How Hearing Works
The way in which our ears screen out sounds so that we can listen selectively only to the noises we want to hear - like a friend's voice across the hubbub of a crowded room - has been revealed by scientists in Australia.
The inner ear, also called the cochlea, is the site where sound waves are converted into brain waves by a population of tiny hair cells that generate nerve impulses as they vibrate in time with the sounds.
Different groups of hair cells are sensitive to different sound frequencies, but how the cochlea is able to tune itself to supress unwanted sounds while amplifying the desired frequencies we want to listen to, was a mystery.
Now, working on mice, Gary Housley, from the University of New South Wales, has unpicked the circuitry that makes this selective hearing possible.
Alongside the inner hair cells, which send sound signals to the brain, are a second population of larger and more numerous outer hair cells. These are responsible for turning up, or down, the sensitivity of the cochlea.
Feeding back through the brainstem, the UNSW team have found that when they are activated these outer hair cells trigger the acoustic equivalent of a knee-jerk reflex, which alters the activity of the outer hair cells along the cochlea and turning down its sensitivity to sounds.
Importantly, the reflex response is sent to both ears, and this has a signficant effect on the way in which sounds are perceived across the head, particularly from the perspective of people using hearing aids.
"Just amplifying sounds indiscriminately and playing them into both ears at the same time is not sympathetic to the way that the brain is wired," explains Housley. "Our findings mean that, in future, hearing aids can incorporate this effect, which could significantly improve the user experience."