Auditory hallucinations and the mismatch negativity: processing speech and non-speech sounds in schizophrenia

In line with emerging research strategies focusing on specific symptoms rather than global syndromes in psychiatric disorders, we examined the functional neural correlates of auditory verbal hallucinations (AHs) in schizophrenia. Recent neuroimaging and behavioural evidence suggest a reciprocal relationship between auditory cortex response to external sounds versus that induced by AHs.

The mismatch negativity (MMN), a well established event-related potential (ERP) index of auditory cortex function, was assessed in 12 hallucinating patients (HP), 12 non-hallucinating patients (NP) and 12 healthy controls (HC). The primary endpoints, MMN amplitudes and latencies recorded from anterior and posterior scalp regions, were measured in response to non-phonetic and phonetic sounds.

While schizophrenia patients as a whole differed from HCs, no significant between-group differences were observed when patients were divided into hallucinated and non-hallucinated subgroups but, compared to NPs and HCs, whose MMN amplitudes were greatest in response to across phoneme change at frontal but not temporal sites, MMN amplitudes in HPs at frontal sites were not significantly different to any of the presented stimuli, while temporal MMNs in HPs were maximally sensitive to phonetic change.

These findings demonstrate that auditory verbal hallucinations are associated with impaired pre-attentive processing of speech in fronto-temporal networks, which may involve defective attribution of significance that is sensitive to resource limitations. Overall, this research suggests that MMN may be a useful non-invasive tool for probing relationships between hallucinatory and neural states within schizophrenia and the manner in which auditory processing is altered in these afflicted patients.