Summary: Researchers found that older mice struggle more than younger ones to suppress certain actively firing neurons when background noise is present. This failure to quiet specific neural activity produces a muddled auditory scene, making it harder for the brain to isolate a target sound and ignore competing noise.
Source: Johns Hopkins University
Researchers at Johns Hopkins Medicine investigated how aging affects the brain’s ability to process sound and found that older mice have a reduced capacity to “turn off” some neurons in the auditory cortex when exposed to ambient noise.
Their experiments suggest that age-related hearing difficulties are not only the consequence of damage to the inner ear’s hair cells but also reflect changes in how the brain filters and represents sounds. When neurons that should be suppressed remain active, the overall soundscape becomes blurred, and distinguishing a single sound—like a voice in a crowded room—becomes more challenging.
“There’s more to hearing than the ear,” says Patrick Kanold, Ph.D., professor of biomedical engineering at The Johns Hopkins University and School of Medicine. He notes that many people over 65 experience problems such as an inability to separate individual conversations in noisy settings.
To study this, Kanold and colleagues recorded activity from 8,078 neurons in the primary auditory cortex of 12 older mice (16–24 months) and 10 younger mice (2–6 months). The team trained the mice to lick a water spout when they heard a tone, first in silence and then with a background of broadband, or “white,” noise.
In quiet conditions, older mice detected the tone and performed the lick response as reliably as younger mice. But when white noise was added, the older mice’s detection performance declined: they were less accurate at detecting the tone and more likely to produce responses at times when no tone was present.
Behavioral differences were mirrored in neural recordings made with two-photon calcium imaging, a technique that uses fluorescent indicators to monitor the activity of many neurons simultaneously. In young mice, hearing a tone amid background noise produced an increase in activity among some neurons and a simultaneous suppression of activity in others. This balance—activation of target-responsive cells coupled with suppression of others—helped create a clear neural representation of the tone.
In most older mice, however, that suppression was weakened. Many neurons that should reduce their firing in response to the tone failed to turn off, leaving the auditory cortex in a generally more active state. The researchers also observed increased prestimulus activity in older animals—up to twice the level seen in young mice—especially in males, which likely contributed to the premature licking responses before the tone began.
The team found that young mice dynamically shifted the balance between active and suppressed neurons to reduce the impact of ambient noise, whereas older mice displayed a more uniformly active population of neurons that remained less modulated by the noisy background. Kanold summarizes: “In older animals, ambient noise seems to make neuron activity more ‘fuzzy,’ disrupting the ability to distinguish individual sounds.”
Importantly, the researchers emphasize that these findings point to the brain’s role in age-related hearing deficits and suggest potential avenues for treatment. Because mammalian brains retain a degree of plasticity, Kanold suggests that training or rehabilitation strategies might help older brains better suppress distracting neural activity and improve the ability to focus on specific sounds in noisy environments.
Further research is needed to map the precise circuits responsible for the suppression deficits, to understand sex differences observed in the study, and to determine how these changes evolve with age. Identifying the neural mechanisms involved will be essential to designing interventions aimed at reducing the “fuzziness” and restoring more selective auditory processing.
Other contributors to this work include Kelson Shilling-Scrivo and Jonah Mittelstadt from the University of Maryland.
About this auditory neuroscience research news
Author: Press Office
Source: Johns Hopkins University
Contact: Press Office – Johns Hopkins University
Image: The image is in the public domain
Original Research: Closed access.
“Decreased modulation of population correlations in auditory cortex is associated with decreased auditory detection performance in old mice” by Kelson Shilling-Scrivo et al., Journal of Neuroscience. DOI: 10.1523/JNEUROSCI.0955-22.2022
Abstract
Decreased modulation of population correlations in auditory cortex is associated with decreased auditory detection performance in old mice
Age-related hearing loss, or presbycusis, affects a large portion of the population and commonly involves difficulty hearing in noisy environments. Presbycusis includes contributions from both the peripheral auditory system (the ear) and central processing (the brain).
To investigate the brain’s role, the researchers performed in vivo two-photon calcium imaging in male and female mice that maintain peripheral hearing into old age (Thy1-GCaMP6s × CBA/CaJ mice). They recorded activity in primary auditory cortex (A1) of young adult (2–6 months) and old mice (16–24 months) while subjects performed a tone-detection task in broadband noise.
Behaviorally, young mice maintained strong detection performance, while older mice showed reduced detection at low signal-to-noise ratios. Imaging revealed that older animals exhibited increased prestimulus activity, diminished attentional gain, and elevated noise correlations across neurons. These increased correlations were present regardless of cell tuning or behavioral outcome and extended over larger cortical regions in older animals.
Neural decoding analyses indicated that elevated prestimulus activity predicted early, incorrect behavioral responses in older mice. Together, the results support a model in which higher and more correlated prestimulus activity in aging animals cannot be fully suppressed, reducing the cortical representation of targets amid distracting stimuli and contributing to poorer auditory detection in noisy settings.