An excerpt of an excerpt:
The psychiatrist Louis Gould wanted to know whether auditory hallucinations in schizophrenia have anything to do with the phenomenon of subvocal speech. Are the experiences that schizophrenics describe as having “voices in their heads” merely the unintentional mutterings of the speech muscles? If so, why would schizophrenics happen to notice their subvocal speech while healthy people do not? Gould designed an experiment using a technique called electromyography, or EMG, which measures muscle activation through time. He gathered a group of schizophrenic and healthy patients and, one by one, recorded their vocal muscle activity. When Gould compared the EMG recordings of schizophrenic patients as they experienced auditory hallucinations to those of nonhallucinating patients, he found that, when the patients were hearing voices, their EMG recordings showed greater vocal muscle activation. This result meant that when the schizophrenics were hearing voices in their heads, their vocal muscles were contracting—they were engaging in subvocal speech.
The immediate source is Slate Magazine, which has, in turn, excerpted NeuroLogic: The Brain’s Hidden Rationale Behind Our Irrational Behavior by Eliezer Sternberg.