Whether you're reading a book to yourself, replaying a conversation that you had with your boss, or recalling something at the grocery store that you forgot — "remember to buy cereal!" — most of us 'hear' an internal monologue inside our heads on a regular basis. But how exactly do we hear this inner voice, when there's no actual sound? It turns out that science has an answer.

The voice that you hear in your head when you go about your day is called "inner speech" by those who study it. And, when it comes to what's happening in our brain and our bodies during inner speech, there are actually a lot of similarities between the words that we say out loud and the voice we hear in our head.

What happens physically during inner speech?

Muscles in your larynx move when you speak out loud. But researchers have also uncovered that tiny muscular movements happen in the larynx when you talk to yourself silently in your head, too. They are only detectable via sensitive measuring techniques like electromyography, however, which is probably why you're not even aware of them.

Thoughty2 - youtube.com

It gets even stranger though. The area of the brain that is active when we speak out loud — the left inferior frontal gyrus, also known as Broca's area — is also active when we 'speak' in our heads. What's more, scientists have shown that disrupting this region of the brain can interfere with our ability to engage in inner speech, much like it can interrupt our ability to speak audibly. This is probably because it's performing a similar function for our bodies, whether we're speaking out loud or just talking to ourselves silently.

Thoughty2 - youtube.com

"Broca’s area has always been viewed as very important for the articulation of speech, but until now its precise role was unclear," said Dr. Nathan E. Chrone, an associate professor of neurology at Johns Hopkins, in a statement on his research on the function of Broca's area to human speech. "We found that rather than carrying out the articulation of speech, Broca’s area is developing a plan for articulation, and then monitoring what is said to correct errors and make adjustments in the flow of speech."

Why our body's physical actions are so similar, regardless of whether we're speaking out loud or inaudibly in our heads, is still unclear.

But we are gaining a better understanding of how we can tell what voices are our own — whether internal or spoken — versus the voices of other people. That process has to do with a brain signal called "corollary discharge." As researcher Mark Scott of the University of British Columbia explains:

"We spend a lot of time speaking and that can swamp our auditory system, making it difficult for us to hear other sounds when we are speaking. By attenuating the impact our own voice has on our hearing — using the ‘corollary discharge’ prediction — our hearing can remain sensitive to other sounds."

Corollary discharge is a essentially copy of a motor signal which allows us to predict our own movements, including vocalizations, and which tells us that we're the ones moving or speaking rather than some someone else. This is why other people can tickle us but we can't tickle ourselves: Our brain determines that another person tickling us is external stimuli and generates a response, whereas if we tickle ourselves, our brain knows that it's an internal stimuli that can be ignored. It's also thought that a malfunction in this process is part of what differentiates those that "hear voices" from everyone else who can distinguish their inner voice as "theirs."

Want to learn more? Watch the video below to find out more about inner speech: