Some research. I doubt that very much.
Agreed. Is there a precise, experimentally-validated definition of “self talk”?
By the same token, under high levels of arousal, a person who is absorbed in thinking may find himself actually verbalizing his thoughts unintentionally--"talking to himself."
Many other symptoms of mental illness can be understood similarly as normal functions carried to excess.
This research btw goes back to the early twentieth century, when people took the scientific study of psychology seriously.