New study sheds light on role of cochlear signaling in hearing loss

A cochlear signal, whose exact role has been unclear since its discovery around 70 years ago, probably gives the brain information about whether or not the ear is working normally. This is the conclusion of a study from the University of Linköping, Sweden. His findings are an important piece of the puzzle in explaining what happens in the ear in hearing loss caused by harmful noise, and may in the long term help in the diagnosis of noise-induced hearing damage.

When the ear is exposed to loud sounds, such as at a concert or in a noisy environment, hearing may be temporarily impaired. Repeated exposure to loud sounds can cause permanent hearing damage. Research indicates that over a billion young people are at risk of damaging their hearing by listening to loud music through headphones and in venues. But although noise damage is a major cause of hearing loss, the exact mechanisms are largely unclear. Pierre Hakizimana of Linköping University is one of the researchers looking 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 struck by sound waves, hair cells transform the vibrations into electrical nerve signals. These signals are directed 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. The AC signal is well studied. It gives the brain information about the intensity and frequency of the sound, i.e. the pitch or gravity of the sound. But the DC signal remained a mystery. Since its discovery around 70 years ago, researchers have wondered what its function might be.

When measuring electrical signals from the hair cells of the cochlea, the DC signal is noticeable because it causes the AC signal to shift slightly in a positive or negative direction. Various studies trying to characterize the DC signal have come to different conclusions as to its polarity. In the present study, Pierre Hakizimana shows that the polarity of the DC signal changes from positive to negative when the cochlea has been exposed to harmful noise. In other words, the signal can provide an indication of the health status of the ear.

It seems that this signal could be a way for the body to inform the brain whether the ear is healthy or not, and thus facilitate the brain’s ability to decode quiet sounds. The brain can amplify a weak signal from the cochlea. If told that the ear is not functioning normally, the brain doesn’t have to expend resources trying to improve the signal to decode sound from an injured ear.”

Pierre Hakizimana, Senior Research Engineer, Department of Biomedical and Clinical Sciences, Linköping University

This finding could hopefully contribute to further research into how the DC signal could be used to diagnose hearing loss caused by harmful noise. This has so far remained unresolved, as it is unclear how to interpret this signal, or how to reliably isolate and measure it in humans.
In his study, Pierre Hakizimana also shows that the DC signal is created by potassium ion channels releasing potassium ions through the membranes of hair cells.

The research was funded by Stiftelsen Tysta Skolan.