New research casts doubt on old ideas about how hearing works

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Summary: A new study challenges old theories about hearing. Researchers have discovered that many cells in the inner ear simultaneously respond to low-frequency sounds. This makes it easier for us to hear low frequency sounds because the brain is able to receive information from multiple sensory cells at the same time.

Source: Linkoping University

The way we experience music and speech differs from what we believed until now. This is the conclusion of a study conducted by researchers from Linköping University, Sweden, and Oregon Health and Science University, USA.

The results were published in Scientists progressand may lead to the design of better cochlear implants.

We are social creatures. The sound of other people’s voices is important to us, and our hearing is directed to experiencing and distinguishing human voices and speech. Sound that arrives at the outer ear is carried by the eardrum to the spiral-shaped inner ear, also known as the cochlea.

The sensory cells of hearing, the outer and inner hair cells, are located in the cochlea. The sound waves cause the “hairs” of the inner hair cells to bend, sending a signal through the nerves to the brain, which interprets the sound we hear.

For the past 100 years, we have believed that each sensory cell has its own “optimum frequency” (a measure of the number of sound waves per second). The hair cell responds most strongly to this frequency.

This idea means that a sensory cell with an optimal frequency of 1000 Hz would respond much less strongly to sounds with a slightly lower or higher frequency. It has also been assumed that all parts of the cochlea work the same way.

Now, however, a research team has discovered that this is not the case for sensory cells that process sound with frequencies below 1000 Hz, which are considered low-frequency sounds. The vowel sounds in human speech lie in this area.

“Our study shows that many inner ear cells respond simultaneously to low-frequency sounds. We believe this makes it easier to experience low-frequency sounds, because the brain receives information from many sensory cells at the same time,” explains Anders Fridberger, professor at the Department of Biomedical and Clinical Sciences in Linköping. University.

Scientists believe that this construction of our auditory system makes it more robust. If some sensory cells are damaged, there are many more that can send nerve impulses to the brain.

It’s not just the vowels of human speech that are found in the low-frequency region: many of the sounds that make up music are also found here. Middle C of a piano, for example, has a frequency of 262 Hz.

The sound of other people’s voices is important to us, and our hearing is directed to experiencing and distinguishing human voices and speech. Image is in public domain

These results may possibly be significant for people with severe hearing loss. The most effective treatment currently available for such cases is a cochlear implant, in which electrodes are placed in the cochlea.

“The design of current cochlear implants is based on the assumption that each electrode should only give nerve stimulation at certain frequencies, in a way that attempts to copy what was believed about how our auditory system works.

“We suggest that changing the stimulation method to low frequencies will be more like natural stimulation, and the user’s hearing experience should be improved as a result,” says Anders Fridberger.

The researchers now plan to examine how their new knowledge can be applied in practice. One of the projects they are studying concerns new methods for stimulating the low-frequency parts of the cochlea.

These results come from experiments on the cochlea of ​​guinea pigs, whose hearing in the low-frequency region is similar to that of humans.

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Funding: This work was funded by the US National Institutes of Health and the Swedish Research Council.

About this auditory neuroscience research news

Author: Karin Soderlund Leifler
Source: Linkoping University
Contact: Karin Söderlund Leifler – University of Linkoping
Image: Image is in public domain

Original research: Free access.
Best frequencies and time delays are similar in low frequency regions of the guinea pig cochlea” by Anders Fridberger et al. Scientists progress


Summary

Best frequencies and time delays are similar in low frequency regions of the guinea pig cochlea

The cochlea maps tones with different frequencies to distinct anatomical locations. For example, a low 5000 hertz tone produces sharp responses at a location about 8 millimeters in the 18 millimeter long guinea pig’s cochlea, but little response elsewhere.

This place code permeates the auditory pathways, where neurons have the “best frequencies” determined by their connections to sensory cells in the hearing organ. However, frequency selectivity in cochlear regions encoding low-frequency sounds has not been systematically studied.

Here we show that low-frequency hearing works on a unique principle that does not involve a location code. Instead, sound-evoked responses and time delays are similar in low-frequency regions of the cochlea.

These findings are a break from theories considered proven for 100 years and have broad implications for understanding information processing in the brainstem and cortex and for optimizing stimulus delivery in hearing implants.

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