In a new study, scientists investigated the mechanism behind our silent internal dialogue (talking to ourselves in our minds) in order to gain a better understanding of how patients with psychosis might hear voices.
First author Dr. Thomas Whitford, an associate professor at the University of New South Wales (UNSW) School of Psychology, said it has long been thought that the auditory-verbal hallucinations experienced in psychosis may arise from abnormalities in our silent internal dialogue.
“This study provides the tools for investigating this once untestable assumption,” says Whitford.
Previous work has shown that when we get ready to talk out loud, our brain creates a copy of the instructions that are sent to our lips, mouth and vocal cords. This copy, known as an efference-copy, is sent to the part of the brain that processes sound to help predict what sound it is about to hear.
This allows the brain to tell the difference between the predictable sounds that we have made ourselves, and the less predictable sounds that are made 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,” Whitford said.
“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.”
For the study, the researchers wanted to determine whether inner speech, an internal mental process, elicits a similar efference-copy as the one that is created when we speak out loud.
The researchers developed a new method for measuring the purely mental action of inner speech. In 42 healthy participants, the researchers measured the degree to which imagined sounds interfered with the brain activity elicited by actual sounds, using electroencephalography (EEG).
The findings show that, just as for vocalized speech, simply imagining making a sound reduced the brain activity that occurred when people simultaneously heard that sound. In other words, people’s thoughts were enough to change the way their brain perceived a sound. 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,” said Whitford.
“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.”
The new findings are published in the journal eLife.
Source: University of New South Wales