The Code of Hearing Deciphered

New findings reveal how our sense of hearing works

Our sense of hearing continues to baffle scientists. Researchers from New York and Göttingen have demonstrated a previously undiscovered division of labor and efficiency in the brain.

Our sense of hearing performs precision work around the clock, and even today, the exact details of how the hearing process works have not yet been fully understood. We know that sound travels like a chain reaction from the outer ear to the auditory nerve. Our outer ear acts as a funnel that captures sound waves and directs them to the eardrum. This thin membrane transmits the vibrations to the ossicles—the malleus, incus, and stapes. At these smallest and most delicate bones in the human body, the sound is amplified so that it can travel through the fluid-filled coiled passages of the cochlea to the so-called basilar membrane. This is where the sensitive hair cells are located, which convert acoustic signals into electrical impulses. The auditory nerve transmits these nerve impulses to the auditory center of the brain, which decodes the stimuli.

At a Glance

• The outer ear is the part of our auditory system that opens outward to capture noises, sounds, and speech, much like a sound funnel. The unique ridges and grooves of this elastic cartilaginous structure act as acoustic resonators that vibrate.
• These vibrations are transmitted through the external auditory canal to the eardrum.
• The outer ear also includes the auricular cartilage and the earlobe.
• The eardrum and the ossicles—the malleus, incus, and stapes—are part of the middle ear.
• The middle ear is connected to the inner ear via the round window. The inner ear’s main components are the cochlea and the vestibular system.

How the Two Halves of Our Brain Work Together When We Hear

But how does our brain decode the signal it receives from the auditory nerve? A group of scientists at New York University’s School of Medicine investigated this question. The neurologists discovered that our two brain hemispheres share the task of auditory processing. Clinical observations from the 19th century had already shown that damage to the left hemisphere affects language processing. If, on the other hand, only the right hemisphere of the brain is impaired, this has no negative effects on hearing and understanding words. Since then, various hypotheses have been proposed regarding how the right and left hemispheres divide up speech and language comprehension. An American research team sought to clarify the exact processes occurring in our brains—with the goal of developing new treatment options for speech disorders.

The left hemisphere processes slow and fast speech, while the right processes high- and low-pitched speech

To this end, the scientists manipulated speech recordings and measured brain activity in a series of studies. The result: Both hemispheres perform different, simultaneous tasks while listening. While the left hemisphere measures how sounds change over time—for example, when speech is slower or faster—the right hemisphere focuses on changes in pitch. This not only provides entirely new insights into the fundamental mechanisms of speech comprehension but also points to new ways of treating hearing disorders.

How Acoustic Signals Enter the Nervous System

Scientists at the Universityof Göttingen, led by Prof. Tobias Moser, have also investigated how acoustic signals are translated into a nerve signal—so that they can reach the brain in the first place. In doing so, they not only refuted previous assumptions but also demonstrated the unique efficiency of our sense of hearing. However, this was only possible because the scientists further refined a particularly precise measurement method—known as nanophysiology—through a complex process. Using this extremely complex measurement method, the researchers were able to measure the smallest functional units of the cells in the inner ear. In doing so, they made a groundbreaking discovery and exposed our previous understanding of hearing as incorrect.

Glutamate connects auditory cells to one another

The conversion of the acoustic signal into a nerve impulse occurs between the hair cells and the auditory nerve cells in the inner ear. At the synapses connecting these nerve cells, microscopic vesicles are released. These so-called synaptic vesicles are filled with the neurotransmitter glutamate and carry information from one nerve to another. Until now, scientists assumed that several of these vesicles were released during each hearing event. However, the latest measurements by the Göttingen scientists have shown that, in the presence of moderate sound, a single vesicle is sufficient to convert acoustic information into a nerve signal. In doing so, the researchers have decoded the fundamental data packet of our sense of hearing and have taken a major step closer to understanding how we hear. At the same time, they have demonstrated that our hearing works extremely efficiently.

Unique Efficiency of Our Hearing Demonstrated

This efficiency, which the Göttingen researchers say is unique in nature, does, however, come at a price: In order for the auditory nerve cells to utilize the release of the neurotransmitter glutamate so efficiently, they must be extremely sensitive. If too much of the neurotransmitter is released, the sensitive cells can suffer irreparable damage. This would be the case if we expose our ears to excessively loud sound pressure. Extreme noise can destroy the sensitive hair cells through glutamate overexcitation, preventing them from converting acoustic signals into nerve impulses and transmitting them to the auditory nerve. Those affected notice this as a decline in hearing ability. Therefore, on the one hand, it is advisable to protect our ears from noise or avoid exposing them to constant noise in the first place—for example, by not listening to music at excessively high volumes through headphones. And second, we should have our hearing tested regularly by a hearing care professional so that we can counteract any decline in our hearing ability directly by wearing hearing aids.

How Artificial Intelligence Helps with Speech Understanding

New scientific findings illustrate that the ear and the brain work together to achieve true feats of mastery. The new technical capabilities of artificial intelligence (AI) can support this unique interaction in many ways for people with hearing loss. For example, Morten Kolbæk at the Center for Acoustic Signal Processing(CASPR) at Aalborg University in Denmark has developed analgorithm that helps hearing aid users understand conversations perfectly in noisy environments. In this case, the AI learns from several thousand examples of voice recordings—with and without background noise—how to clearly isolate a voice from a noisy background. This algorithm mimics the performance of the interaction between the brain and the ear. It enables hearing aid users to better cope with acoustically challenging situations.

There are already hearing aids that use AI to adjust the hearing settings and microphone control to the specific listening situation. Whether the wearer is standing in a crowded train station trying to understand a public address announcement or sitting at home in a quiet living room, the hearing aids automatically adjust to ensure optimal hearing. In the process, the AI learns to correctly assess new situations by analyzing various noise profiles.

Can you hear and understand speech clearly even in noisy environments? Find out by taking a quick online hearing test right here. You’ll get an immediate indication of how well you can hear.