The Physiology of Hearing Voices in Schizophrenia

​Hearing voices in schizophrenia or auditory verbal hallucinations, is fundamentally caused by a breakdown in the brain's ability to recognize a person's own inner thoughts or speech as self-generated, combined with abnormal activity in the auditory cortex. This misattribution makes internal speech feel like real voices coming from an external source. 
 
The Role of Corollary Discharge
The primary physiological mechanism implicated is a failure in the corollary discharge system. 
 
Failure of Corollary Discharge: Normally, when you speak aloud or form inner speech, your brain sends a "corollary discharge" (an efference copy of the motor command) to the auditory cortex. This signal predicts the sound and dampens the brain's response, allowing you to distinguish your own voice from external sounds. In individuals who hear voices, this mechanism is "broken" or impaired; the self-generated inner speech is not suppressed.
 
Misattribution of Inner Speech: Because the inner speech is not tagged as "self-generated," the brain misinterprets it as an external voice or sound. This explains why the voices feel so real and are perceived as distinct from one's own thoughts.
 
Overactive Auditory Cortex: Instead of the normal dampened response, brain regions involved in processing sound and language (like the superior temporal gyrus, including the primary auditory cortex) show an enhanced or overactive response to the internally generated speech. This heightened activity can make the voices feel more salient. 
 
Brain Regions and Connectivity
Neuroimaging studies consistently show altered activity and connectivity in specific brain regions involved in language and auditory processing. 
 
Auditory Cortex Overactivity: When patients are actively hallucinating, there is increased activity in the primary and secondary auditory cortices, located in the superior temporal gyrus (STG). These are the same areas that process real external sounds.
Altered Connectivity: There is evidence of abnormal structural and functional connections between the frontal and temporal brain regions, specifically:
 
Reduced grey matter volume in the STG, particularly in the left hemisphere.
 
Altered connectivity between language-production areas in the frontal lobe (Broca's area) and language-perception areas in the temporal lobe (Wernicke's area). This prevents the proper flow of information needed to "tag" inner speech as self-produced.
 
Dysfunctional gating mechanisms in the thalamus may also contribute to a failure to filter auditory information properly. 
 
Neurochemical Factors
Neurotransmitter systems are also believed to be involved: 
 
Dopamine Hypothesis: While complex, the general theory suggests excessive dopamine activity in certain brain pathways may contribute to positive symptoms like hallucinations.
 
Glutamate Dysfunction: Growing evidence suggests that abnormalities in glutamatergic transmission, particularly involving the NMDA receptor, may mediate the neural processing deficits that lead to hallucinations. 
 
In summary, the physiology of hearing voices involves a complex interplay of faulty neural signalling, structural abnormalities in speech-related brain regions, and neurotransmitter dysregulation, resulting in the brain misinterpreting internal thoughts as compelling, external voices. 
 
References
Di Biase, M.A., Zhang, F., Lyall, A., Kubicki, M., Mandl, R.C.W. et al. (2019, February 14). 50(3):403-412. Neuroimaging Auditory Verbal Hallucinations in Schizophrenia Patient and Healthy Populations. Psychological Medicine.
 
Ford, J.M., Dierks, T., Fisher, D.J., Herrmann, C.S., Hubl, D, et al. Neurophysiological Studies of Auditory Verbal Hallucinations. Schizophrenia Bulletin.
 
Kompus, K, Falkenberg, L.E., Bless, J.J., Johnsen, E., Krokrn, R.A., et al. The Role of the Primary Auditory Cortex in the Neural Mechanism of Auditory Verbal Hallucinations. Frontiers in Human Neuroscience.
 
Van Lutterveld., R., Sommer, I.E.C. & Ford, J.M. (2011, May 16). The Neurophysiology of Auditory Hallucinations – A Historical and Contemporary Review. Frontiers in Psychiatry.
 
Tracy, D.K. & Shergill, S.S. (2013, April 26). 3(2):642-669. Mechanisms Underlying Auditory Hallucinations – Understanding Perception Without Stimulus. Brain Science.