Loud noises change how brain processes speech

Date: Aug-05-2014 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 loud noise damages sound-receiving hair cells in the ear

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.

Severe hearing loss led to changes in the brain's auditory cortex

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.

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.

Written by Catharine Paddock PhD

View all articles written by Catharine, or follow her on:

Courtesy: Medical News Today Note: Any medical information available in this news section is not intended as a substitute for informed medical advice and you should not take any action before consulting with a health care professional.