New research throws doubt on old ideas of how hearing works

Contrary to what was previously assumed, humans interpret music and speech differently. Researchers from Linköping University in Sweden and Oregon Health and Science University in the United States came at this result in their study. The findings, which have been published in Science Advances, could improve cochlear implant design.

We are sociable beings. We value hearing other people's voices, and we use our hearing to recognize and identify human speech and voices. The ear drum transmits sound from the outer ear to the spiral-shaped cochlea, which is the inner ear. The cochlea is home to the outer and inner hair cells, which are the hearing sense cells. The inner hair cells' "hairs" flex as a result of the sound waves, sending a signal to the brain, which interprets the sound we hear, via the nerves.

We have assumed that each sensory cell has its own "optimal frequency" for the past 100 years (a measure of the number of sound waves per second). This frequency elicits the strongest responses from hair cells. According to this theory, a sensory cell with an optimum frequency of 1000 Hz would react to noises with a frequency that is little lower or higher much less powerfully. Additionally, it has been presumpted that the cochlea functions uniformly throughout. However, it has now been found by a research team that this is not the case for sensory cells that interpret low-frequency sound, or sound with frequencies under 1000 Hz. These are the vowel sounds used in human speech.

"Our research demonstrates that numerous inner ear cells respond concurrently to low-frequency sound. Since the brain receives information from numerous sensory cells simultaneously, we think that this facilitates the perception of low-frequency sounds more than it otherwise would "explains Anders Fridberger, a professor at Linköping University's Department of Biomedical and Clinical Sciences.

The structure of our hearing system, according to scientists, makes it more durable. Numerous remaining sensory cells can still transmit nerve impulses to the brain even if some of them are damaged.

Not only do many of the sounds that make up music reside in the low-frequency range, but also the vowel sounds used in human speech. For instance, the middle C on a piano has a frequency of 262 Hz.

These findings may one day be important for those with severe hearing loss. A cochlear implant, in which electrodes are inserted into the cochlea, is now the most effective treatment for such situations.                                                            

"The current generation of cochlear implants are built on the presumption that each electrode should only stimulate the nerve at specific frequencies in an effort to mimic how our hearing system works. We propose that altering the stimulation technique at low frequencies will make it more like natural stimulation, and the user's hearing experience should be improved in this way "Anders Fridberger says.

The researchers will now investigate the practical use of their newly acquired information. One of the projects they are looking into is finding new ways to activate the cochlea's low-frequency regions.

These findings are based on research done on the cochlea of guinea pigs, whose low-frequency hearing is comparable to that of humans. Swedish Research Council and the U.S. National Institutes of Health provided funding for this project.

Linköping University