The case of a woman who started hearing speech-impaired voices after a bike accident, has lent support to the idea that auditory hallucinations can be caused by people misidentifying their own inner voice as not belonging to them.
The 63-year-old woman was unconscious for a week and was only able to communicate in short phrases after coming around, probably because of the extensive damage she suffered to the left-hand side of her brain (a condition known as aphasia).
A few months later she developed signs of epilepsy and started hearing voices that weren't there – at first the voices sounded like her own but from the outside, then she heard the voices of hospital staff. In both cases, these hallucinatory voices spoke in very simple and short sentences mirroring her own real-life speech deficit. When her seizures were controlled with drugs, her auditory hallucinations stopped, suggesting epilepsy was the underlying physiological cause of her hearing voices.
The fact the woman heard voices that shared her speech impairment provides unique support for the popular hypothesis that people with psychosis who hear voices do so because they are misidentifying their own inner speech as coming from outside the self.
Hubl, D., Hauf, M., van Swam, C., Muri, R., Dierks, T. & Strik, W. (2007). Hearing dysphasic voices. The Lancet, 370, 538.