As they say in rock n’ roll, “if it’s too loud, you’re too old”. That could very well be true, but according to a recent study done by neuroscientists at the University of Texas at Dallas, loud noises alter how the brain processes speech, potentially increasing the difficulty in distinguishing speech sounds. In a paper published this week in Ear and Hearing, researchers demonstrated how noise-induced hearing loss affects the brain’s recognition of speech sounds. Noise-induced hearing loss (NIHL) reaches all corners of the population, and affects an estimated 15 percent of Americans between the ages of 20 and 69.
Exposure to intensely loud sounds leads to permanent damage of the hair cells, which in turn act as sound receivers in the ear. Once they’re damaged, these hair cells don’t grow back, which leads to NIHL. As people have made machines and electronic devices more powerful, there arises a whole lot more potential to cause permanent damage. Even the smaller MP3 players are able to reach volume levels that are extremely damaging to the ear in just a matter of minutes. Before the study, scientists didn’t have a clear understanding of the direct effects of NIHL on how the brain responds to speech.
To simulate two different types of noise trauma that clinical populations face, UT Dallas scientists exposed rats to moderate or intense levels of noise for an hour. One group heard a high-frequency noise at 115 decibels that induced moderate hearing loss, and the second group heard a low-frequency noise at 124 decibels, causing severe hearing loss. In comparison, the American Speech-Language-Hearing Association lists the maximum output of an MP3 player or the sound of a chainsaw at about 110 decibels and the siren of an emergency vehicle at 120 decibels. Regular exposure to sounds more than 100 decibels for more than a minute at a time could lead to permanent hearing loss.
Researchers observed how the two types of hearing loss affected speech sound processing in the rats by recording the neuronal response in the auditory cortex a month after the noise exposure. The auditory cortex, one of the main areas that processes sounds in the brain, is organized on a scale, not unlike a piano. Neurons at one end of the cortex respond to low-frequency sounds, while other neurons at the opposite end react to higher frequencies.
In the group with severe hearing loss, less than one third of the tested auditory cortex sites that normally respond to sound reacted to stimulation. In the sites that did respond, there were noted unusual patterns of activity; neurons reacted more slowly, the sounds had to be louder and the neurons responded to frequency ranges more narrow than before. In addition, the rats couldn’t tell the speech sounds apart in a behavioral task that they could successfully complete before the hearing loss.
In the group with moderate hearing loss, the area of the cortex responding to sounds didn’t change, although the reaction of the neurons did. A much larger area of the auditory cortex responded to low-frequency sounds. Neurons reacting to high frequencies needed more intense sound stimulation and responded much slower than those in normal hearing animals. However, in spite of these changes, the rats still could discriminate the speech sounds in a behavioral task.