Schizophrenia is a chronic brain disorder that alters a person’s perceptions, thoughts, and behaviors. The most common and distressing symptom is auditory verbal hallucinations (AVH), the perception of voices in the absence of any external sound. These voices can manifest as commands, running commentaries, or conversations, and they are experienced as being undeniably real. Understanding the origins of these voices reveals a complex interplay of failed self-monitoring, abnormal brain activity, and neurochemical imbalances.
The Failure to Recognize Inner Speech
A leading cognitive explanation for hearing voices is the misattribution hypothesis: the brain fails to correctly identify a person’s own inner thoughts as self-generated. Every person possesses “inner speech,” a silent monologue of thoughts. For people with schizophrenia, this internal dialogue is mistakenly processed as input originating from an external source.
This failure of self-recognition is connected to a mechanism called corollary discharge. Normally, when the brain prepares to speak or move, it sends a predicted copy of the command—the corollary discharge—to the sensory processing areas. This signal acts as an internal filter, dampening the brain’s response to self-generated sensation so it is not perceived as surprising or external.
In individuals who experience AVH, this internal predictive signal is significantly impaired. Electrophysiological studies using EEG have shown that the normal suppression of auditory brain responses to self-generated speech is either reduced or reversed. Instead of muting the self-generated sound, the auditory cortex shows an amplified response, treating the inner speech as novel, external sound.
This breakdown in the inner voice filter causes the self-generated thought to lose its “sense of self” tag. Because the brain’s internal prediction system is faulty, the thought is not recognized as belonging to the person. This misattribution blurs the boundary between internal thought and external perception, allowing the person to experience their own inner speech as an alien voice.
Language Processing Centers and Brain Activity
Neuroimaging studies (fMRI and PET scans) provide physical evidence of what occurs in the brain during an auditory hallucination. When a person reports hearing voices, the brain regions responsible for language processing become highly active. This confirms the experience’s reality within the brain, even without an external stimulus.
Specifically, hearing voices correlates with increased activity in areas associated with both speech production and comprehension. The left inferior frontal gyrus (Broca’s area) is connected to speech production and is often hyperactive during AVH. This suggests the brain is activating the motor component of language, similar to how it prepares to speak aloud.
The left superior temporal gyrus, which includes Wernicke’s area, the primary center for language comprehension, also shows significant activation. This co-activation of production and comprehension centers explains why the voices are perceived as intelligible speech. The brain is engaging the same circuits it uses to process a conversation.
Furthermore, studies indicate that the functional connectivity between these language areas is often altered. Reduced information flow between the temporal lobe’s comprehension area and the frontal lobe’s production area is evident. This partial disconnection within the language network may prevent the frontal lobe from properly monitoring and tagging the self-generated speech, contributing to the misattribution and the perception of the voice as external.
Neurochemical Drivers of Hallucinations
The cognitive and anatomical abnormalities that lead to hearing voices are rooted in dysregulation of the brain’s primary chemical messengers. Dopamine has long been implicated, particularly a hyperfunction within the mesolimbic pathway, which projects to the striatum. This excessive dopamine activity can heighten the brain’s sensitivity to internal and external stimuli.
Elevated dopamine levels can cause the brain to place undue significance on otherwise irrelevant internal events, such as a fleeting thought or an internal fragment of speech. This over-reliance on internal expectation over sensory input results in the perceptual distortions that manifest as voices. Antipsychotic medications primarily work by blocking dopamine receptors, which reduces this over-responsiveness.
Glutamate, the brain’s main excitatory neurotransmitter, also plays a part, particularly through the hypofunction of its N-methyl-D-aspartate (NMDA) receptors. Drugs that block these receptors can induce psychotic symptoms, suggesting a link between glutamate imbalance and schizophrenia symptoms. This chemical disruption affects the communication pathways necessary for proper sensory gating and self-monitoring.
The dopamine and glutamate systems are interconnected, and their combined dysregulation disrupts the delicate balance required for normal brain function. This chemical environment sets the stage for abnormalities in language processing and the failure of the corollary discharge mechanism, enabling the experience of hearing voices.
Targeting the Mechanisms Through Treatment
Current treatment approaches are designed to intercept the neurological and cognitive mechanisms that drive auditory verbal hallucinations. Antipsychotic medications directly address the neurochemical component by modulating neurotransmitter activity.
By primarily acting as antagonists at dopamine receptors, they reduce the excessive signaling and the brain’s over-responsiveness to internal thoughts. This chemical intervention aims to restore typical functioning in the mesolimbic pathway, which reduces the intensity and frequency of the voices.
However, medication alone is often insufficient, necessitating psychological approaches to address the cognitive aspect. Cognitive Behavioral Therapy for Psychosis (CBTp) is frequently used alongside medication.
This form of talk therapy directly addresses the misattribution hypothesis by helping individuals re-evaluate the source and meaning of the voices. CBTp teaches coping strategies and encourages a shift in the interpretation of the voice, allowing the person to reduce the distress and disruption caused by the auditory experience.