For people with hearing loss, trying to hold a conversation in a noisy environment, such as a busy cafe, can feel exhausting. But why is this?
When someone has a hearing loss, the signals that the ears send to the brain become less clear. It’s like reading text that is out of focus – everything looks blurry. The brain may have to work harder to make sense of these unclear signals. Our research aims to understand what goes on in a person’s brain as they struggle to understand speech.
Measuring brain activity with infrared light
We measure brain activity using a technique called functional near-infrared spectroscopy (fNIRS). fNIRS works by shining infrared light into the head to measure how much oxygen the brain is using. This can tell us how hard different parts of the brain are working – the more oxygen they use, the harder they are working. fNIRS opens up exciting new possibilities for studying how the brain copes with hearing loss, because it allows us to see what goes on in a person’s brain as they listen through devices like hearing aids or cochlear implants in a natural way.
Mimicking hearing loss in hearing volunteers
As a first step, we wanted to test whether fNIRS could measure if a person’s brain was more active when they listened to speech in difficult conditions. To do this, we tested young volunteers who did not have hearing loss. The volunteers listened to speech that was either clear, or that had been digitally modified to mimic what someone with a hearing loss might hear. While the clear speech was very easy to understand, the modified speech was much harder to understand, especially at first. With practice, volunteers were able to understand about 85% of the modified speech.
A part of the brain that works harder when listening gets tough
Based on earlier research, we had some clues about which parts of the brain have to work harder when listening gets tough. We were particularly interested in a part of the brain called the left inferior frontal gyrus (LIFG), located towards the front of the head on the left-hand side. We found that the LIFG was more active when people listened to the modified speech (which was hard to understand) than when they listened to the clear speech (which was easy to understand). This suggests that the LIFG may work harder to help us uncover the meaning of speech that is difficult to understand.
The critical role of attention
Interestingly, we saw more brain activity in the LIFG only when people paid attention to and tried to understand the modified speech. If they ignored the speech and listened instead to another (non-speech) sound we played in the background, brain activity in the LIFG did not increase. This suggests that the hard work being carried out within the LIFG reflects the conscious effort a person is using to try to understand what they are hearing.
What does it all mean?
Our findings confirm that the LIFG is more active when people pay attention to speech that is difficult to understand. We have also shown that we can measure this using fNIRS, a promising new technique that is compatible with hearing aids and cochlear implants.
However, many questions remain. How exactly does this part of the brain work to help us understand speech in difficult listening situations? How does brain activity in the LIFG relate to the sense that listening is more or less ‘effortful’? Will we see the same patterns in people who have a real hearing loss, and do hearing aids or cochlear implants make things better or worse?
Dr Ian Wiggins, who led the research at the National Institute for Health Research (NIHR) Nottingham Biomedical Research Centre, has recently been awarded a 12-month Pauline Ashley Fellowship from Action on Hearing Loss. Ian will use his fellowship to study brain activity in people who use hearing aids.
At the same time as measuring brain activity, Ian will use an eye-tracking camera to record the size of the pupils. The pupils grow larger when we perform complex mental tasks, so the size of the pupils can be used as a measure of how much ‘listening effort’ a person is putting in in order to understand speech. Ian will look at how people with hearing aids are affected by realistic levels of background noise, as we all encounter in daily life.
He aims to understand how the brain has to work harder to compensate for the hearing loss, and how this relates to people’s everyday experiences of listening effort and fatigue. Improving our understanding of these issues is crucial to ensuring that future hearing aids can be designed to not just make sounds audible, but rather to make listening as easy as possible on the brain and the person.
Find out more
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The research described here was published recently in the journal Hearing Research. You can read the full text on ScienceDirect (link opens in a new window).
You can also find out more about the research we’re funding in our biomedical research section.
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