When someone talks to themselves, it can often look as if they are not quite all there, mentally. The image of a person carrying on a conversation when no one else is around can be seen as a symptom of some form of mental illness, but it’s a practice that benefits the brain.
How the brain processes speech is a frequent subject of neuroscience research and a study by scientists at the University of New South Wales suggests that the inner dialogue that most of us practice in our heads, impacts the way the brain works.
Associate Professor Thomas Whitford at UNSW is the first author of the work published in the journal eLife, and he explained that when we speak out loud, the brain creates an “efference copy” of the electrical signals that tell the mouth, tongue, and lips to move and talk. Speaking out loud is external speech, the silent words in the brain are inner speech. The team in New South Wales wanted to know if an efference copy was also produced by the brain when an internal monologue was going on, the way it is when words are spoken aloud.
Having this internal copy of words and sounds that we are about to speak helps the brain predict what it will hear and then process that sound by identifying it as our own words and voice. It’s thought that when people with certain mental illnesses have auditory hallucinations, i.e., hearing voices, that it’s is a disruption in the brain concerning recognizing our inner speech as our own. People who suffer from schizophrenia and report hearing voices fully believe that what they hear is a separate entity from themselves. They are likely not able to understand that these auditory hallucinations are just their own inner monologue, and their brain responds to that with stress and fear since they don’t recognize the voices as being self-generated.
To see if the brain worked the same way with silent inner speech as it does with external speech, the team recruited healthy volunteers to undergo electroencephalogram (EEG) screenings. While their brain activity was being recorded, they listened to speech sounds via headphones. The participants were asked to mentally repeat the same sound or a different sound, and the EEG recorded the brain’s processing of these exercises. As it does when a person speaks out loud, the brain did make an efference copy of the speech sounds that the study subjects mentally repeated. When they were asked to imagine speaking a sound that was not the same as the one they were hearing, there was a different pattern of brain activity, because the brain has to work harder processing the unexpected.
Professor Whitford explained how the brain processes words it expects and those it doesn’t, stating, "The efference-copy dampens the brain's response to self-generated vocalizations, giving less mental resources to these sounds, because they are so predictable. This is why we can't tickle ourselves. When I rub the sole of my foot, my brain predicts the sensation I will feel and doesn't respond strongly to it. But if someone else rubs my sole unexpectedly, the exact same sensation will be unpredicted. The brain's response will be much larger and creates a ticklish feeling."
Being able to distinguish the brain activity that happens during inner speech and external speech could be a significant step forward in treating patients who have a mental illness that includes auditory hallucinations. Professor Whitford summarized the findings, stating, "We all hear voices in our heads. Perhaps the problem arises when our brain is unable to tell that we are the ones producing them." Check out the video below for more information on the voices we all have inside our minds.