a chart showing how loud sounds are in decibels

Decibel scale, via the University of Texas at Dallas

Around 15% of American adults have noise-induced hearing loss (NIHL). Scientists already understand the mechanism of hearing loss, but how NIHL affects the brain’s ability to respond to speech is not well understood. Researchers from the University of Texas at Dallas are changing that. The research team studied how NIHL altered the brain’s response to speech sounds in rats. The investigation revealed that NIHL resulted in various deficits in processing speech, including slower processing time. This study could help scientists understand how hearing loss affects auditory processing in humans.

The American Speech-Language-Hearing Association advises that ongoing exposure to sounds louder than 100 decibels can result in permanent hearing loss. The researchers exposed two groups of rats to moderate and intense noise trauma. One group of rats received exposure to high-frequency noise at 115 decibels to induce moderate hearing loss. The other group of rats received exposure to low-frequency noise at 124 decibels to induce severe hearing loss.

A month after the exposure, the researchers observed how the hearing loss affected the neurons in the rats’ auditory cortices—the part of the brain that processes sound—while they exposed the rats to speech sounds. Both before and after their noise-induced hearing loss, the rats completed a behavioral task in which they distinguished speech sounds.

In both groups of rats, the neurons of the of auditory cortex responded more slowly to speech sounds compared to their ability to distinguish speech before the NIHL. The rats with moderate hearing loss had altered responses to high- and low-frequency sounds. At high frequencies, the rats needed more intense sound stimulation to process the sounds. Low frequencies elicited responses from a larger area of the cortex. The rats with moderate hearing loss were still able to distinguish speech sounds. In contrast, the rats with severe hearing loss were not able to distinguish speech sounds. The auditory cortices of these rats had less than one-third of the area responding to stimulation than before the NIHL.

“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,” stated study authors Dr. Michael Kilgard and Margaret Fonde Johnson.

This research is published in the journal Ear and Hearing.

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