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What is the brain’s role in age-related hearing loss?

A team of scientists from Johns Hopkins Medicine investigating how the brain works amid age-related hearing loss has recently discovered that old mice are less capable than young ones to turn off certain actively firing brain cells in the midst of ambient noise. This inability creates a “fuzzy” sound stage which makes it difficult for the brain to focus on one type of sound, such as spoken words, while filtering out surrounding noise. 

Although scientists have long associated inevitable age-related hearing loss to deteriorating hair cells in the inner ear, this new study suggests that the brain has much to do with this condition, and thus, that it may be possible to treat this type of hearing loss by re-training the brain to tamp down wildly firing neurons.

The researchers recorded the activity of 8,078 neurons from the auditory cortex of 12 old mice (16 to 24 months old) and 10 young mice (two to six months old). In a first experiment, they conditioned the mice to lick a water spout when hearing a specific tone. Then, they made the mice perform the same exercise while playing “white noise” in the background.

While, without the ambient noise, both young and old mice licked the water spout when hearing the tone, in the presence of the ambient noise the old mice were worse at detecting the tone and licking the spout than their younger counterparts. Moreover, the young mice tended to lick the spout at either the onset or end of the tone, while older mice licked it at the start of the auditory cue, but sometimes also before the tone was presented, suggesting that they thought a tone was present when it was not.

Next, in order to assess how auditory neurons performed during these hearing tests, the scientists employed a method called “two-photon imaging” – which uses fluorescence to identify and measure the activity of hundreds of neurons at the same time – to get a glimpse into mice’s auditory cortex.

The analysis revealed that, under normal conditions (when the brain worked correctly in the presence of ambient noise), some neural activity increased when the mice heard the tone, while other neurons were repressed. However, in the older mice, there was more neural activity overall, and the neurons that were supposed to turn off when the tone was played in the presence of ambient noise failed to do so. Moreover, just before the auditory cue, there was up to twice as much neural activity in old mice, indicating that their brains may be “firing” or behaving as if a tone is present when it is not.

“In older animals, ambient noise seems to make neuron activity more ‘fuzzy,’ disrupting the ability to distinguish individual sounds,” said study senior author Patrick Kanold, a professor of Biomedical Engineering at Johns Hopkins.

Fortunately, due to its flexible learning potential, the mammalian brain could be “taught” to address this auditory fuzziness in older animals, including humans. “There may be ways to train the brain to focus on individual sound amid a cacophony of noise,” Kanold concluded.

The study is published in the Journal of Neuroscience.

By Andrei Ionescu, Staff Writer

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