By Khaleel Razak, Ph.D.
Many older adults notice changes in their ability to understand speech, which can affect quality of life. These types of changes in auditory processing affect individuals with or without measurable hearing loss. To better distinguish between the effects of hearing loss and aging, my graduate student compared cortical responses in two strains of mice with differing levels of age-related hearing loss, reporting our results in Hearing Research in October 2021.
Previous studies had already demonstrated in animal models that noise-induced hearing loss results in increased central gain, which compensates for the lower levels of input, without any concomitant improvement in temporal processing. Until our study, it remained unclear whether the same was true in the case of presbycusis (age-related hearing loss), which develops over longer time-frames. We tested temporal processing and central gain in mice with severe or mild hearing loss with age using electrophysiological responses from awake and freely moving mice.
Schematics of noise stimuli presented to mice. Credit: Rumschlag, Razak/Hearing Research
We show that severe presbycusis leads to increased gain in the auditory cortex, but with reduced temporal fidelity. Data from the mice with more moderate hearing loss demonstrated age-related changes in temporal processing without concomitant increase in cortical gain. However, in the mice with moderate hearing loss, cortical temporal processing deficits were seen only when tested with more challenging sounds (shorter gaps and shallower modulation). This indicates that even mild hearing loss with aging may result in a decline in temporal processing under challenging conditions, such as environments with increased noise.
This has implications for treating age-related changes in auditory processing in humans. Sound amplification via hearing aids or cochlear implants may be insufficient; interventions may have to be multifaceted, involving amplification devices, behavioral training, and pharmacological approaches, as well as more detailed treatment plans that identify the most appropriate time points for each type of intervention.
Our data from mice are largely consistent with EEG studies of aging humans, suggesting that the EEG measures in mice can be used as translation-relevant biomarkers to test pharmacological intervention and behavioral training studies aimed at reducing age-related auditory processing deficits in humans.
A 2009 and 2018 Emerging Research Grants recipient, Khaleel Razak, Ph.D., is a psychology professor at the University of California, Riverside. His 2018 grant was generously funded by the General Grand Chapter Royal Arch Masons International.
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