Wendy Van Zuijlin in Phys.Org:
New research showing that talking to ourselves in our heads may be the same as speaking our thoughts out loud could help explain why people with mental illnesses such as schizophrenia hear voices. As far our brains are concerned, talking to ourselves in our heads may be fundamentally the same as speaking our thoughts out loud, new research shows. The findings may have important implications for understanding why people with mental illnesses such as schizophrenia hear voices. UNSW Sydney scientist and study first author Associate Professor Thomas Whitford says it has long been thought that these auditory-verbal hallucinations arise from abnormalities in inner speech – our silent internal dialogue. "This study provides the tools for investigating this once untestable assumption," says Associate Professor Whitford, of the UNSW School of Psychology. Previous research suggests that when we prepare to speak out loud, our brain creates a copy of the instructions that are sent to our lips, mouth and vocal cords. This copy is known as an efference-copy. It is sent to the region of the brain that processes sound to predict what sound it is about to hear. This allows the brain to discriminate between the predictable sounds that we have produced ourselves, and the less predictable sounds that are produced by other people.
"The efference-copy dampens the brain's response to self-generated vocalisations, giving less mental resources to these sounds, because they are so predictable," says Associate Professor Whitford. "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." The study, published in the journal eLIFE, set out to determine whether inner speech – an internal mental process – elicits a similar efference-copy as the one associated with the production of spoken words.