By Carolyn McClaskey, Ph.D.
Credit: @mbennettphoto/Unsplash
Changes in sound volume over time, known as temporal modulations, are common features of sounds that are important for hearing. The ability to perceive these temporal modulations, or temporal processing, is known to decline with aging and hearing loss. This negatively impacts overall sound perception, especially speech understanding.
However, neuroscience research also shows evidence of exaggerated or enhanced brain responses to temporal envelopes (patterns of sound volume change over time) in aging and hearing loss. This may arise from changes in brain signaling and overactivity of neurons.
This review paper published in Hearing Research in February 2024 describes the physiological changes to the brain's encoding of temporal envelopes shown to occur with age and hearing loss. The paper also discusses the role of disinhibited (reduced inhibitory neurotransmission) and neural hyperactivity (increased nerve cell activity) in contributing to these changes.
Note that the term “inhibition” is used to mean both the kind of inhibition that involves executive functioning or inhibitory control, as well as the more cellular-level inhibitory neurotransmission that is the focus of this review paper.
Studies in both humans and animal models suggest that as we age or experience hearing loss, our brains develop stronger neural responses to both periodic rhythmic modulations and modulations in speech. But this exaggeration is primarily seen for low-frequency modulations (below 80 hertz).
Although these results are generally taken as evidence of amplified envelope encoding in the cortex (the outer layer of the brain) and weakened encoding in the midbrain and brainstem, additional evidence suggests exaggerated envelope responses may also occur at deeper levels of the brain, in subcortical auditory pathways, though only for low modulation rates.
A better understanding of how temporal envelope encoding changes—how our brains process patterns—with aging and hearing loss, and when neural responses are exaggerated versus diminished, can aid in developing treatments and devices to improve age- and hearing loss-related hearing difficulties.
Emerging Research Grants scientist Carolyn McClaskey, Ph.D., is a research assistant professor in the department of otolaryngology–head and neck surgery at the Medical University of South Carolina. Her 2023 grant, renewed in 2024, is generously funded by Royal Arch Research Assistance.
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