A newly published study in Trends in Hearing provides evidence that age-related hearing difficulties experienced by people with clinically normal hearing may arise primarily from declining inhibition in the brain rather than loss of connections between sensory hair cells and auditory nerve fibers in the inner ear.
Hearing Difficulties with Age
As humans age, many report increasing difficulty understanding speech amid background noise and rapidly changing sounds, even when they have normal hearing thresholds on standard clinical audiometric evaluations.
Researchers have proposed two key factors that may contribute to these age-related auditory deficits:
- Cochlear synaptopathy – progressive loss of synaptic connections between hair cells in the cochlea and auditory nerve fibers. This is thought to degrade encoding of sound details sent to the brain.
- Reduced inhibition – decline in inhibitory neurotransmitters like GABA in central auditory pathways. This could impair temporal processing and the ability to filter out distractions.
The relative contribution of these two mechanisms to common auditory complaints has been unclear.
About the Study
To address this question, a team of researchers from Spain and Germany led by Dr. Enrique A. Lopez-Poveda tested 30 adults aged 21-59 years old with clinically normal audiometric thresholds and no cognitive impairment.
The researchers measured participants’ ability to detect brief silent gaps between sounds, recognize speech amid steady background noise, and detect small changes in tone frequencies. Performance on all these basic auditory skills worsened with age.
The team also measured wave I of the auditory brainstem response (ABR) to rapid click sounds. The amplitude and slope of the wave I potential reflects activity in auditory nerve fibers and has been used as an indicator of cochlear synaptopathy. As expected, wave I responses declined with increasing age, consistent with loss of connections between hair cells and auditory nerve fibers.
Finally, participants completed the Stroop color-word test, a standard assessment of the ability to inhibit cognitive interference when processing competing visual inputs. Performance on this test of inhibition deteriorated with age.
Key Findings
The critical finding was that speech recognition and auditory temporal processing abilities were closely associated with performance on the Stroop test but not ABR wave I slope.
After accounting for individual differences in Stroop inhibition scores, age was no longer a significant predictor of speech recognition thresholds or gap detection performance.
This suggests that declining cognitive inhibition, rather than loss of cochlear nerve fiber connections, may be the primary driver of difficulties understanding speech amid noise and processing rapid acoustic changes as we age – even when hearing sensitivity is clinically normal.
In particular, the extent of cochlear synaptopathy, as indirectly assessed via ABR, was not significantly associated with real-world functional auditory abilities on the tasks measured.
Implications
These findings suggest that enhancing inhibitory neurotransmitters such as GABA in the central auditory system could offer an effective strategy to enhance hearing abilities among older individuals. Instead of solely relying on hearing aids for peripheral amplification, therapies aimed at improving central inhibitory function might significantly assist older adults with normal audiometric thresholds in navigating challenges related to understanding speech in noisy settings.
The study highlights the crucial role played by central auditory processing mechanisms and cognitive functions, underscoring that successful auditory perception in older age goes beyond peripheral hearing sensitivity. Age-related auditory complaints seem to be strongly associated with declining cognitive inhibition, emphasizing its significance in hearing-related challenges.
Although further investigation is necessary, the study implies that strengthening inhibitory capacity in the brain might be as crucial as addressing the decline in cochlear sensory connections when addressing age-related hearing decline. This research outlines the substantial impact of reduced cognitive inhibition on the common hearing difficulties experienced by aging adults, suggesting that enhancing central inhibition could be a promising target for improving real-world listening abilities and overall quality of life in older age.
However, certain limitations should be considered. The study’s sample size was relatively small, involving only 30 participants. Additionally, the assessment of cochlear synaptopathy and cognitive inhibition relied on indirect proxy metrics rather than direct measurements, signaling the need for further research in larger populations using more direct quantification methods.
Reference:
- Gómez-Álvarez M, Johannesen PT, Coelho-de-Sousa SL, Klump GM, Lopez-Poveda EA. The Relative Contribution of Cochlear Synaptopathy and Reduced Inhibition to Age-Related Hearing Impairment for People With Normal Audiograms. Trends in Hearing. 2023;27. doi:10.1177/23312165231213191
