In this video, Dr Russel Barkley states the following:
I can put electrodes all over your face and I can ask you to cite your Canadian pledge of allegiance in your mind. You still move your face. The muscle movements are so small that they're inperciptible to you and to others. But they're measurable. The human face is moving any time you engage in verbal thought. So is your larynx, by the way.
I am having a hard time locating a source for this though, and have never heard of it. Does anyone know if this is true?