Body and mind

New discovery: how the ear deciphers speech

New research shows how the inner ear transforms acoustic pressure waves and frequencies so that the brain can understand them as speech. The discovery paves the way for new diagnostic opportunities, which may also eventually help people to restore lost hearing.

A new study by researchers from Sweden, Denmark, the United States and India shows what happens when the inner ear transforms the multitude of frequencies and pressure waves in speech so that the brain can understand it and people can thereby understand what is being said.

The research shows that the hair cells in the inner ear actually distort the sound so that the brain can perceive it through meaningful electrical signals.

The discovery may pave the way for new diagnostic methods to identify the causes of hearing loss. Over time, the discovery may also be used to develop new treatments that can restore lost hearing without using hearing aids or implants.

“Today, we can measure hearing loss, but it is often difficult to tell whether the problem is in the sensory cells, the neurons or other cell types in the inner ear that support the function of the sensory cells. If we do not know what causes hearing loss, more effective treatments are difficult to find. Our discovery enables new diagnostic methods to determine the exact causes of hearing loss and thereby to start to develop new treatments,” says Anders Fridberger, Professor and Head, Department of Clinical and Experimental Medicine, Linköping University.

The research results, which also include contributions from researchers from Danish companies Oticon and Interacoustics, were recently published in Nature Communications.

The brain uses electrical signals to understand speech

Understanding the new research results requires understanding what speech is.

Speech is sound that enters the ear as acoustic frequencies and pressure waves. Speech has a physical signature that can be observed, for example, by speaking into a microphone and recording the sound on a computer. The sound is visualized as numerous lines that rapidly rise and fall – similar to measuring earthquakes with a seismograph.

All the frequencies and the rapid changes in the computerized signature of the speech are called the fine structure of the sound, and researchers call the shape of these lines in their overall structure the acoustic envelope. The ear transforms this envelope into understandable signals that the brain can perceive as speech.

“We have known that the brain uses the acoustic envelope to understand what people are saying, but we have not known how the inner ear extracts information about the envelope,” explains Anders Fridberger.

Cells converting sound into electrical signals

In the new study, the researchers conducted several studies on both mice and people, inserting electrodes into the inner ear to determine how various cells in the ear respond to speech.

The inner ear has many parts, including the cochlea, which contains very sensitive auditory hair cells. Sound entering the ear sets these hair cells in motion.

The results of the studies show that the inner ear distorts the sound before the brain processes it. One might think that distorting sound would be undesirable, but the results show that the ear must transform acoustic pressure waves and frequencies into electrical signals that the brain can understand.

The research also shows that the inner hair cells decode the acoustic envelope.

This discovery suggests that speech initiates a unique type of electrical signal in the brain that other sounds cannot.

Each hair cell has a sensitive ion channel that opens and closes. Speech activates these ion channels, the hair cells distort the sound and transform it into electrical signals that are transferred to the brain, which then deciphers the electrical signals as speech,” explains Anders Fridberger.

Improving diagnosis of hearing loss

The discovery is an important contribution to understanding how the inner ear works.

The inner ear is encased in thick bone, making access difficult when studying injuries that may explain hearing loss.

However, this can start to change now.

“We believe our results will improve the diagnostic procedures for various types of hearing loss, and this is really needed. The immediate applications have not yet been developed, but we hope we can apply the discovery in practice soon,” says Anders Fridberger.

Another researcher behind the study, Thomas Lunner, Senior Researcher at Oticon, says that the discovery can also improve how hearing aids are calibrated.

“So far, it has only been possible to investigate the state of the outer hair cells, such as in screening newborns. Our research can help to create the first method for diagnosing the state of the inner hair cells, which can potentially help us make better individualized hearing aids,” says Thomas Lunner.

A mechanoelectrical mechanism for detection of sound envelopes in the hearing organ” has been published in Nature Communications. In 2015, the Novo Nordisk Foundation awarded a grant to Anders Fridberger for the project Clinical Testing of a New Strategy for Treating Hearing Loss.

Anders Fridberger
The sensory cells in the ear are amazing creatures capable of detecting sound-evoked motions smaller than a billionth of a meter. I am a researcher working to understand how this is possible, and trying to find ways of combating hearing loss. As a medical student, I started a research project in the lab of Åke Flock and Mats Ulfendahl at Karolinska Institutet. When seeing the organ of Corti through an operating microscope, I was struck by the beauty of the almost perfect spiral of glittering Hensen cells and sensory cells, whose function at the time was very poorly known. The inner ear continues to fascinate me and I still want to know how it works.