15 However, one study found evidence of selective atrophy of the hippocampal head in Parkinson’s disease patients with VH. These studies did not report involvement of the amygdala or the hippocampus, perhaps because the hallucinatory experiences were not emotional because people with Parkinson’s disease typically have retained insight into the origin of VH. Trait studies of people with Parkinson’s disease reveal decreased visual cortical activation to external visual stimulation in people with visual hallucinators, 12–14 perhaps suggesting competition for visual resources. 11 Amygdala activity was not reported, perhaps due to the lack of negative content in the specific VH. 10 For example, a symptom-capture study of VH in 1 person with schizophrenia found activity in higher visual areas corresponding to the content of the hallucinations (faces, bodies, scenes), as well as the hippocampus, possibly related to retrieval of visual images from memory. 10 In addition, in people with psychosis, areas involved in emotional experience and memory retrieval, such as the amygdala and hippocampus, are also activated by hallucinations.
State studies suggest involvement of sensory specific areas in the visual hallucinatory experiences of people with Charles Bonnet syndrome 9 and in the auditory hallucinatory experiences in SZ. Trait studies involve comparing groups who tend to have hallucinations to groups who do not, 6, 7 or involve using hallucination severity scores as regressors in analyses of brain activity across groups. State studies, or “symptom-capture studies,” involve participants signaling the start and end of a hallucination, and brain activity during periods with and without hallucinations is compared. 5īrain imaging studies of hallucinations typically take 2 forms: “state” and “trait”. 3 Indeed, common instruments for assessing hallucinations in SZ either neglect the distinction between AH and VH 4 or primarily focus on AH. This may reflect their prominence in neurological conditions with known etiology, such as Lewy Body Dementia, temporal lobe epilepsy, Parkinson’s disease, and Charles Bonnet syndrome. 1 Although visual hallucinations (VH) are experienced by about half of those, 2 VH are studied less frequently by clinical neuroscientists interested in schizophrenia. Auditory verbal hallucinations (AH) are experienced by about 75% of people with a diagnosis of schizophrenia (SZ).
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