Is the brain talking back? The science behind hearing voices in schizophrenia spectrum disorders

When people think of schizophrenia, a common association is hearing voices or experiencing hallucinations.¹ This perception of a voice in the absence of an external speaker is clinically referred to as an auditory verbal hallucination, a hallmark symptom in the diagnosis of schizophrenia.^2,3 It is estimated that 60–80% of individuals with schizophrenia spectrum disorders experience auditory verbal hallucinations.^4,5 These experiences can impair daily functioning, disrupt social interactions, and reduce quality of life.² There is a need to further understand the mechanisms behind auditory verbal hallucinations in schizophrenia spectrum disorders, and why they affect some individuals but not others.

The theory: Failure of speaking-induced suppression

A long-standing hypothesis is that auditory verbal hallucinations arise from dysfunction in speaking-induced suppression (SIS).⁶ SIS is a type of corollary discharge mechanism, in which the brain uses a copy of a motor signal to predict and suppress the sensory effects of self-generated actions.⁶ In the context of speech, this means that self-generated sounds generate a smaller neurophysiological response in the auditory cortex than the same sounds produced externally.⁶

SIS is associated with a reduction in the amplitude of the N1 component of the auditory-evoked potential in response to self-generated speech.⁶ In healthy individuals, this process functions normally, allowing the brain to recognise internally generated speech so that an individuals’ internal voice feels and sounds internal.⁶ In individuals with schizophrenia, SIS (i.e. the brain’s ability to “turn down” its response to its own voice) is impaired.⁶ As a result, the N1 component is not suppressed as much as expected during self-generated speech.⁶ Consequently, internally generated speech may be perceived as coming from an external source, which can manifest as hearing voices.⁶

The evidence: The brain may misread its own inner speech

A recent study from the University of New South Wales, Sydney, provides further support for the hypothesis that auditory verbal hallucinations in schizophrenia spectrum disorders are linked to dysfunction in the SIS corollary discharge mechanism during inner speech.⁶

The study included three groups: 1) healthy controls, 2) patients with schizophrenia spectrum disorders experiencing current auditory verbal hallucinations, and 3) patients with schizophrenia spectrum disorders not experiencing auditory verbal hallucinations.⁶ Participants were asked to imagine producing a sound (inner speech) while simultaneously hearing an external sound.⁶ The imagined and external sounds were either the same (matched) or different (mismatched).⁶

In healthy controls, the expected pattern of SIS was observed: the amplitude of the N1 component of the auditory-evoked potential was reduced when the external sound matched the imagined sound.⁶ In contrast, patients with schizophrenia spectrum disorders experiencing current auditory verbal hallucinations displayed an inverted SIS pattern.⁶ In this group, the amplitude of the N1 component was enhanced when the external and imagined sounds matched.⁶ This suggests an enhancement of responses to internally generated speech, which may contribute to difficulty distinguishing between internal and external sounds, ultimately leading to hallucinations.⁶

However, not all patients with schizophrenia spectrum disorders displayed this inverted SIS pattern.⁶ Those without current auditory verbal hallucinations demonstrated an in-between pattern of SIS.⁶ In this group, suppression of the N1 component was observed only when the external sound did not match the imagined sound.⁶ This finding suggests that corollary discharge dysfunction may exist along a spectrum, with non‑hallucinating individuals showing partial impairment compared with both healthy individuals and those experiencing hallucinations.⁶ Further investigation is needed to better understand why some individuals develop auditory verbal hallucinations while others do not.⁶

The outcome: Inner SIS may serve as a potential biomarker for schizophrenia spectrum disorders and auditory verbal hallucinations

Recent evidence provides strong support for the concept that, in patients with schizophrenia spectrum disorders experiencing auditory verbal hallucinations, internal speech may be misperceived as externally generated sound – a phenomenon referred to as inner SIS.⁶ In the absence of established biological biomarkers for schizophrenia, inner SIS may represent a promising neurophysiological candidate.^6,7 It may have potential clinical utility in supporting the assessment of treatment response, serving as an objective outcome measure, and helping to predict the transition to psychosis in at-risk individuals.⁶