Neuroscience: Amateur Hour
Neuroscience: Amateur Hour
Episode 24: The Neuroscience of Deafness
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Citations and relevant papers are below!
CDC. Genetics of Hearing Loss | CDC. Centers for Disease Control and Prevention. Published February 18, 2015. https://www.cdc.gov/ncbddd/hearingloss/genetics.html#:~:text=50%25%20to%2060%25%20of%20hearing
Hello! My name is Barbara and this is Neuroscience: Amateur hour.
*intro music*
Hello! Today’s episode is part two of my little series on the neuroscience of language and speech. Two weeks ago we covered the neuroscience of speech production, specifically the brainstem circuit involved in the physical manipulation of our laryngeal muscles.
To review, the laryngeal motor cortex sends out a command signal for the laryngeal muscles in the throat to contract that travels down to the nucleus ambiguous in the brainstem. This brain nucleus houses the cell bodies of the motor neurons that will eventually connect directly onto the laryngeal muscles.
It’s the movements of the laryngeal muscles plus the air we expel from our lungs that results in sound coming from our mouths. We are further able to use our mouths and tongues to manipulate that sound to produce specific pitches and tones that make up the words and syllables that in turn make up language as we know it. The same language that Oliver Wendell Holmes described as the “the blood of the soul into which thoughts run and out of which they grow”.
But what happens if someone doesn’t have the ability to hear language? If they’re born deaf or they become deaf at some point in their life? How do the brain regions involved in language production and auditory processing change?
So that’s what this episode is about! The neuroscience of deafness. Let’s find out more together!
I know that there is a large variety of deafness, over 400 kinds actually but the kind I’m going to focus on today is the kind that occurs before birth - where an individual is never exposed to sound in their life. Otherwise known as congenital hearing loss.
What would be the cause of this kind of deafness? Well in a large proportion of cases it's genetic. Scientists have identified a few genes that are linked to prenatal deafness, including the gene - GJB2 which encodes a protein called connexin 26. Connexin 26 is fundamental in the functioning of the cochlea, the snail-shaped part of the ear that is responsible for translating changes in pressure (that’s what sound is) into neural signals. In fact, any gene whose mutation or alteration could result in changes in the structure or function of the ear may result in deafness. If the ears don’t work - you can’t hear haha.
But genetics are only part of the story. Prenatal deafness in a child can occur from environmental factors such as complications during the mom’s pregnancy from infections like rubella, toxoplasmosis and herpes. There’s also some drugs, known as ototoxic drugs, which can damage a baby’s hearing system before birth.
So what’s going on in the brain of deaf individuals? Scientists have been able to show that early auditory deprivation alters cortical and subcortical brain regions primarily linked to auditory and language processing which in turn results in behavioral consequences.
How does auditory deprivation alter brain structure? When a child’s brain is developing, connections from across the brain, like spiderwebs. Neurons will bridge the different brain regions involved in processing information to those involved in generating emotions etc etc. And in order for the brain to properly form the connections it needs to, we need to experience things within a “sensitive period”.
In the case of learning how to properly process sound, we need to be able to hear sounds. The sensitive period we’re talking about is when the brain is maximally plastic, maximally able to adjust and configure its structures and circuits to the individual’s experiences. In the case of auditory processing - this sensitive period lasts about three and half years.
Although plasticity remains in some, but not all children until approximately age 7. I bring this up because these kinds of experiments are super important for understanding when best to place a cochlear implant in a congenitally deaf child. More on that towards the end of the episode though.
Before I move on - let’s dive a little deeper into neuroplasticity! Neuroplasticity is fundamentally the brain’s ability to change and adapt as the result of experience. There’s two broad types. The first is functional plasticity - the brain’s ability to move functionality from a damaged part of the brain to undamaged areas. The second is structural plasticity, the brain’s ability to actually change its physical structure as a result of learning.
For a long time, it was thought this neuroplasticity was only present during early childhood development but as we understand more and more - we’ve come to know that our brains are neuroplastic, to a smaller degree, throughout our lives. And the awesome thing is that neuroplasticity is extremely variable across individuals and throughout the lifetime. Which of course makes it kind of annoying to study sometimes.
But back to how this relates to deafness. Several studies have demonstrated that there is a developmental decrease of synaptic plasticity in the auditory cortex after early deafness. The auditory cortex is the part of the temporal lobe that processes auditory information in humans and many other vertebrates. In short - early deafness means less plasticity in the part of the brain responsible for processing sound.
The organization of the auditory cortex is in its own way pretty damn cool. The primary auditory cortex which is often referred to as A1 actually has a precise topographical map of the cochlea, a tonotopic map. This means that as you move from front to back in this brain region, you’ll see a representation of sound of increasing frequency. Pretty fucking cool, right? We also see this kind of organization in the visual cortex and somatosensory cortex so it makes sense here too.
So if someone who has been deaf their entire life - this tonotopic map will not develop correctly. So what’s that brain real estate being used for? Turns out that the vision part of your brain takes over.
I found this paper from 2017 that wanted to understand what principles governed large-scale reorganization in the brain like this. They drew from findings in the visually impaired that several visual regions preserve their task specificity but might switch to tactile or auditory input. So in their experiment they asked deaf and hearing adults to discriminate between temporally complex sequences of stimuli while being imaged in an fMRI so that researchers could look at their brain activity.
They saw that the same auditory regions were activated when deaf subjects performed the visual version of the task and when hearing subjects performed the auditory version of the task. Simply put, in deaf humans, the high-level auditory cortex switches its input from sound to vision.
What????? No wasted neural real estate. The brain is the literal embodiment of that meme - the free real estate meme lol. My first question after learning this of course is - wait do deaf people see better that hearing people? It does fall in line with the well known idea that if you lose a sense, the others will get stronger to compensate and with the functional plasticity we talked about towards the beginning - that would make sense, right?
And while that idea sounds good, it turns out that deaf individuals exhibit both better and worse visual skills than hearing controls. Researchers have shown that in cases where you only look at deafness, you do see some enhancements in visual cognition. They’re not as universal as one would expect, in fact, they’re limited to the aspects of vision that are attentionally demanding and would normally benefit from auditory-visual convergence.
Think things like attentive listening - you need to pay attention to what that person is saying and look at them as well. In examples such as this where - you integrate multiple sensory modalities, a hearing impaired individual may very well have the advantage in the visual part of such a task.
But the auditory cortex is one thing - what about the brain regions that are involved in language processing?
As I’m sure you all well know - deaf individuals will sometimes learn and use sign language to communicate with others. That’s not universally true - from my understanding it depends on the resources you have throughout your life. If you’re born into a community with other deaf members, you may pick up sign language from your loved ones or learn it at school but not everyone has the resources to do that.
Sign language, and i'll be primarily talking about american sign language because that’s what I have the closest association to is incredibly cool because it is its own language, with stronger similarities to Japanese or Navajo than to english. It relies on someone seeing and processing gestures and body movements that convey letters and words and sentiments.
And because it is its own language - it's a fantastic way of studying language and language processing. If you remember our previous conversations about Broca’s area, that’s the part of the left hemisphere of the brain that is thought to be vital in producing spoken words. If you lesion this area, people aren’t able to produce words. It’s geographically very close to the regions that control the movements of our tongues and lips so that makes sense.
But in a twist, if you damage broca’s area in a deaf signer, they will in fact have trouble producing signs, or communicating to others. Those with damage to Wernicke’s area (another region thought to be involved in processing incoming language rather than producing it) have trouble comprehending signed language.
So the same brain regions are in fact necessary for language production and processing in both hearing and deaf folks. But the specialization of these cortical networks for language processing does not appear to be driven either by the acoustic requirements for hearing a spoken language or by the articulatory requirements for speaking. It seems likely then that it is the specialized requirements of language processing itself, including, for instance, composition, syntax and understanding coherent concepts that determine the final form of specialized language circuits in the brain.
Doesn’t matter if you’re hearing or you’re deaf - language circuits will form regardless of the means of communication.
The deaf community is a pretty incredible one and for a child that is born congenitally deaf, many choose to stay deaf. There are some who strongly argue against implants, arguing the point of view that deafness doesn’t need to be fixed.
But there are also some who choose to get cochlear implants - these are small electronic devices that have a microphone and speech processor that ultimately converts sound into electric impulses sent to the auditory nerve, much like a normal ear. If this cochlear implant is implanted at a young age, within that sensitive period we talked about earlier, the auditory cortex and its tonotopic map will develop like normal - to an extent and the neuroplasticity of the auditory cortex after implantation is a hot topic to this day.
But that is a bite-sized look at the neuroscience of deafness. I hope that you enjoyed the episode and you’ve learned something new! This episode and I’m gonna work on my backlog as well will be published with the transcript along with the actual episode. This episode has forced me to reevaluate my efforts to make my work accessible to everyone, including deaf individuals and I am going to work on it.
I’ve cited all my relevant sources and papers in the show notes (for some reason there are a butt ton this time) and you should keep an eye out on Instagram for some cool figures I think are pertinent.
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Happy researching! Hope to see you again!