Increased feelings of external influence during instructed imaginations in patients with psychotic disorder

Feelings of external influence are a hallmark of psychotic disorders, but mitigated forms can also be observed in healthy persons. A newly developed paradigm designed to induce feelings of external influence through verbal information (influence will be attempted/not-attempted) and physical setup (presence of tDCS-device or hand touch) was carried out in a pilot study in 21 patients with psychosis and in 22 healthy controls. A higher prevalence of induced feelings of external influence was observed in patients with psychosis. Furthermore, in healthy controls the impact of verbal information outweighed the impact of physical setup, whereas in patients the setup had a stronger influence. Moreover, the intensity of induced feelings of external influence correlated with measures of psychotic symptom severity (PANSS positive score, SAPS, AMDP ego-disorder) as well as response latencies during the estimation process. These findings indicate that feelings of external influence can be reliably induced in healthy controls and in patients with psychotic disorders. Stronger effects of physical setup in patients might be related to disorder-specific biases regarding the impact of initial perceptual impressions of features of the physical environment on belief formation, whereas the attenuated response to explicit verbal information about attempted influence might reflect reduced trust in other people's statements.