How the ear can inform the brain when hearing is impaired

Cochlear signals, whose exact role has been unclear since their discovery some 70 years ago, may provide information to the brain whether the ear is functioning normally or not. This is the conclusion of a study from Linköping University, Sweden.

Her findings are an important piece of the puzzle in explaining what happens in the ear in noxious noise-induced hearing loss, and in the long term may contribute to diagnosing noise-induced hearing injury. This paper is published in the journal Cellular and Molecular Life Sciences.

When the ear is exposed to loud noises, such as at a concert or when in a noisy environment, hearing can be temporarily impaired. Repeated exposure to loud noises can cause permanent hearing damage.

There are studies showing that more than one billion young people risk damaging their hearing by listening to loud music with headphones and in public places. But although noise damage is a leading cause of hearing loss, the exact mechanism is largely unclear. Pierre Hakizimana at Linköping University is one of the researchers whose aim is to find out how this damage occurs and whether it can be prevented.

The inner ear, or cochlea, has about 15,000 hair cells. When exposed to sound waves, hair cells convert the vibrations into electrical nerve signals. These signals are carried to the brain, which interprets them, and only then can we hear the sound.

The hair cell signal consists of two parts, called AC and DC. AC signal is well researched. This gives the brain information about the loudness and frequency of the sound, i.e. how high or low the pitch of the sound is. But the DC signal remains a mystery. Since its discovery some 70 years ago, researchers have wondered what it does.

When measuring the electrical signal from the cochlear hair cells, the DC signal is visible because it causes a slight shift in the AC signal in either the positive or negative direction. Various studies trying to characterize DC signals have come to different conclusions regarding their polarity. In the current study, Pierre Hakizimana demonstrated that the polarity of the DC signal changes from positive to negative when the cochlea is exposed to noxious noise. In other words, these signals can provide an indication of ear health status.

“It appears that this signal could be a way for the body to tell the brain whether the ear is healthy or not, and in that way facilitate the brain’s ability to decode faint sounds. The brain can amplify weak signals from the cochlea. If it is informed that the ear is not functioning normally, the brain does not need to spend resources trying to boost the signal to decode the sound from the injured ear,” said Pierre Hakizimana, principal research engineer in the Department of Biomedical and Clinical Sciences at Linköping University.

It is hoped that this discovery will contribute to new research into how DC signals can be used to diagnose hearing loss caused by noxious noise. So far unsolved, because it is not yet known how to interpret these signals, or how to isolate and reliably measure them in humans.

In his study, Pierre Hakizimana also showed that DC signals are created by potassium ion channels that release potassium ions through the hair cell membrane.