We spend a lot of time listening to ourselves talk inside of our heads. We listen to our inner voice each time we read a book, decide what to have for lunch, or imagine how we are going to get our view across in a meeting.
In fact, estimates suggest that we spend at least a quarter of our waking hours attending to our inner voice.
But how is the brain able to tell the difference between the inner voice and the external voice that makes the sounds that other people hear?
Tools exploring this area of brain processing could be very useful to improving our understanding of disorders that involve "hearing voices," such as schizophrenia.
Schizophrenia and hallucinations
Schizophrenia is a long-term, severe, and disabling mental illness that affects around 1.1 percent of adults in the United States. In most cases, symptoms start in late adolescence or early adulthood.
The disorder disrupts thinking, emotions, perception, and behavior, making it hard to get along with people, keep a job, and manage day-to-day life. Experts suggest that this highly complex disease is probably a collection of several disorders.
A common symptom of schizophrenia is "auditory verbal hallucinations," or hearing voices that other people do not hear. For some time, it has been assumed that these hallucinations are a result of problems with inner speech.
"This study," explains first study author Thomas Whitford, an associate professor in the School of Psychology at the University of New South Wales in Australia, "provides the tools for investigating this once untestable assumption."
'Efference copy dampens brain response'
When we speak our thoughts out loud, our brain sends out instructions that tell the vocal chords, tongue, and lips how to move to produce the intended sound. When this happens, the brain also makes a copy of the instructions. This is known as the "efference copy."
The efference copy allows the brain regions that process sound to predict what they are about to receive. If the efference copy matches the sounds that we actually hear when we speak, then the sound processing response is more subdued.
"The efference copy dampens the brain's response to self-generated vocalizations," says Prof. Whitford, "giving less mental resources to these sounds, because they are so predictable."
He explains this effect with another example: trying to tickle ourselves feels much less ticklish than when someone else does it. When we try to tickle ourselves, the brain produces an efference copy of the instructions to the fingers, allowing it to predict the exact sensations, resulting in a somewhat dampened response.
But if someone else does the tickling, there is no reference copy, and the unpredicted act — even if it involves the same sensations — creates a bigger response in the brain and the feeling of being tickled.
Does inner speech make efference copies?
The purpose of the new study was to find out whether or not the brain makes an efference copy for inner speech in the same way that it does for words that are spoken out loud.
The team recruited 42 healthy volunteers and used electroencephalography (EEG) to track their brain activity as they listened to vocal speech on headphones.
As they listened, the participants had to make either the same or a different sound in their heads to the sound that they could hear.
The EEG recordings showed that a specific type of brain response was "dampened" when the inner speech sound was the same as the external one. However, when the inner speech sound did not match the external one, the dampening did not occur — the response was much bigger.
The researchers conclude that this indicates that the brain makes an efference copy for inner speech in the same way as it does for external speech.
They suggest that the method they used in the study could be useful for exploring what happens in the brain during auditory verbal hallucinations.
"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."
Prof. Thomas Whitford