Neuroimaging Findings in Patients with Hallucinations: Evidence from Neurodegenerative and Psychiatric Conditions

Hallucinations are severe and disabling symptoms experienced in various neurodegenerative and psychiatric conditions. Over the years, neuroimaging techniques have allowed the investigation of the neural mechanisms underpinning this symptomatology. The present chapter summarises positron emission tomography and single-photon emission computed tomography studies of hallucinations in patients with neurodegenerative and psychiatric disorders. The majority of these studies focused on visual hallucinations (VH) in Lewy body disease (LBD) and auditory verbal hallucinations (AVH) in schizophrenia, given their clinical relevance and high prevalence in these conditions. Impairments in bottom-up and top-down mechanisms appeared to be involved. Dysfunctions in brain areas implicated in sensory processing have been found in regions consistent with the hallucination sensory modality, especially in occipito-temporal areas in LBD with VH, and in primary and associative auditory cortices in schizophrenia with AVH, as well as in language-related regions in the latter. Top-down control mechanisms may also play a role that may be linked to impaired activity in frontoparietal areas in LBD and in the anterior cingulate cortex in schizophrenia. These findings suggest that similar mechanisms may underlie different types of hallucinations across diagnoses. Top-down control mechanisms over sensory pathways might foster the genesis and emergence of these symptoms.