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The world is a noisy place. The human brain relies on limited cognitive resources to process the massive amount of sensory information we encounter in each moment, focusing on what’s important and filtering out what’s not. When a person has a sensory issue such as hearing loss, information processing, across the board, can significantly suffer. Jonathan Peelle, Ph.D., an associate professor of otolaryngology at the Washington University School of Medicine in St. Louis, says that hearing loss or impairment is extremely common in older people—and when speech is hard to hear, the brain has to work that much harder to comprehend and remember what has been said. His work investigating the links between hearing loss and cognition has found that hearing impairment can lead to changes in frontal lobe activity. Here, he discusses the cognitive challenges involved with hearing issues, that even slight deficits in “normal” hearing levels can lead to changes in how sound is processed, and why this work may offer us new insights into how hearing may be linked to the development of dementia.
Why think hearing loss might be related to cognitive decline?
Let me back up one step because there are two independent, but related, issues to consider here. One, your brain has to work harder to understand what you’re hearing in the moment when you have hearing issues. Even if your hearing is fine, if you are in an environment with a lot of background noise, you have to work harder, as a listener, to understand what someone is saying. It takes a lot more effort. So, with hearing loss, the question we wanted to look at was how hearing loss might affect cognitive processing in general.
Nearly 80 percent of people over the age of 70 have some amount of hearing loss. What do we know about how hearing loss ties in with age-related cognitive decline?
As a listener, the challenge you face at any moment is one of acoustic challenge, which could be background noise or hearing loss. But you also have your cognitive abilities to help you. If you can’t hear something as well, your brain will try to help fill in some of those gaps. So if you have hearing loss, but you are also very high-functioning cognitively speaking, you may not struggle at all because you have a lot of short-term memory capacity and processing speed to compensate for what you have trouble hearing. For daily communication, you can function very well and remember what you’ve heard. But if you take someone with identical hearing who isn’t as high functioning cognitively, they have less ability to compensate. Without that faster processing or stronger working memory, they might struggle to understand what they are hearing or, even if they can understand it, have trouble remembering it.
A decline of cognitive abilities is kind of a general thing that happens in aging. But you will see huge individual differences from one listener to the next in both hearing ability and cognition. You need to look at both abilities.
You’ve found that people even with slight hearing deficits show differences in cognitive abilities. Did those results surprise you?
Yes. We’ve now done a couple of studies where we looked at young adults with self-reported good hearing. And, when we measure it, they are within the normal range. Yet, even though they are normal, there is still a good bit of variability. You have some people who have great hearing, some who are just fine, and others who are a little worse than the others in that category. And those people who are in the bottom of the category? Well, if they went to an audiologist to be checked, the audiologist would say they are fine. But even within this group of young adults who all have “normal” hearing, we still see the degree of hearing ability correlates with how much work the brain has to do when they are listening to speech. Those with worse hearing have brains that have to work a little bit harder. We see extra activity in the frontal lobes when they are listening to speech compared to someone whose hearing is even just a little bit better. They are 100 percent accurate on the tests we give them to make sure they understand what they are hearing. They’re not complaining about hearing difficulty. Behaviorally, they are all doing great. But we still see differences in brain activity in those who have a little more trouble hearing.
If someone can’t hear at all and needs a hearing aid, it’s not surprising that their brains would have to do a lot more work to compensate for that. For me, what’s more interesting is that even these tiny variations in hearing ability, well within the normal range, also affect how the brain processes speech. This is important because there is a lot of epidemiological-type research now showing that hearing difficulty in young people, due to listening to loud music or environmental noise exposure, are showing brain differences in processing speech. You can imagine, over time, this may have profound effects on the brain and how it handles cognitive processing.
What can we do now to help protect our hearing?
Prevention is definitely the easiest way to avoid these cognitive issues. There’s only so much you can do to rescue hearing once it’s been lost. A well-fit hearing aid can be really, really helpful but there are types of auditory processing that you just can’t make up for even with a hearing aid. For example, it can help make the sound louder but it can’t help you with a lot of the little fine-grained things the auditory system does in order to help you process sounds.
So, most importantly, what you can do is limit your noise exposure. Limiting the time you spend in loud environments, and wearing the right ear protection when you are in them, is really helpful. This includes, by the way, not listening to music over headphones really, really loudly. It might seem socially awkward for young adults to do but it can help prevent a lot of damage.
It’s important because you can’t count on reversing the damage even with a hearing aid. Protection is the number one way to address this. There are a lot of studies looking at the effects of hearing loss on cognitive processing now, and it’s well accepted that, if you have hearing impairment, your brain will just have to work that much harder to understand what you hear. If you’ve got a well-fit hearing aid that’s helping you, it’s probably going to help with your cognitive processing, but it’s not going to fix any damage that’s already been done.
As an interesting tangent: When people get fit for a hearing aid, they will be asked, “Oh, could you understand this sentence? Could you understand this word?” If the answer is yes, it’s considered a fit. But today, no one uses a measure of cognitive challenge to see if the hearing aid is really doing its job. It’s reasonable to suggest that, at some point, it would be great to incorporate measures of cognitive effort into hearing healthcare. Especially since if the aid can make hearing less effortful, you are much more likely to wear your hearing aid.
Your work has also compared speech degradation to semantic ambiguity, or situations where it’s not clear what a particular word might mean. Tell me more.
You have domain-specific regions of the brain that care more about auditory processing or visual processing, but then you also have domain-general regions in charge of higher-level executive functions like attention that deal with lots of different types of sensory tasks. Some types of language processing, like semantic ambiguity, seem to rely on these more domain-general regions that aren’t language specific. As it turns out, when speech is degraded, these domain-general regions also are activated. Our motivation was to try to show a way that hearing difficulty can interfere with things outside actually understanding the words you are hearing, whether it’s the memory of what you just heard or, in this case, understanding the deeper conceptual meaning of what you just heard. We were looking for overlaps in these higher-level brain networks—because all that extra activity, that extra processing during degraded speech or hearing loss, is happening in the frontal lobes.
Is there something special about the auditory system that might lead to cognitive decline? Or would you expect to see similar effects in the vision or somatosensory systems?
I think the specifics might be a bit different but the general principles would be the same. In fact, there’s been some work in vision, artificially degrading what people are reading, that shows that the brain has to work harder to process that information. Interestingly, we also see things like this with balance. Age-related difficulties with balance during walking is fairly common. And there’s a lot of evidence that these balance issues increase the cognitive burden on people while they’re walking. A lot of these sensory deficits all tap into higher-executive processing, which will interfere with people trying to do all the things they would like to do.
What are the implications?
Sensory difficulties, like hearing loss or vision problems, are not just about those senses. They also have effects on your overall cognitive processing and the overall effort associated with interacting with people and the world around you. So many of our interactions with others involves understanding speech and producing speech. When it’s difficult, it can limit our understanding—but also our social interactions—which likely has all sorts of other health implications.
But it’s not just people who are complaining about hearing loss. Even mild levels can affect your daily life. So broader awareness of hearing loss prevention, and the importance of regular check-ups with an audiologist, are important.
This also has implications for dementia in aging. There are large epidemiological studies that show a small—but significant—relationship between hearing loss and the development of dementia. We don’t know for sure why that is. But one possibility is that, if you have hearing loss, it changes the way your brain is processing speech. Those changes, after years and years, may reorganize your brain in ways that contributes to the cognitive difficulties. It may have long term consequences for other things you’d like to do with your brain—like planning or decision-making. By better understanding how hearing problems can lead to changes in brain structure and function, we may be able to actually find ways to reduce this risk factor for dementia.
Jonathan Peelle and Arthur Wingfield, The neural consequences of age-related hearing loss. Trends in Neuroscience 39 (2016), 486-497 doi: 10.1016/j.tins.2016.05.001