Exposure to loud noise doesn't just damage ears, but even changes the way the brain handles speech sounds, a new study shows.
Noise-induced hearing loss (NIHL) affects around 15 percent of Americans aged between 20 and 69 years, states the National Institute of Deafness and Other Communication Disorders (NIDCD).
The researchers have found that exposure to loud noise doesn't just damage ear hair cells, but also alters brain's response to speech. The study, conducted by neuroscientists at the University of Texas at Dallas, was based on animal models.
"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, according to a news release. "Even the smaller MP3 players can reach volume levels that are highly damaging to the ear in a matter of minutes."
For the study, researchers exposed rats to loud noise to induce hearing loss. One set of rats was exposed to low-frequency noise at 124 decibels and had severe hearing loss. The second group was exposed to high-frequency noise at 115 decibels, which induced moderate hearing loss.
Researchers then looked at how hearing loss affected speech processing in brains of test rats.
The auditory cortex, the region where speech in processed, is organized on a scale. Neurons at one end of the cortex respond to high-frequency sounds, while the other end responds to low-frequency sounds.
The team found in rats suffering from severe hearing loss, less than a third of neurons in auditory complex responded to speech sounds. During behavior tests, these rats were less likely than others to follow speech sounds.
Rats suffering from moderate hearing loss, too, had poor neuronal response in auditory cortex. However, they could still differentiate between speech sounds during behavior tasks.
"Although the ear is critical to hearing, it is just the first step of many processing stages needed to hold a conversation," Kilgard said in a news release. "We are beginning to understand how hearing damage alters the brain and makes it hard to process speech, especially in noisy environments."
The study is published in the journal Ear and Hearing. NIDCD supported the research.