One-third of people worldwide experience age-related hearing loss, also known as presbycusis. Hearing loss due to presbycusis is a symptom of noisy settings. The aging ear and brain are the two parts of presbycusis that can be distinguished. Presbycusis’ connection to the aging brain is still poorly understood.
Johns Hopkins Medicine researchers said they discovered that older mice were less able than younger mice to “switch off” some actively firing brain cells in the presence of background noise. They were searching for answers about how the brain functions amid age-related hearing loss. As a result, scientists claim, the sound stage becomes “fuzzy,” making it challenging for the brain to concentrate on one type of sound, such as spoken words, and filter out surrounding “noise.”
Patrick Kanold, Ph.D., professor of biomedical engineering at The Johns Hopkins University and School of Medicine, said, “There’s more to hearing than the ear. Most people will experience hearing loss after age 65, like the inability to pick out individual conversations in a bar or restaurant.”
Scientists recorded the activity of 8,078 brain cells, or neurons, in the auditory cortex brain region of 12 old mice (16–24 months old) and 10 young mice (2–6 months old). They started conditioning the mice to lick a water spout when they heard a tone. They then performed the same exercise while playing “white noise” in the background.
The aged mice licked the water spout when they heard the tone, just as well as the young mice did when there was no background noise.
The old mice performed worse than the young mice at detecting the tone and licking the spout when the scientists applied the white noise. Additionally, the young mice tended to lick the spout at the beginning or finish of the tone. Older mice licked it at the beginning of the tone cue, but they also displayed licking before the tone being provided, suggesting that they mistook its absence for a tone.
Next, the scientists used a method known as two-photon imaging to see inside the auditory cortex in the mice to observe how auditory neurons directly functioned during such hearing tests. Fluorescence is used to detect and monitor hundreds of neurons’ activity simultaneously.
When the mice heard the tone, some neurons’ activity rose under normal circumstances when brain circuitry functioned properly amid background noise. At the same time, other neurons got repressed or turned off. However, in the majority of the old mice, the ratio tipped in favor of having a majority of active neurons, and the neurons that were intended to turn off when the tone was played in the presence of background noise failed to do so.
Scientists also found that just before the tone cue, there was up to twice as much neuron activity in old mice than in young mice, especially among males, causing the animals to lick the spout before the tone started.
Kanold said, “A possible reason for that result in the old mice, the brain may be ‘firing’ or behaving as if a tone is present when it’s not.”
“The experiments with ambient noise also revealed that young mice experienced shifts in the ratio of active to inactive neurons, while older mice had more consistently active neurons overall. Thus, young mice could suppress the effects of ambient noise on neural activity while old mice could not.”
“In older animals, ambient noise seems to make neuron activity more ‘fuzzy,’ disrupting the ability to distinguish individual sounds.”
“On the upside, we believe that because of the mammalian brain’s flexible learning potential, it can be “taught” to address the fuzziness in older animals, including humans. There may be ways to train the brain to focus on individual sound amid a cacophony of noise.”
More research is needed to precisely map the connection between the inability to shut off specific neurons and hearing loss amid ambient sound, including the brain circuits involved and how they change with age, as well as the potential differences between male and female animals.
- Kelson Shilling-Scrivo, Jonah Mittelstadt and Patrick O. Kanold. Decreased Modulation of Population Correlations in the Auditory Cortex Is Associated with Decreased Auditory Detection Performance in Old Mice. Journal of Neuroscience 7 December 2022, 42 (49) 9278-9292; DOI: 10.1523/JNEUROSCI.0955-22.2022