Talking to ourselves in our heads may have fundamental similarities with speaking our thoughts out loud, new research from the University of New South Wales shows. The findings could have substantial implications for understanding why people with mental illnesses such as schizophrenia hear voices.
As you read this text, the chances are you can hear your own inner voice narrating the words. You may hear your inner voice again when silently considering what to have for lunch, or imagining how a phone conversation this afternoon will play out.
Estimates suggest that we spend at least a quarter of our lives listening to our own inner speech. But to what extent does the brain distinguish between inner speech and the sounds we produce when we speak out loud?
Efference Copy Effect
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. 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,”
says Associate Professor Whitford, of the UNSW School of Psychology.
But does the inner speech in our heads also generate an efference copy?
To find out, Whitford et al. tracked the brain activity of 42 healthy volunteers as they listened to speech sounds through headphones. While listening to the sounds, the volunteers had to produce either the same speech sound or a different speech sound inside their heads.
Electroencephalography (EEG) showed that a specific type of brain activity (amplitude of the N1 component of the auditory-evoked potential elicited by the auditory phoneme) decreased whenever the inner speech sound matched the external speech sound. This decrease did not occur when the two sounds were different.
Inner speech experiment: N1 component analysis. (A) Waveforms showing the auditory-evoked potentials elicited by the audible phonemes in the Match condition (blue line), Mismatch condition (red line) and Passive condition (black line). (B) Box-and-whiskers plots showing the amplitude of the N1 component elicited by the audible phonemes in the Match, Mismatch and Passive conditions. (C) Scatterplots showing the within-subjects difference scores (in terms of N1-amplitude) for the three contrasts-of-interest in the inner speech experiment; namely Match minus Mismatch, Match minus Passive, and Mismatch minus Passive. Credit: Thomas J Whitford, et al CC-BY
This suggests that the brain produces an efference copy for inner speech similar to that for external speech.
“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.
Symptoms such as hearing voices are thought to reflect problems with producing and interpreting inner speech.
Funding for the work was provided by the Australian Research Council, National Health and Medical Research Council, and U.S. Department of Veterans Affairs.
The technique the team has developed will enable researchers to test this long-held but hitherto untestable idea. The results should increase our understanding of these symptoms and may eventually lead to new treatments.
Thomas J Whitford, Bradley N Jack, Daniel Pearson, Oren Griffiths, David Luque, Anthony WF Harris, Kevin M Spencer, Mike E Le Pelley Neurophysiological evidence of efference copies to inner speech eLife, 2017; 6 DOI: 10.7554/eLife.28197