One third of people worldwide experience age-related hearing loss, also known as presbycusis. Hearing loss due to presbycusis is a symptom of noisy environments. The aging ear and brain are the two parts of presbycusis that can be distinguished. The connection of presbycusis to the aging brain is still poorly understood.
Johns Hopkins Medicine researchers said they found that older mice were less able than younger mice to “turn off” some actively firing brain cells in the presence of background noise. They were looking for answers about how the brain functions amid age-related hearing loss. As a result, scientists claim, the sound image becomes “blurred,” making it challenging for the brain to focus on one type of sound, such as spoken words, and filter out surrounding “noise.”
The study indicates that the brain has a lot to do with the condition, and it may be possible to treat such hearing loss by retraining the brain to suppress the wild-firing neurons.
Patrick Kanold, Ph.D., professor of biomedical engineering at Johns Hopkins University and School of Medicine, said: “Hearing is more than the ear. Most people will experience hearing loss after age 65, such as the inability to pick up 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 began 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 elderly 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. In addition, the young mice tended to lick the spout at the beginning or end of the tone. Older mice licked it at the start of the tone cue, but they also showed licking before the tone was given, suggesting they mistook its absence for a tone.
Next, the scientists used a method known as two-photon imaging to peer into the auditory cortex of the mice to observe how auditory neurons functioned directly during such hearing tests. Fluorescence is used to detect and track the activity of hundreds of neurons simultaneously.
When the mice heard the tone, the activity of some neurons spiked under normal conditions when brain circuits functioned properly amid background noise. At the same time, other neurons were suppressed or turned off. However, in the majority of the old mice, the ratio flipped in favor of a majority of active neurons, and the neurons 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 as in young mice, especially in males, causing the animals to lick the spout before the tone started.
Kanold said, “One possible reason for that result in the old mice is that the brain ‘fires’ or behaves as if a tone is present when it isn’t.”
“The environmental noise experiments also revealed that young mice experienced shifts in the ratio of active to inactive neurons, while older mice had more consistently active neurons overall. For example, young mice were able to suppress the effects of ambient noise on neural activity, while old mice could not.”
“In older animals, environmental noise appears to make neuron activity more ‘blurred,’ interfering with the ability to distinguish individual sounds.”
“On the other hand, we believe that due to the flexible learning potential of mammalian brains, it can be ‘taught’ to handle the blurriness 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 accurately map the link between the inability to turn off specific neurons and hearing loss to ambient noise, including the brain circuits involved and how they change with age, as well as the possible differences between males and females.
- Kelson Shilling-Scrivo, Jonah Mittelstadt, and Patrick O. Kanold. Decreased modulation of population correlations in the auditory cortex is associated with reduced auditory detection performance in aged mice. Journal of Neuroscience 7 Dec 2022, 42 (49) 9278-9292; DOI: 10.1523/JNEUROSCI.0955-22.2022