Reduced mismatch negativity to frequency deviants and altered non-oscillatory gamma-band power in first episode psychosis

Psychotic disorders are debilitating mental health conditions that affect around 3% of the population. Traditional dopaminergic antipsychotic medications are generally effective in treating positive psychotic symptoms (such as hallucinations), however, they fail to substantially improve functioning for around 30-40% of patients, and show less efficacy for negative and cognitive symptoms. These limitations are hypothesised to result from glutamatergic system dysfunction which leads to excitation/inhibition (E/I) imbalances. Using electroencephalographic (EEG) measures relating to E/I imbalance, specifically gamma power (30-90 Hz) and mismatch negativity (MMN), may provide biomarkers for symptom severity in individuals in early psychosis. Accordingly, the primary objective of the current study was to determine how resting-state gamma power and MMN amplitude differ in individuals with a first episode of psychosis (FEP) compared to healthy controls (HC); and if these measures scale with symptom severity. To address this, we recorded 64-channel EEG data from young people (15-25 years) experiencing FEP (N = 31) and healthy controls (N = 27). During a resting-state task, we examined total gamma power, and periodic (oscillatory) and aperiodic (non-oscillatory) contributions to gamma band power at frontal, central and posterior (Fz, Cz and Oz electrodes) locations. In addition, the MMN to frequency (pitch) and duration deviants was examined at frontal locations (Fz electrode) using a jittered interstimulus interval paradigm. Finally, we correlated the FEP group measures with their symptom severity at time of EEG collection and six weeks later. Results for gamma power showed a significant decrease in aperiodic low gamma (30-45 Hz) activity at frontal (Fz) locations for the FEP group. For the MMN, we found no differences in the duration MMN, however, the frequency MMN was significantly reduced (between 174 ms and 236 ms) for those with FEP. Finally, we found no significant correlations with any measures and symptom severity or psychosocial functioning. Our findings suggest that localised E/I imbalances may be present in those with a FEP, however, these changes do not index current or near-future symptom severity or decline in functioning.