Background noises don't just cover up conversation, they may actually scramble brain activity

Oct 5 2004

Background noises don’t just cover up conversation, they may actually scramble brain activity, a discovery that helps explain why even perfectly loud speech can be hard to understand in a noisy room, say University of Florida researchers writing in the current Proceedings of the National Academy of Sciences.

The insight from experiments with rats could influence the design of hearing devices, MP3 music players and virtually any audio transmission technology, say the scientists.

“Some people have a tremendously difficult time understanding speech in a noisy environment and we’ve all had the experience of hearing someone tell us something, but we can’t tell what it is they are saying,” said Purvis Bedenbaugh, an assistant professor of neuroscience with the UF College of Medicine and a member of UF’s McKnight Brain Institute. “This research is a first step toward looking at why that would be.”

Scientists examined how brain cells in alert rats responded to specific sounds while one of three standardized noises played in the background. They discovered that brain activity actually decreased in the presence of background noise. Furthermore, background noise didn’t simply cover up sounds, it interfered with the brain’s ability to process or interpret information about a sound, even though the sound was heard. Essentially, the brain couldn’t understand what the ear told it.

The phenomenon may play a role in auditory processing disorder, a problem first noticed in children in the 1970s. The lack of coordination between the ear and brain that characterizes the disorder is expected to be widespread, although it is difficult to diagnose, according to the American Academy of Audiology.

“Some people have a lot of trouble hearing in background noise,” said Eugene Martin, a doctoral associate in the neuroscience department who participated in the research. “Now we know a little bit more about why. The design of hearing aid devices and cochlear implants — any device that involves auditory perception — will benefit from deeper understanding of the organ which serves that perception, which is the brain.”

Researchers explored the effects of background noises by recording brain activity detected by electrodes implanted in the auditory thalamus of the rats. Beeps and sharp shushing noises were the target sounds, akin to specific words in conversation, and scientists used standardized sounds for background noise similar to static, the conversational murmur at a busy restaurant and the disjointed whir of a rewinding tape recorder.

Each of the background noises changed the brain’s electrical activity, suggesting brain circuits received a message, but the message was scrambled.

Similar interactions occur during human audio reception and noise processing, which kindles the understanding that noise is more than just a nuisance. Noise specifically interferes with the way the brain processes information, according to Michael Merzenich, a professor of otolaryngology at the Keck Center for Integrative Neuroscience at the University of California San Francisco.

“Noise interference is a fundamental aspect of many impaired populations,” he said. “Children struggling with language and reading often have problems specifically in the presence of noise. Go to the other end of the age spectrum, and again you find the loss of ability to operate effectively in noisy environments. Commonly what older people want to do is turn up the volume when they have trouble understanding, but fundamentally it is important to know that noise interferes with the processing of speech, not just the reception.”

http://www.napa.ufl.edu