People who have any kind of hearing loss usually have difficulty hearing conversation when they are in noisy environments – environments where there is a lot of “background noise” – such as at large gatherings in which many people are all speaking at the same time, or at loud concerts (they can hear the music but cannot hold a conversation easily in such a situation), or outdoors near a busy intersection or highway, where there is a lot traffic noise.
In a way, this is nothing new. There are even people who do not have hearing impairments who find it difficult to function in such situations. But recently researchers have discovered that background noise actually causes the ears of those with hearing impairments to work differently than the ears of those who do not have such impairments.
According to Kenneth S. Henry, a postdoctoral researcher at Purdue University’s Department of Speech, Language, and Hearing Sciences, “When immersed in the noise, the neurons of the inner ear must work harder because they are spread too thin. 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.”
Temporal coding refers to the rate at which neurons fire responses to the brain, in response to stimuli, in this case the stimuli being sound. In a person with a hearing impairment in which there are fewer neurons in the inner ear, then they must work much harder at bringing meaningful sound to the brain. More neurons will allow an individual to process the sounds more easily, thus allowing them to distinguish between the sounds, and to make sense of what is heard. Fewer neurons will be overworked and will fail to allow the hearing impaired individual to process the sounds easily, thus disallowing them to distinguish between the sounds, rendering them unable to make sense of what they are hearing.
In the past, hearing studies were done in quiet environments, using chinchillas to measure response to noise stimuli. Chinchillas are used because they exhibit a similar range of hearing as humans. In the more recent research, however, background noise was used to stimulate what people might hear in a room full of people. Some of the chinchillas used in the study had normal hearing while others had a cochlear hearing loss.
Michael G. Heinz, associate professor at Purdue University said, “Previous studies on how the inner ear processes sound have failed to find connections between hearing impairment and degraded temporal coding in auditory nerve fibers, which transmit messages from the inner ear to the brain. 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 fibers respond to sound.”
The inner ear filters sound into a number of channels, via the auditory system, which 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 fibers are particularly distracted by background noise,” said Heinz.
The findings were published in Nature Neuroscience.
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