THE ESSENTIAL
- Deafness is the reduced ability to perceive sounds (decreased hearing).
- According to Inserm, more than 65% of French people aged 65 and over have hearing loss.
- We speak of mild deafness if the hearing loss is between 21 and 40 dB, average between 41 and 70 dB and severe between 70 and 90 dB. Deafness is considered total beyond 90 db.
It has long been established that the hearing loss observed after age 65 is related to the destruction of hair cells in the inner ear over time. Researchers at Johns-Hopkins University wanted to better understand how the brain works in the face of this disorder. By studying mice, they discovered that the brain could also be involved.
Older rodents had more difficulty turning off certain brain cells when they were in a noisy environment. This “sound blur” would prevent the brain from focusing on one type of sound like words.
Hearing problem: the neuronal activity of the mice was recorded
For this study published in The Journal of Neuroscience At the beginning of December, the scientists recorded the activity of neurons located in the auditory cortex of 12 old mice (between 16 and 24 months) and 10 young mice (between 2 and 6 months). These rodents were conditioned to lick water when they heard a sound. The same exercise was then performed with “white noise” in the background.
During quiet phases, older animals performed the task as well as younger ones. In contrast, when white noise was introduced, older mice performed worse at detecting sound and licking the drip than younger mice. To understand how auditory neurons functioned during these tests, the researchers observed the brains of rodents using two-photon excitation microscopy.
Age-related hearing loss: neurons do not “turn off”
Examinations showed that when the brain circuits functioned correctly in the presence of ambient noise, part of the activity of neurons increased when the mice heard the sound and, at the same time, other neurons were repressed or deactivated. In most old mice, the balance swung towards predominantly active neurons, and those that were supposed to turn off when the tone was played against a loud background, did not.
The team also discovered that just before the sound of the signal, there was up to twice as much neuronal activity in the “elders”, in particular in the males. This led the animals to lick the water before the sound started. The results also show that the young people had the ability to suppress the effects of ambient noise on the activity of neurons, while the older ones did not.
“In older animals, ambient noise appears to blur neuron activity, impairing the ability to distinguish individual sounds”, explains Prof. Patrick Kanold, professor of biomedical engineering at Johns-Hopkins University. For the scientist, his discovery opens the door to a treatment for age-related hearing loss. “There may be ways to train the brain to focus on an individual sound amidst a cacophony of noise”, he assures. However, more research is needed to accurately map the link between the inability to turn off certain neurons and hearing loss amidst ambient sound.