Transcranial direct current stimulation improves action-outcome monitoring in schizophrenia spectrum disorder

Patients with schizophrenia spectrum disorder often demonstrate impairments in action-outcome monitoring. Passivity phenomena and hallucinations, in particular, have been related to impairments of efference copy-based predictions which are relevant for the monitoring of outcomes produced by voluntary action. Frontal transcranial direct current stimulation has been shown to improve action-outcome monitoring in healthy subjects. However, whether transcranial direct current stimulation can improve action monitoring in patients with schizophrenia spectrum disorder remains unknown. We investigated whether transcranial direct current stimulation can improve the detection of temporal action-outcome discrepancies in patients with schizophrenia spectrum disorder. On 4 separate days, we applied sham or left cathodal/right anodal transcranial direct current stimulation in a randomized order to frontal (F3/F4), parietal (CP3/CP4) and frontoparietal (F3/CP4) areas of 19 patients with schizophrenia spectrum disorder and 26 healthy control subjects. Action-outcome monitoring was assessed subsequent to 10 min of sham/transcranial direct current stimulation (1.5 mA). After a self-generated (active) or externally generated (passive) key press, subjects were presented with a visual outcome (a dot on the screen), which was presented after various delays (0-417 ms). Participants had to detect delays between the key press and the visual consequence. Symptom subgroups were explored based on the presence or absence of symptoms related to a paranoid-hallucinatory syndrome. In general, delay-detection performance was impaired in the schizophrenia spectrum disorder compared to the healthy control group. Interaction analyses showed group-specific (schizophrenia spectrum disorder versus healthy control group) and symptom-specific (with/without relevant paranoid-hallucinatory symptoms) transcranial direct current stimulation effects. Post hoc tests revealed that frontal transcranial direct current stimulation improved the detection of long delays in active conditions and reduced the proportion of false alarms in undelayed trials of the passive condition in patients. The patients with no or few paranoid-hallucinatory symptoms benefited especially from frontal transcranial direct current stimulation in active conditions, while improvement in the patients with paranoid-hallucinatory symptoms was predominantly reflected in reduced false alarm rates in passive conditions. These data provide some first evidence for the potential utility of transcranial direct current stimulation in improving efference copy mechanisms and action-outcome monitoring in schizophrenia spectrum disorder. Current data indicate that improving efference copy-related processes can be especially effective in patients with no or few positive symptoms, while intersensory matching (i.e. task-relevant in passive conditions) could be more susceptible to improvement in patients with paranoid-hallucinatory symptoms.