Background noise causes the ears of those with hearing impairments to work differently, U.S. researchers say.
"When immersed in the noise, the neurons of the inner ear must work harder because they are spread too thin," said Kenneth S. Henry, postdoctoral researcher at Purdue University's Department of Speech, Language and Hearing Sciences. "It's comparable to turning on a dozen television screens and asking someone to focus on one program. The results can be fuzzy because these neurons get distracted by other information."
"The study confirmed that there is essentially no change, even for those with hearing loss, in terms of how the cochlear neurons are processing the tones in quiet surroundings, but once noise was added, we did observe a diminished coding of the temporal structure," Henry said.
While previous hearing studies were done in quiet environments, the researchers measured a variety of physiological markers in chinchillas in both quiet and noisy environments. Some of the chinchillas used in the study had normal hearing while others had a cochlear hearing loss.
Chinchillas are used because they have a similar hearing range to humans, and background noise was used to simulate what people would hear in a crowded room.
Change in response to sound
"Previous studies on how the inner ear processes sound have failed to find connections between hearing impairment and degraded temporal coding in auditory nerve fibres, which transmit messages from the inner ear to the brain," said Michael G. Heinz, associate professor at Purdue University.
"The difference is that such earlier studies were done in quiet environments, but when the same tests are conducted in a noisy environment, there is a physical difference in how auditory nerve fibres respond to sound."
The auditory system in the inner ear filters sound into a number of channels that are tuned to different frequencies, and those channels vary based on their frequency tuning. In a normal system, the channels are sharp and focused, but they get broader and more scattered with hearing impairment.
"Now we know that a major physiological effect of hearing loss is that the auditory nerve fibres are particularly distracted by background noise," said Heinz.
The findings were published in Nature Neuroscience.