Conflict Processing in Schizophrenia: Dissociable Neural Mechanisms Revealed by the N2 and Frontal Midline Theta

Abstract

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 origin from ACC, i.e., the N2 component of event-related potentials (ERPs) and frontal midline theta (FM{theta}) 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 time in incongruent trials than in congruent trials) between HC and SZ. Trial-averaged ERP and spectral analysis suggested that both N2 and FM{theta} 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 in HC was totally absent in SZ. By contrast, the trial-by-trial modulation of FM{theta} (larger power in slower trials) was observed and localized in ACC in both groups, despite a relatively smaller magnitude in SZ. Notably, such trial-by-trial FM{theta} modulation was only present in incongruent trials that demanded conflict processing. Taken together, our results not only support the idea that FM{theta}, not N2, serves as the neural substrate of conflict processing in SZ, but also provide novel insights into the functional roles of ACC during executive control, enriching the current neurocognitive models of SZ.