A region of the brain known as auditory cortex is involved in processing and understanding sounds in hearing people. In our latest paper, we show that in deaf people the auditory cortex is involved in what we call working memory – a cognitive process that allows us to manipulate, store and update information. This is very exciting, because it shows that regions of the brain can change their function from sensory processing to higher-order cognitive functions, meaning that the brain has more potential for change that we previously thought.
This blog explains our experiment and findings, and highlights its relevance.
STUDYING DEAFNESS AND THE BRAIN
One of the reasons we are interested in studying the deaf brain is that we can contribute evidence to guide and improve health and education for deaf individuals, and to promote Deaf Awareness. However, studying deafness can also tell us a lot about how the brain works, and what the brain has the potential to do. Why is this? The brain is the organ that we use to make sense of all the information that we get through our eyes, ears and touch. For example, all the images that reach our eyes are processed in what we call the visual cortex. In hearing people, the part of the brain that is involved in making sense of understanding sounds is called auditory cortex. But what happens to the auditory cortex in deaf people? Does is not get used at all or does it do something else? If the latter, what is its function? These are the kinds of questions that they deaf brain can help us to answer.
“AUDITORY CORTEX” AND THE DEAF BRAIN
Previous research, from our group and others, shows that in deaf people the auditory cortex is involved in making sense of our visual and somatosensory (touch) world. But how does this happen? What changes in the brain so that a region that is involved in hearing starts responding to vision and touch?
One of the theories is that, well… not much is changed. I know this seems a bit strange, so I’ll give you an example. We know that there are specific parts of the brain that process different attributes of a sensory signal – for example, certain parts of the brain are involved in understanding the location of sounds, others in processing sound movement. What previous studies have shown is that, in deaf individuals, those regions that are involved in processing location and movement of sounds are involved in processing location and movement of visual images. Therefore, those parts of the “auditory cortex” still have the same function – location and movement – but now in a different sensory modality (vision instead of hearing). In short, this evidence suggest that brain function is preserved.
How could this be the case? An everyday analogy could be that of a food processor. In a food processor, whenever you put carrots or tomatoes, it will give you slices. A food processor doesn’t care about the identity of the food, they will treat it as something that needs to be sliced. In the same way, you could think about neurons in the brain not caring about what the information is or where is coming from, they see it as electrical signals. In this way, those neurons will be identifying movement, visual or auditory.
CAN THE BRAIN CHANGE ITS FUNCTION?
However, preservation of function may only be part of the story in humans. We were interested in understanding how the deaf brain processed higher-level functions such as working memory. In our experiment, participants did a working memory task and, for comparison, they also did a colour task. We saw that what is usually called the auditory cortex was activated by the working memory task in deaf people, but not in hearing people. Furthermore, the activation was specific for working memory, but not for the control colour task.
These results are puzzling, as they suggest that instead of preserving its function, some portions of the auditory cortex may shift their role towards higher cognitive processes. What does this mean? Let’s use again our food processor analogy. Functional change means that the food processor makes slices when we put tomatoes. However, when we put carrots, it doesn’t make slices anymore; it does something else entirely, let’s say soup. The only way this can happen is by either using a different blade or a different processor altogether. In the same way, if a brain region changes what it does from auditory processing to working memory, then something has to change – either the way the neurons work, or the type of neurons that are involves.
These results are just the first step, and we still need to conduct further research to check that there is indeed change of function. We are now investigating this effect further, trying to understand if indeed the auditory cortex is involved in working memory and higher-order cognitive functions, and whether functional preservation and functional changes are co-existing phenomena of crossmodal plasticity. Get in touch if you want to volunteer to take part in this research, and stay tuned for further results!
You can also have a look at this revised version of our blog, published by Action on Hearing Loss