Lower P300 amplitudes for internally-generated events in patients with schizophrenia

Patients with schizophrenia demonstrate difficulty differentiating internally-generated from externally-generated events. An excessive deployment of attention to external events as well as poor processing of self-generated events has been proposed to explain this misattribution. This study was done to understand the neurophysiological basis of source monitoring bias in schizophrenia.

Seventeen patients and fourteen healthy controls completed a two-stage source monitoring task involving solving a three-letter jumbled word either by the participant or a computer following which they had to identify the correct source of the word - solved by self (internal source monitoring, ISM) or by computer (external source monitoring, ESM) - while EEG was recorded. P300 amplitude and latency were computed separately for ISM and ESM conditions.

P300 amplitude for ISM condition was lower in patients with schizophrenia compared to controls over right and left parietal regions. Additionally, P300 amplitude for ISM condition was lower compared to ESM condition in patients over right and left parietal regions. However, P300 amplitude for ESM condition did not differ significantly between the groups. In contrast to P300 amplitude, there was no significant difference in P300 latency in ISM or ESM condition within or between the groups.

P300 amplitude was lower for internally-generated but not externally-generated events in patients with schizophrenia. These findings suggest that patients allocate less attentional resources to internally generated events resulting in poorer processing and consequent misattribution of their source of origin. This study provides the first specific electrophysiological evidence of impaired source monitoring in schizophrenia.