A new study looking at the brain activity of mice is hoping to shed some light on why some people’s hearing starts to go as they grow older. Researchers from Johns Hopkins Medicine looked at the brain activity of older mice compared to younger ones and found that older mice were less able to silence certain actively firing brain cells when ambient noise was playing, making it more difficult for them to focus on specific sounds compared to younger mice. “There’s more to hearing than the ear,” Patrick Kanold, professor of biomedical engineering at The Johns Hopkins University and School of Medicine and one of the authors of the study, said in a press release. Although previous research has established that age-related hearing loss is connected in part to changes in hair cells in the inner ear, this new research published in The Journal of Neuroscience in December is making the case that the brain itself plays a role in this hearing loss as well. Researchers looked at brain activity in 12 mice that were 16-24 months old compared to 10 mice that were just 2-6 months old. In total, they looked at the activity of more than 8,000 neurons across all mice. In order to see if the mice were able to accurately pick out a sound that held meaning while other, more distracting noises were being played, researchers conditioned the mice to lick a water spout when they heard a specific tone. They then played the tone for the mice with and without ambient noise playing in the background. All of the mice performed well in recognizing the tone and licking the water spout when it was the only sound. But when ambient noise was introduced, the older mice had more trouble picking out the tone and licked the spout less often than the younger mice. Younger mice also tended to recognize the tone immediately, while some of the older mice licked the spout before the tone had been played, indicating they incorrectly thought they were hearing it. So what was going on in the brain that might be connected to these results? When researchers looked at the neurons, they saw that when no ambient noise was playing, certain neurons would activate at the sound of the tone, while other neurons would become repressed, or turn off temporarily, similar to turning down the radio to better hear someone speaking. However, in the older mice, more of the neurons stayed active regardless of the tone, indicating that the older mice were less able to turn off some neurons that were meant to not be firing when the tone was playing. Their higher level of firing neurons also might have caused them to think they were hearing a tone when they weren’t, researchers noted. “In the old mice, the brain may be ‘firing’ or behaving as if a tone is present, when it’s not,” Kanold said. “In older animals, ambient noise seems to make neuron activity more ‘fuzzy,’ disrupting the ability to distinguish individual sounds.” If these findings are true for humans as well, it could be good news for those struggling to hear things clearly as they age, researchers suggested. “There may be ways to train the brain to focus on individual sound amid a cacophony of noise,” Kanold said. However, this research only established a correlation — more research needs to be done to pinpoint the mechanisms driving this inability to shut off certain neurons and whether it is truly causing worse hearing comprehension.
