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."
This study showed that inner speech produces a similar efference-copy.
The researchers found that, just as for vocalized speech, simply imagining making a sound reduced the brain activity that occurred when people simultaneously heard that sound. People's thoughts were enough to change the way their brain perceived sounds. In effect, when people imagined sounds, those sounds seemed quieter."By providing a way to directly and precisely measure the effect of inner speech on the brain, this research opens the door to understanding how inner speech might be different in people with psychotic illnesses such as schizophrenia," says Associate Professor Whitford.