Cognition, language and the brain of deaf individuals

This article was originally published in the 2022 magazine of the British Association of Teachers of the Deaf (BATOD)

Being a researcher who studies the brain, I’m frequently asked whether the brains of deaf and hearing people are the same. To answer this fully would take a whole book, but in a nutshell, the overall structure is the same, and for most part, the way it works is also the same. There is, however, one part that is different in function: the auditory cortex.  This is the part of the brain that allows hearing people to experience and understand sounds. In people who are deaf from birth, this hearing part of the brain, rather than being redundant, is known to be used for other functions such as vision and touch. However, our recent work at DCAL suggests that, in deaf individuals, the function of the “auditory cortex” goes beyond vision and touch. In our latest study, we show that the auditory cortex of deaf individuals is also involved in higher-order cognitive functions, which are the mental processes that allow people to learn and interact with the world.  In this research, we also found that language skills, either in sign or spoken language, predict performance in cognitive tasks, highlighting the importance of early access to good-quality  language for the development of cognitive skills. 

One way to study the human brain is to use Functional Magnetic Resonance Imaging (fMRI). This technique allows us to measure brain activity in humans in a non-invasive way. In our recent work, we wanted to understand if deaf people use the auditory cortex for cognitive processes we call ‘executive functions’. In simple terms, executive functions can be defined as the mental processes that we use for organising our thoughts and actions, generally with a goal in mind. Everyday examples of this could be remembering and following the rules of a playground game, or deciding when to cross the street at a busy junction. To study executive functions, we did a study where deaf and hearing people took part in some standard psychology tasks while we scanned their brains in an MRI scanner.  

The tasks tested four executive functions: switching, working memory, planning and inhibition. Switching refers to the ability to adapt to changes in the rules of a task. Here, people were asked to follow a set of button-pressing rules, and from time to time, the rules will change and people will have to quickly adapt to keep scoring correctly. Working memory is the cognitive ability that we use to keep and manipulate information in our mind. In this task, people were shown three displays, one at a time, and their task was to remember all of them in combination. Planning allows us to achieve goals by playing in our mind the steps and consequences of certain actions. To test this, we used a task called Tower of London, where participants had to plan how to move beads to a target configuration following a set of rules. Inhibition in this case refers to the ability to stop a default response. We tested this with a task where some items elicited responses that are default or more automatic, while other items required responses that are less automatic.

In our analysis of the brain data, we found that the auditory cortex of deaf individuals was activated during the switching task, but not during the other tasks. Switching is not a function typically located or found in the auditory cortex in hearing individuals. These findings show that the brain adapts its function to the sensory experience of the person. This is an exciting finding, because it changes the way we think about the function of different brain regions. It shows that the function of different brain regions is not ‘fixed’ and that it is very much influenced by sensory experience.  We also found a link between how much activity there was in the auditory cortex during the switching task, and how fast people responded. This link was only found in deaf participants, and not in the hearing group. This is important, because it shows that our brain findings are relevant for people’s behaviour.

These findings are important for our understanding of the brain, but perhaps, for teachers of the deaf, the most interesting results of this study come from our behavioural tests. We know that deaf individuals have different language-learning trajectories, and in many cases, deaf people have a late exposure to accessible language.  In some cases, this late language access results in language deprivation. Previous research has shown that language proficiency is linked to how well people perform in executive tasks, so we wanted to take this into account in our study. The issue is that deaf individuals are, in many cases, bilingual in a spoken and a sign language. In the UK, often (but not always),  those languages are English and BSL. Measuring proficiency in one language is not enough, as people can have limited BSL but full English or vice versa. To account for this, we tested participants whose first language was BSL or English, and we measured grammar skills in both these languages. We then chose the value of the best language, so that we had d a measure that reflected the person’s best language, independently of whether it was spoken or signed.

We did not find links between brain activity and language during our tasks. However, what we did find was that participants who did better in the grammar tasks performed better in the switching task. This link between language skills and other aspects of cognition highlights the importance of having access to language from an early age. It shows that accessible language is not only important for language development, but also for many other aspects of cognition. What is interesting about our study is that it shows that this link exists independently of the type of language. It does not matter if your language is spoken or signed, what matters is access to language, so that it can support the development of other languages and cognitive functions. 

Link to our research paper:

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