BALTIMORE — This time of year, it’s common to find yourself in a crowded room filled with voices competing for dominance. Many older adults struggle to hear words in these loud environments. Now, researchers from Johns Hopkins Medicine appear to have discovered why this happens on a neurological level. Their study found that older adult mice were less capable of “turning off” certain actively firing brain cells when confronted with ambient noise.
Consequently, study authors say the rodents developed a “fuzzy sound stage,” making it harder for their minds to focus on just one sound, like a voice, while filtering out other noises.
While scientists have previously linked inevitable hearing loss during old age to hair cells in the inner ear that eventually become damaged or destroyed over time, this latest work indicates the brain is heavily involved as well. Moreover, the findings suggest it may be possible to treat this kind of hearing loss by re-training the mind to suppress these wildly firing neurons.
“There’s more to hearing than the ear,” says Patrick Kanold, Ph.D., professor of biomedical engineering at Johns Hopkins University and School of Medicine, in a media release.
Prof. Kanold adds most people will experience some kind of hearing loss after age 65. For example, having trouble picking out individual conversations in a crowded bar or restaurant.
What’s going on in the brain to cause hearing loss?
The research team recorded the activity of 8,078 brain cells, or neurons, in the auditory cortex brain region of 12 old mice (16 to 24 months-old) and 10 young mice (2 to 6 months-old). To start, the rodents were conditioned to lick a waterspout whenever they heard a tone. Next, the same exercise was repeated but now “white noise” played in the background.
Without ambient noise, the old mice licked the waterspout just as well as the young mice when they heard the tone. However, when researchers introduced the white noise, the old mice were generally worse at detecting the tone and licking the spout than their younger counterparts. Young mice also tended to lick the spout at the beginning or the ending of the tone, whereas the old mice licked it at the start of the tone cue, but also started licking before the tone chimed. This suggests they may have believed the tone was present even when it wasn’t.
Moving on, researchers set out to see how auditory neurons performed directly during hearing tests. This was accomplished using a technique called two-photon imaging that allowed the team to see directly into the auditory cortexes of the mice. This approach uses fluorescence to identify and measure the activity of hundreds of neurons simultaneously.
Under normal conditions, when their brain circuitry was working properly in the presence of ambient noise, some neuron activity increased whenever the mice heard the tone and, simultaneously, other neurons turned off. In most of the older mice, on the other hand, most neurons were active, and the neurons that were supposed to turn off when the tone was played in the presence of a noisy background failed to do so.
Can you train the brain to hear better?
Additionally, study authors note that just before the tone cue, old mice displayed up to twice as much neuronal activity than young mice. This was especially true for males, and usually led the animals to lick the spout before the tone began.
A possible reason for that result, Prof. Kanold theorizes, is that “in the old mice, the brain may be ‘firing’ or behaving as if a tone is present, when it’s not.”
The young mice also exhibited changes in their ratio of active to inactive neurons, while older mice had more consistently active neurons overall. Thus, young mice appear much more capable of suppressing the effects of ambient noise on neural activity.
“In older animals, ambient noise seems to make neuron activity more ‘fuzzy,’ disrupting the ability to distinguish individual sounds,” Prof. Kanold adds.
On a positive note, Prof. Kanold believes there may be a way to train the brain to address the sound fuzziness in older animals, and even among humans too.
“There may be ways to train the brain to focus on individual sound amid a cacophony of noise,” the professor concludes.
Moving forward, more research is necessary to ascertain the precise connection between the inability to shut off certain neurons and hearing loss amid ambient sound. Future projects should explore the specific brain circuits involved and how they change with age, as well as any potential differences between males and females.
The study is published in the journal JNeurosci.