The low-frequency oscillation model of hallucinations in neurodegenerative disorders and in delirium
Visual hallucinations in neurodegenerative disorders and in delirium are composed of fully developed objects of normal shape and size whereas fragmented or distorted objects and geometric patterns are uncommon. Usually, patients hallucinate other people or frightening animals whereas emotionally irrelevant objects are rarely hallucinated. Commonly, the same image repeats itself on different occasion and patients hallucinate one or very limited kinds of objects. The characteristic of hallucinations suggest that they result from activation of preexisting and shaped by experience neuronal representation of external objects. A consistent EEG finding in Parkinson’s and Alzheimer’s disease, dementia with Lewy Bodies, and in delirium is increased power in delta and theta frequencies with the degree of EEG slowing paralleling the frequency of occurrence of hallucinations. Therefore, I propose a theoretical model of hallucination that is based on current concepts in neuroscience and on electrophysiological findings in clinical and basic research. According to the proposed model the prolonged depolarization, associated with low-frequency oscillations, randomly activate neurons which, in turn, send impulses by forward and backward connections. It is expected that emotionally relevant objects are represented by networks with strongest synaptic connections and that the stronger object representation the higher probability that random activation of neurons will activate the entire network by means of reciprocal connections. Activated representation is further reinforced by attentional processes and enters the content of consciousness leading to hallucination. The proposed model explains the phenomenology of hallucinations occurring in neurodegenerative disorders and in delirium. The model can be also relevant for hypnagogic hallucinations experienced during the sleep onset and in schizophrenia.
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