A central feature of psychosis is the disintegration of the sense of self. A healthy person, during the course of the day, talks to themselves, and rehashes past or hypothetical arguments with friends and family. The healthy person may even speak out loud during these episodes*, but they know all these thoughts and imagined voices are exactly that, coming from their own head, under their control. In contrast, the voices that psychotic people describe is that very often (actually, in my experience, usually) they are people that the person can identify - usually friends and family. Again in my experience, voices of parents are the most common. What's more, I've witnessed more than once a person who had badly decompensated and told me in the emergency department that he could hear his friend's voice talking to him and wasn't able to reality-test that the voice must be coming from his own brain; but as he reconstituted over several days with medication, the voice eventually became an internal monologue he was having with his friend, with full recognition by the patient that it was indeed an internal monologue - just like anyone else rehashing an argument in the shower.
An interesting study in Psychological Science by Mark Scott from UBC gives evidence that our capacity for inner speech is related to our ability to tune out our own voices when speaking; the neural correlate of this suppression is called corollary discharge. This of course leads to speculations about whether this mechanism underlies the origins of language and cognitive modernity, but psychiatrists and clinical psychologists will find it immediately interesting for another more immediately practical reason: as a way to measure and possibly target auditory hallucinations in psychosis. Of note, this study had no direct measurement of neural correlates, instead relying on the Mann effect, a phenomenon of context-dependent perception of vocal sound (McDonald-McGurk is another example.) That said, a 2011 by Greenlee et al at U of Iowa showed with intracranial electrode measurements in humans that corollary discharge in hearing speech is unsurprisingly located in the auditory cortex.
*More than once, while I was out running on what I thought was a deserted trail, I have been caught talking to myself by an alarmed trail user coming the other way. Invariably as soon as I see them I act like I was singing to myself the whole time. Somehow this never seems to comfort them.
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