The brain's "inner voice" begins to talk when listening to "boring" speakers, a study has found.
The brain "talks over" a monotonous speaker to make their quotes more vivid, researchers from the University of Glasgow's Institute of Neuroscience and Psychology found.
The scientists studied 18 participants and scanned their brains using functional magnetic resonance imaging (fMRI) while they listened to audio clips of short stories containing direct or indirect speech quotations.
The direct speech quotations were either spoken "vividly" or "monotonously" and the results showed that listening to monotonous quotations increased brain activity.
Dr Bo Yao, the principal investigator of the study, said: "You may think the brain need not produce its own speech while listening to one that is already available. But, apparently, the brain is very picky on the speech it hears.
"When the brain hears monotonously-spoken direct speech quotations which it expects to be more vivid, the brain simply 'talks over' the speech it hears with more vivid speech utterances of its own. By doing so, the brain attempts to optimise the processing of the incoming speech, ensuring more speedy and accurate responses."
Previous research has found that the brain "talks" when silently reading.
Professor Christoph Scheepers said: "Direct speech quotations are generally assumed to be more vivid and perceptually engaging than indirect speech quotations as they are more frequently associated with depictions of voices, facial expressions and co-speech gestures.
"When the brain does not receive actual stimulation of auditory speech during silent reading, it tends to produce its own to enliven written direct speech quotations - a phenomenon commonly referred to as the 'inner voice' during silent reading. Now it appears the brain does the same even when listening to monotonously-spoken direct speech quotations."
The study, titled Brain "Talks Over" Boring Quotes, is published in the NeuroImage journal this month.