How our Brains “Hear” Inner Speech

Most people hear themself speak even if they’re not saying words out loud. They could be reading the paper or thinking through their schedule for the day.

This internal the monologue you “hear” inside your head is an ever-present but still unexamined phenomenon. A new study examines a possible brain mechanism that could explain how we hear this inner voice in the absence of actual sound.

Mark Scott, researcher at the University of British Columbia, found in two different experiments evidence that a brain signal called corollary discharge, a signal that helps us distinguish the sensory experiences we produce ourselves from those produced by external stimuli, is an important part in our experiences of internal speech.

Auditory Perception and Corollary Discharge

Corollary discharge is a kind of predictive signal made by the brain that helps to explain, among other things, 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.

The same mechanism plays a role in how our auditory system processes speech. 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,” said Scott. “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 speculates 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.

Cancelled Out Perceptions

If indeed corollary discharge does underlie our experiences of inner speech, then the sensory information coming from the outside world should be cancelled out by the internal copy produced by our brains if the two sets of information match, like when we tickle ourselves. This was the hypothesis at least.

In fact, this is exactly what the data showed. The impact of an external sound was considerably 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.

The findings offer evidence that internal speech uses a system that is chiefly 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.

The study was published in the journal Psychological Science.