due to over-exposure to audio devices, including smartphones, besides the damaging levels of sound at noisy entertainment venues such as nightclubs, bars and sporting events, according to WHO.

Data showed that among teenagers and young adults aged 12-35 years, nearly 50% are exposed to unsafe levels of sound from the use of personal audio devices and around 40% are exposed to potentially damaging levels of sound at entertainment venues.

WHO recommends safe levels of sounds as exposure below 85 decibels (dB) for eight hours or 100dB for 15 minutes.