A transdiagnostic comparison of hallucinations

Hallucinations present in a wide range of clinical disorders, including psychiatric, neurological and other medical disorders. Hallucinations also occur in a minority of the general population. As hallucinations are not unique to one disorder, there has been growing consensus that hallucinations should be investigated using a broader transdiagnostic approach. Transdiagnostic research is currently hindered due to the lack of a suitable measurement tool. In part I of the thesis, a new transdiagnostic questionnaire - the Questionnaire for Psychotic experiences (QPE) - was developed. The QPE enables assessment of severity, frequency and phenomenology of psychotic experiences across disorders. The results showed that participants with various disorders reported hallucinations, which confirms that hallucinations should be considered a transdiagnostic phenomenon. A comparison of hallucinations across disorders confirmed previous findings that similar phenomenological characteristics can exist between disorders, and that phenomenological characteristics can differ within a diagnosis. This suggests the existence of subtypes of hallucinations that transcend diagnosis.In part II of the thesis, the neural mechanism of hallucinations was investigated across participants with a schizophrenia-spectrum disorder, bipolar disorder and non-clinical individuals. Previous studies report similar phenomenological characteristics of hallucinations across these participants, suggesting a similar underlying neural mechanism. Our results do not support this hypothesis. Non-clinical individuals and schizophrenia patients with hallucinations show similar alterations of the functional connectome, whereas differential alterations were reported for bipolar disorder patients with hallucinations. This suggests a different underlying neural mechanism for hallucinations in bipolar disorder as compared to non-clinical individuals and patients with schizophrenia