Deafness and Neural Plasticity Lab

A link between dementia and hearing loss in older adults has been suggested by previous research (1). In a recent article (2), I reviewed the human neuroscience evidence on the effects that hearing loss and ageing have on brain structure and function, to ultimately try to understand why hearing loss is associated with poorer cognition only in older adults. The original post was published in Soundbite (December, 2016), the e-newsletter of  Action on Hearing Loss/RNID.

Why is understanding speech in noisy backgrounds so difficult?

We’ve all been there – noisy pub, impossible to follow the conversation… You might need to pop outside to have a fair chance of talking to your friend, or just shout into each other’s ears. As older people know well, this gets worse as you age – understanding conversation starts becoming challenging at dinner parties and you keep on turning up the volume on the TV.

Difficulties in understanding conversations in noisy conditions are a reality for most older people, but why is this? Clearly, one of the explanations is that your ears are not as good as they used to be. We know that on average, tests that evaluate the function of your ears (audiograms) show that younger people can hear quieter sounds than older people. However, this doesn’t explain all the difference. Studies have shown that older adults with identical results in hearing tests vary greatly in their ability to understand speech against a noisy background (3). How can this be possible? One of the reasons is that a lot of the process of understanding speech takes place in the brain. In other words, your ability to understand speech is dictated by the health of your ears, but also by the health of your brain.

How does this work? Usually, when listening to a conversation, many parts of the brain are involved. You engage not only the part of the brain known as the auditory cortex, which is involved in processing sounds, but also other brain regions that help in extracting the meaning of words, understanding what information is important, deciding what to pay attention to and how to respond.

Now, when younger adults are in noisy environments, such as a pub on a Saturday night, additional effort is necessary to listen and understand successfully. This additional effort in brain terms involves strongly engaging frontal areas of the brain that are involved in deciding what information is relevant and what actions to deploy in response to it (4).

As we get older, it’s harder to listen all the time

What seems to be an issue for many older people is that these additional frontal areas are engaged for longer periods or all the time while listening, and not only in noisy environments. This means that for older adults all listening involves additional effort. The reason for the constant engagement of these additional resources could be age-related damage to the auditory cortex, as studies have shown that damage in this region is associated with extra usage of frontal areas during speech comprehension (5). The trouble is that hearing loss is also associated with structural damage in the auditory cortex, and also involves additional engagement of the frontal regions (6). As a consequence, mental effort (referred to in the scientific literature as ‘cognitive load’) in older adults with hearing loss is constantly extremely high, deeming all listening effortful, and reducing the amount of spare resources that the brain has for other tasks, such as memory or planning.

What is most troubling is that there is a link between hearing loss in older adults and dementia (1), making the effects of aging on cognitive decline worse. Scientists are still trying to understand the reasons for this, but one possibility is having to direct excessive cognitive resources to understanding speech. This constant effortful listening means that the available cognitive resources are reduced, and this might be what accelerates cognitive decline in older adults with hearing loss.

Therefore, hearing loss in older age is not only problematic because it makes communication with our friends and family more difficult, but also because it could accelerate dementia, potentially through the increased brain effort that has to be devoted to understanding sounds and, crucially, speech.

Early diagnosis and treatment could help

Many of the detrimental effects of hearing loss on brain function are found in the early stages, when losses are mild. Therefore, we should aim efforts at preventing or treating hearing loss at the very early stages. It would be interesting to investigate whether early diagnosis and treatment of hearing loss can prevent the cognitive decline associated with hearing loss in older age. In the meantime, to make things easier for your brain, you can get your hearing checked, get hearing aids if needed, and when possible, turn off the background noise!

References

1. Lin, F. R., Metter, E. J., O’Brien, R. J., Resnick, S. M., Zonderman, A. B., & Ferrucci, L. (2011). Hearing loss and incident dementia. Arch Neurol, 68, 214–220.
2. Cardin, V. 2016. Effects of aging and adult-onset hearing loss on cortical auditory regions. Front Neurosci. 10: 199.
3. Schneider, B. A., Daneman, M., & Pichora-Fuller, M. K. (2002). Listening in aging adults: from discourse comprehension to psychoacoustics. Canad J Exp Psychol, 56(3), 139–52.
4. Erb, J., & Obleser, J. (2013). Upregulation of cognitive control networks in older adults’ speech comprehension. Front Syst Neurosci, 7, 116.
5. Harris, K. C., Dubno, J. R., Keren, N. I., Ahlstrom, J. B., & Eckert, M. a. (2009). Speech Recognition in Younger and Older Adults: A Dependency on Low-Level Auditory Cortex. J Neurosci, 29(19), 6078–6087.
6. Peelle, J. E., Troiani, V., Grossman, M., & Wingfield, A. (2011). Hearing loss in older adults affects neural systems supporting speech comprehension. J Neurosci, 31, 12638–12643.