So when you look at the mask, what you see is a projection of a character. Let’s call that your outward voice. This is also the most classic way to think about the voice, it’s a way of projecting yourself in the world. The mechanism for this projection is well understood. Your lungs contract your diaphragm and that creates a self-sustained vibration of your vocal fold, that creates a sound.
And then the way you open and close the cavities in your mouth, your vocal tract is going to transform the sound. So everyone has the same mechanism. But voices are quite unique. It’s because very subtle differences in size, physiology, in hormone levels are going to make very subtle differences in your outward voice. And your brain is very good at picking up those subtle differences from other people’s outward voices.
In our lab, we are working on teaching machines to understand those subtle differences. And we use deep learning to create a real-time speaker identification system to help raise awareness on the use of the shared vocal space — so who talks and who never talks during meetings — to increase group intelligence. And one of the difficulties with that is that your voice is also not static. We already said that it changes with every person you talk to but it also changes generally throughout your life.
At the beginning and at the end of the journey, male and female voices are very similar. It’s very hard to distinguish the voice of a very young girl from the voice of a very young boy. But in between, your voice becomes a marker of your fluid identity.
Generally, for male voices there’s a big change at puberty. And then for female voices, there is a change at each pregnancy and a big change at menopause. So all of that is the voice other people hear when you talk.
So why is it that we’re so unfamiliar with it? Why is it that it’s not the voice that we hear? So, let’s think about it. When you wear a mask, you actually don’t see the mask. And when you try to observe it, what you will see is inside of the mask. And that’s your inward voice. So to understand why it’s different, let’s try to understand the mechanism of perception of this inward voice.
Because your body has many ways of filtering it differently from the outward voice. So to perceive this voice, it first has to travel to your ears. And your outward voice travels through the air while your inward voice travels through your bones. This is called bone conduction. Because of this, your inward voice is going to sound in a lower register and also more musically harmonical than your outward voice.
Once it travels there, it has to access your inner ear. And there’s this other mechanism taking place here. It’s a mechanical filter, it’s a little partition that comes and protects your inner ear each time you produce a sound. So it also reduces what you hear.
And then there is a third filter, it’s a biological filter. Your cochlea — it’s a part of your inner ear that processes the sound — is made out of living cells. And those living cells are going to trigger differently according to how often they hear the sound. It’s a habituation effect. So because of this, as your voice is the sound you hear the most in your life, you actually hear it less than other sounds.
Finally, we have a fourth filter. It’s a neurological filter. Neurologists found out recently that when you open your mouth to create a sound, your own auditory cortex shuts down. So you hear your voice but your brain actually never listens to the sound of your voice. Well, evolutionarily that might make sense, because we know cognitively what we are going to sound like so maybe we don’t need to spend energy analyzing the signal. And this is called a corollary discharge and it happens for every motion that your body does.
The exact definition of a corollary discharge is a copy of a motor command that is sent by the brain. This copy doesn’t create any motion itself but instead is sent to other regions of the brain to inform them of the impending motion. And for the voice, this corollary discharge also has a different name. It is your inner voice. So let’s recapitulate.
We have the mask, the outward voice, the inside of the mask, your inward voice, and then you have your inner voice. And I like to see this one as the puppeteer that holds the strings of the whole system. Your inner voice is the one you hear when you read a text silently, when you rehearse for an important conversation. Sometimes is hard to turn it off, it’s really hard to look at the text written in your native language, without having this inner voice read it. It’s also the voice that refuse to stop singing the stupid song you have in your head. And for some people it’s actually impossible to control it. And that’s the case of schizophrenic patients, who have auditory hallucinations, who can’t distinguish at all between voices coming from inside and outside their head.
So in our lab, we are also working on small devices to help those people make those distinctions and know if a voice is internal or external. You can also think about the inner voice as the voice that speaks in your dream. This inner voice can take many forms. And in your dreams, you actually unleash the potential of this inner voice. That’s another work we are doing in our lab: trying to access this inner voice in dreams.