Conflict Processing in Schizophrenia: Dissociable neural mechanisms revealed by the N2 and frontal midline theta

Deficits in executive control have long been regarded as one of the hallmark cognitive characteristics in people with schizophrenia (SZ), and current neurocognitive models of SZ generally regard the dysfunctional anterior cingulate cortex (ACC) as the possible neural mechanism. This however, contrasts with recent studies showing that conflict processing, a key component of executive functions that relies on ACC, remains relatively intact in SZ. The current study aimed to investigate this issue through two well-known electrophysiological signatures of conflict processing that have been suggested to originate from ACC, i.e., the N2 component of event-related potentials (ERPs) and frontal midline theta (FMθ) oscillations. We recorded 64-channel scalp electroencephalography from 29 SZ (17 women; mean age: 30.4 years) and 31 healthy control subjects (HC; 17 women; mean age: 29.1 years) performing a modified flanker task. Behavioral data revealed no significant differences in flanker conflict effects (lower accuracy and longer reaction times in incongruent trials than in congruent trials) between HC and SZ. Trial-averaged ERP and spectral analysis suggested that both N2 and FMθ were significantly impaired in SZ relative to HC. Furthermore, by sorting incongruent trials according to their reaction times within individual subjects, we found that the trial-by-trial modulation of N2 (larger amplitude and longer latency in slower trials) which was observed and localized in ACC for HC was totally absent for SZ. By contrast, the trial-by-trial modulation of FMθ (larger power in slower trials) was observed and localized in ACC for both groups, despite a smaller magnitude in SZ, which suggested that FMθ, not N2, might serve as the neural substrate of conflict processing in SZ. Taken together, our results enrich the current neurocognitive models of SZ by revealing dissociable neural responses between N2 and FMθ during conflict processing in SZ.