Inner speech · Grey Matter

Inner speech is the experience of saying words in one's head without sound or movement, the voice people use to rehearse a conversation, read silently, count or plan, and it is the signal a speech implant may pick up when it was built to read only attempted speech.

Inner speech. Inner speech is the experience of saying words in one's head without sound or movement, the voice people use to rehearse a conversation, read silently, count or plan, and it is the signal a speech implant may pick up when it was built to read only attempted speech.

It works as a quiet form of speaking. Imaging finds that it recruits much of the network used to speak aloud, including Broca's area and parts of the motor and auditory cortex, and it supports working memory, as when a phone number is repeated silently to keep it. In 2025 recordings from arrays in the motor cortex of four people with paralysis (two with ALS, two after brainstem strokes) showed that imagined words produce patterns similar to attempted ones, only weaker; sentences were decoded from them in real time with up to 74 % accuracy on a vocabulary of about 125,000 words.

People differ widely in how much of it they have. When volunteers were interrupted at random moments and asked to describe what was in their minds, inner speech appeared in about a quarter of the moments on average, ranging from almost never to almost always between individuals.

It is separable from attempted speech. The two patterns overlap but differ enough that a decoder can learn to ignore the silent one, or to stay off until the user thinks of a chosen unlock phrase, which the 2025 system recognised more than 98 % of the time.

It could be an easier input. Attempting to speak is slow and tiring for people with partial paralysis, so a speech neuroprosthesis that read imagined words would ask less of them, at the cost that it could also catch words they never meant to share.

Inner speech is speaking with the output turned off, and the motor cortex still drafts it.

A device that reads that draft can serve the user or overhear them, which is why the safeguard has to be part of the design; see mental privacy.

Questions: Why is the motor cortex active when someone only imagines saying a word? Imagining a movement runs part of the same preparation as making it, only weaker and without the final command reaching the muscles, and speech behaves the same way. Implants in the speech part of the motor cortex of four people with paralysis found that imagined words produced patterns much like those of attempted words, at lower strength, clear enough to decode sentences from them in real time. The difference between the two is consistent enough that a decoder can tell them apart and listen only to the attempt. Why does inner speech sound like a voice when no sound is heard? When people speak, the motor areas send a copy of each command to the auditory cortex, which predicts the sound about to be heard so that one's own voice can be told apart from someone else's. In inner speech the command is prepared and the copy is sent, but the mouth never moves, so the auditory cortex is left with the prediction alone. Recordings of imagined speech show auditory activity following the motor planning by a fraction of a second, which is a plausible origin for the heard quality of the inner voice.