Listening to your favorite music at extremely high volumes may be great for ambience and entertainment, but you won't have fun for long. A study released this week shows that exposure to excessively loud sounds can permanently damage the brain's ability to recognize speech.

Previous studies have shown that loud sounds damage hair cells irreversibly, leading to noise-induced hearing loss (NIHL). This study went a step further to show that NIHL can directly damage speech responses. Plugging your earphones in and ramping up the volume isn't just dangerous for your hearing anymore.

UT Dallas scientists exposed rats to varying frequencies and decibels of noise for an hour. One group was exposed to high-frequency noises at 115 decibels and the other group was exposed to low-frequency noises at 124 decibels. The former group experienced moderate hearing loss, while the latter group experienced severe hearing loss.

If you're wondering about your own listening experience, you can check out a list of audio outputs and decibel levels at the American Speech-Language-Hearing Association's website. The highest volume on an MP3 player is the same as the output of a chain saw, at 110 decibels. The siren on an ambulance or police car is about 120 decibels. Studies show that listening to sounds at these volumes for longer than a minute can lead to NIHL, as per the National Institute of Deafness and Other Communication Disorders (NIDCD).

Dr. Michael Kilgard, corresponding author of the paper, which was published in Ear and Hearing, was concerned about the increasing prevalence of devices that emit powerful noises and have tremendous potential to damage hearing. Prior to his study, scientists had no clear idea about how NIHL can affect the brain's response to speech.

After inducing hearing loss in the experimental rats, Kilgard and colleagues attempted to study the effect on the rats' auditory responses to stimulation. They took neuronal recordings in the auditory cortex a month after the exposure to debilitating sounds. This region of the brain operates on a continuous range--one end of the cortex is dedicated to processing low-frequency sounds while the other end processes high frequencies.

Less than a third of the rats that experienced severe hearing loss exhibited normal responses to stimulation in their auditory cortices. Their electrical patterns were abnormal, the reactions of the neurons were slower than usual and the cortices responded only to sound stimulations that were louder and frequencies that were narrower than typically necessary. These rats were also unsuccessful in recognizing speech sounds that they previously had no trouble distinguishing.

The rats with moderate hearing loss exhibited a change in neuronal reactions. A larger part of their auditory cortices responded to low-frequency sounds, while their high-frequency neurons needed more sound stimulation than usual and responded more slowly. These rats were, however, still able to distinguish speech 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." In the meantime, turn it down a notch.

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