The study of hearing in old and young mice suggests that the brain can be trained to filter out background sounds.
Searching for answers to how the brain works amid age-related hearing loss, Johns Hopkins Medicine researchers found that old mice were less able than young mice to “turn off” certain brain cells that fire in actively in the midst of ambient noise. The result, they say, creates a “fuzzy” sound stage that makes it difficult for the brain to focus on one type of sound — such as spoken words — and filter out the surrounding “noise.”
Scientists have long linked inevitable age-related hearing loss to hair cells in the inner ear that become damaged or destroyed over time.
But Johns Hopkins researchers say their new study, described Dec. 7 in Journal of Neuroscienceshow that the brain has a lot to do with the condition, and it may be possible to treat such hearing loss by re-training the brain to suppress wild neurons.
“There’s more to hearing than hearing,” says Patrick Kanold, Ph.D., professor of biomedical engineering at Johns Hopkins University and School of Medicine. Kanold notes that most people will experience some type of hearing loss after age 65, such as not being able to pick out individual conversations in a bar or restaurant.
Kanold and his team recorded the activity of 8,078 brain cells, or neurons, in the brain region of the auditory cortex of 12 old mice (16-24 months old) and 10 young mice (2-6 months old).
First, the researchers conditioned rats to lick a mouthful of water when they heard a tone. Then, the same exercise was performed while playing “white noise” in the background.
Without ambient noise, old mice licked the water spout just as well as young mice when they heard the tone.
When the researchers presented white noise, in general, the old mice were worse at detecting the tone and licking the mouthpiece than the young mice.
Also, young mice tended to lick the mouthpiece at the beginning or end of the tone. The older rats licked it at the onset of the tone signal, but also showed licking before the tone was presented, indicating that they thought a tone was present when there was not.
Then, to see how auditory neurons work directly during such listening tests, the researchers used a technique called two-photon imaging to look at the auditory cortex in mice. The technique uses fluorescence to identify and measure the activity of hundreds of neurons simultaneously.
Under normal conditions, when the brain circuit was functioning properly in the presence of ambient noise, some of the neurons’ activity increased when the mice heard the tone, and, at the same time, other neurons were suppressed or turned off. However, in most old mice, the balance was tipped to have mostly active neurons, and neurons that were supposed to turn off when the tone was played in the presence of a noisy background failed to do so.
In addition, the researchers found that just before the tone signal, there was up to twice as much neuronal activity in old mice than young mice, especially in males, causing the animals to lick their throats before the tone began.
One possible reason for this result, Kanold says, is that “in old mice, the brain can ‘live up,’ or act as if a tone is present, when it is not.”
Ambient noise experiments also revealed that young mice experienced changes in the ratio of active to inactive neurons, while older mice had more active neurons overall. Thus, the young mice could suppress the effects of ambient noise on neural activity, while the old mice could not, the researchers say.
“In older animals, ambient noise appears to make neuronal activity more ‘fuzzy,’ impairing the ability to distinguish individual sounds,” says Kanold.
On the other hand, Kanold believes that because of the flexible learning potential of the mammalian brain, it can be “taught” to handle ambiguity in older animals, including humans.
“There may be ways to train the brain to focus on individual sounds among a cacophony of noise,” he says.
Kanold notes that more research is needed to precisely map the link between the inability to shut down certain neurons and ambient sound hearing loss, including the brain circuits involved and how they change with age, as well as possible changes between male and female animals. .
Reference: “Diminished modulation of population correlations in auditory cortex is associated with reduced auditory detection performance in aged mice” by Kelson Shilling-Scrivo, Jonah Mittelstadt, and Patrick O. Kanold, December 7, 2022 Journal of Neuroscience.
Other contributors to the research are Kelson Shilling-Scrivo and Jonah Mittelstadt from the University of Maryland.
Funding for the research was provided by the National Institutes of Health (P01AG055365, RO1DC009607, RO1DC017785).
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