A new study shows for the first time that prolonged exposure to loud noise changes how the brain processes speech, suggesting the damage such exposure causes is not limited to physical changes in the ear itself.

According to the National Institute of Deafness and Other Communication Disorders (NIDCD), the organization that funded the study, prolonged exposure to noise levels at 85 decibels and above increases people’s risk for hearing loss.

And it is a sobering thought that many devices children use today have noise levels much higher than this threshold – for example an MP3 player on its loudest setting is giving out sound at 105 decibels, which is 100 times more intense than 85 decibels.

Repeated exposure to intensely loud noise eventually causes permanent damage to the hair cells in the ear that act as sound receivers – they convert sound energy into electrical signals that travel to the brain.

Once damaged, the hair cells do not grow back, leading to noise-induced hearing loss (NIHL), a condition that affects around 15% of Americans between the ages of 20 and 69.

Now for the first time, neuroscientists at the University of Texas (UT) at Dallas, writing in the journal Ear and Hearing, describe how after studying noise-induced hearing loss in rats, they discovered that it also affects the brain’s recognition of speech sounds.

Co-author Dr. Michael Kilgard, Margaret Fonde Jonsson Professor in the School of Behavioral and Brain Sciences at UT Dallas, says:

As we have made machines and electronic devices more powerful, the potential to cause permanent damage has grown tremendously. Even the smaller MP3 players can reach volume levels that are highly damaging to the ear in a matter of minutes.”

Until this study, it was not clear how NIHL might affect the brain’s ability to respond to speech.

For their investigation, Dr. Kilgard and colleagues exposed two groups of rats to moderate or intense levels of noise for an hour. One group was exposed to high-frequency noise at 115 decibels – this induced moderate hearing loss. The other group developed severe hearing loss after being exposed to low-frequency noise at 124 decibels.

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Regular exposure to sounds greater than 100 decibels for more than a minute at a time may lead to permanent hearing loss, according to the National Institute of Deafness and Other Communication Disorders.
Image credit: University of Texas at Dallas

A month after this exposure, the team found both types of hearing loss affected how brain circuits in the auditory cortex responded to speech sounds. This part of the brain, one of the main areas that process sound, is organized on a scale, much like a piano, with brain cells at one end responding to low-frequency sound while at the other end they process high-frequency sound.

The team found fewer than a third of the auditory cortex sites they tested responded to stimulation in the rats that developed severe hearing loss. And in the sites that did respond, the brain cells responded more slowly and the sounds had to be louder, and in narrower frequency ranges, to elicit a reaction.

Also, the rats with severe hearing loss were less able to distinguish different speech sounds in a behavioral task they had successfully completed before experiencing severe hearing loss.

In the group of rats that developed moderate hearing loss, the team did not see the same extent of change in the auditory cortex as they saw in those whose hearing was severely impaired, but they did find that a larger area of the auditory cortex responded to low-frequency sounds, and brain cells responding to high-frequency sounds needed more intense stimulation and reacted more slowly than they did in animals with normal hearing.

However, despite these physical changes, the rats with moderate hearing loss were able to complete the speech discrimination task as well as they had before suffering hearing damage.

Dr. Kilgard says the study shows:

Although the ear is critical to hearing, it is just the first step of many processing stages needed to hold a conversation. We are beginning to understand how hearing damage alters the brain and makes it hard to process speech, especially in noisy environments.”

Meanwhile, Medical News Today recently learned how engineers at the University of Texas at Austin are working on next-generation hearing aids that emulate a fly’s ability to pinpoint sound so the devices help the wearer distinguish conversations more clearly against background noise.