Off the beaten path: hallucinations beyond psychosis

This thesis explored hallucinations in a broader context, going beyond psychosis and diagnostic boundaries with a special focus on deafferentation hallucinations, which are hallucinations caused by sensory impairment. While there is limited research on the precise mechanisms of deafferentation, it is suggested that this phenomenon may be caused by an imbalance between top-down and bottom-up processing in the brain. This idea is supported by the findings in this thesis. Here changes in spontaneous activity in higher order brain areas were linked to the occurrence of hallucinations whereas the complexity of the hallucination is associated with changes in spontaneous activity in brain areas known for bottom-up mechanisms. The lack of research may affect clinical settings as our survey revealed that clinicians have low confidence in their ability to assess and treat deafferentation hallucinations.Even though they are a symptom of many disorders, hallucinations can share similarities in the way they are perceived. In fact, based on the transdiagnostic comparison performed in this thesis general hallucination characteristics are not specific to any particular diagnosis, but rather showed similarities across different disorders. With regard to temporal dynamics of brain activity, no significant differences between clinical and non-clinical voice hearers could be shown, yet hallucination periods exhibited a pattern distinct from rest periods across these two groups. Further the role of the content of hallucinations was assessed in the general Dutch population. This study revealed four content-based subtypes of hallucinations: complex visual hallucinations, grief-related hallucinations, auditory hallucinations, and tactile hallucinations