Inner speech speaks volumes about the brain

July 22, 2013

Inner voices, by James Tissot (credit: Wikimedia Commons)

Do you talk to yourself? If so, researcher Mark Scott of the University of British Columbia can help. He’s found evidence that a brain signal called corollary discharge plays an important role in these experiences of internal speech.

This is a signal that helps us distinguish the sensory experiences we produce ourselves from those produced by external stimuli.

It’s a kind of predictive signal generated by the brain that helps to explain, for example, why other people can tickle us but we can’t tickle ourselves. The signal predicts our own movements and effectively cancels out the tickle sensation.

This explains why we don’t overload our brain when we speak.  “By attenuating the impact our own voice has on our hearing — using the ‘corollary discharge’ prediction — our hearing can remain sensitive to other sounds,” Scott said.

But Scott also speculated that the internal copy of our voice produced by corollary discharge can be generated even when there isn’t any external sound, meaning that the sound we hear when we talk inside our heads is actually the internal prediction of the sound of our own voice.

Curiously, Scott found that the impact of an external sound was significantly reduced when participants said a syllable in their heads that matched the external sound. Their performance was not significantly affected, however, when the syllable they said in their head didn’t match the one they heard.

These findings provide evidence that internal speech makes use of a system that is primarily involved in processing external speech, and may help shed light on certain pathological conditions.

“This work is important because this theory of internal speech is closely related to theories of the auditory hallucinations associated with schizophrenia,” Scott concludes.

This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada to Bryan Gick, Janet F. Werker and Eric Vatikiotis-Bateson.