Noise-Induced Hearing Loss Affects Brain’s Response to Speech Sounds

A new research has found that prolonged exposure to loud noise alters brain's response to speech.

The researchers at the University of Texas, Dallas, for the first time showed how noise-induced hearing loss affects the brains recognition of speech sounds. They state that continuous exposure to loud noise alters the manner in which brain processes speech, gradually increasing the difficulty to distinguish speech sounds.

Nearly 15 percent of the American population aged between 20-69 years are affected with noise-induced hearing loss (NIHL), according to the National Institute of Deafness and Other Communication Disorders (NIDCD).

Exposure to intensely loud sounds triggers long term damage to hair cells that act as sound receivers in the ear. Once these hair cells are damaged, the hair cells do not grow back again causing NIHL.

"As we have made machines and electronic devices more powerful, the potential to cause permanent damage has grown tremendously," said Dr. Michael Kilgard, co-author and Margaret Fonde Jonsson Professor in the School of Behavioral and Brain Sciences. "Even the smaller MP3 players can reach volume levels that are highly damaging to the ear in a matter of minutes."

Prior to this study, not much was known about the direct effects of NIHL or how the brain responds to speech.

In order to stimulate the noise trauma that most of the population face, researchers exposed rats to moderate or intense levels of noise for almost an hour. One group of rats heard a high frequency noise at 115 decibels including moderate hearing loss, the second group of rats were exposed to low-frequency noise at 124 decibels leading to severe hearing loss.

According to NIDCD, regular exposure to sounds of more than 100 decibels for more than one minute causes permanent hearing loss.

In this study, the researchers observed how two types of hearing loss affected speech sound processing in rats by recording neuronal response in auditory cortex for a month after exposure to loud noise.

Auditory cortex is one of the key areas that processes sounds in the brain. Neurons at one end of the cortex respond to low frequency sounds and the other neurons present at the opposite end respond to sounds of higher frequency.

The group that had severe hearing loss, less than one-third of the tested auditory cortex sites that respond to sound, reacted to stimulation. They noticed unusual patterns of activity in sites that responded to the sound.

Additionally, the rats could not tell the speech sounds apart in a behavioral task they could successfully complete before the hearing loss.

In those with moderate hearing loss, the area of the cortex responding to sounds didn't change but the neurons' reaction did. Larger region of the auditory cortex responded to low-frequency sounds.

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

The study was documented in the Ear and Hearing.