A Canadian researcher says he's identified a kind of brain signal that could explain why we "hear" speech in our heads even in the absence of actual sound.

Internal speech a person "hears" inside their head is a ubiquitous but largely unexamined phenomenon, Mark Scott of the University of British Columbia said.

Experiments suggest a brain signal called corollary discharge, which helps us distinguish the sensory experiences we produce ourselves from those produced by external stimuli, plays an important role in our experiences of internal speech, he said.

Corollary discharge is a type 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, he said.

The signal predicts our own movements and effectively cancels out the tickle sensation.

The same mechanism is involved in how our auditory system processes speech, Scott said; when we speak, an internal copy of the sound of our voice is generated in parallel with the external sound we hear.

"We spend a lot of time speaking and that can swamp our auditory system, making it difficult for us to hear other sounds when we are speaking," Scott explained. "By attenuating the impact our own voice has on our hearing -- using the 'corollary discharge' prediction -- our hearing can remain sensitive to other sounds."

That internal copy of our voice produced by corollary discharge can be generated even when there isn't any external sound, he suggested, so the sound we hear when we talk inside our heads actually may be the internal prediction of the sound of our own voice.

Scott has published the results of his experiments in the journal Psychological Science.