The brain recognizes the shape of the ears from sound reflections
It is not difficult for people with intact hearing to locate sounds, to differentiate between several speakers and to hide certain sounds or to focus attention on them. This is made possible in particular by the shape of the ears. The shape of the ears reflects the sound in the inner ear in a very special way, which allows the brain to draw the appropriate conclusions and to calculate the position of the sound in the room.
Exactly how this process works was previously unclear. Now neuroscientists from the University of Leipzig and the University of Montreal have deciphered which processes take place in the brain. These findings could be used to develop better hearing aids, as the researchers announced in a press release from the University of Leipzig.
Big or small - the shape of your ears is crucial
In their study, the Leipzig research team found out to what extent the shape of the ears and their beads influences how sound waves are transmitted to the inner ear. These reflection patterns are analyzed by the brain, which can be used, for example, to identify whether a sound is coming from above, below, left or right.
Sounds from above and below are more difficult to locate
In particular, how the sounds coming from above or below are located by the brain has so far been unclear. "Sounds from different directions hit the outer areas of our ears differently," explains Marc Schönwiesner, professor at the Institute of Biology at the University of Leipzig. The irregular shape of the auricle reflects the sound in the ear canal. This creates a short echo that changes the timbre.
The brain learns based on small differences
"Our brain can learn these small differences and associate them with different directions," says Schönwiesner. The timbre is the property of a tone, which is determined by the volume of the individual frequencies contained in the tone. It is the reason why the same note, such as a high C, sounds different from a violin than from a recorder.
Course of the study
In the study, the ear shape was changed in 15 subjects with silicone inserts. These should then locate tones in a sound laboratory, both with and without silicone in the auricle. Although the tones always remained the same, there were clear differences in the tests. With the respective natural ear shape, the tones could be located very precisely, whereas the test subjects with the silicone pieces used hardly succeeded.
Without the natural shape of the ear, the sound could not be located
“When we played a tone above their heads, they suddenly believed that it came from below,” reports Schönwiesner. In subsequent tests without the use of silicone, the test subjects were able to continue their previous listening efforts.
In further test series, tones were played to the test persons while they were in the magnetic resonance tomograph (MRT). This allowed the scientists to observe what is going on in the brain while the sound is being processed. The focus of the neuroscientists was on the activities in the hearing cortex, the area of the cerebral cortex that is responsible for hearing.
The higher the source of the sound, the worse the location
The scientists found that the higher the source of a sound is above the head, the less active the neurons are. The researchers could even read the source of the sound from the evaluation of the brain waves. The MRI also showed that changes in the shape of the ears led to a clearly disorganized behavior of the neurons.
The subjects got used to the new ear
After the participants in the study had been wearing the silicone inserts for a long time and thus moved through everyday life, brain activity had returned to normal and reacted to the new ear shape as if it were their own. "We can hear with our own individually designed ears because our brain knows their shape," summarizes Schönwiesner. If this changes, it will take some time to adjust. That is also the case, for example, when we grow.
A new generation of hearing aids?
"In Germany, about 17 percent of the population is currently suffering from hearing impairments," said Schönwiesner. With an increasing tendency, because our environment is getting louder and at the same time people are getting older. Currently, hearing aid manufacturers and doctors estimate that up to 25 percent of hearing aids are not in use because patients often underestimate that the brain needs time to get used to. Instead, many hearing impaired people would expect immediate improvement. "If we better understand the process of getting used to it, we may be able to speed it up so that patients can receive targeted advice," concludes Schönwiesner. (vb)