The Low-Frequency Oscillation Model of Hallucinations in Neurodegenerative Disorders and in Delirium

Electroencephalography (EEG) found in dementia with Lewy bodies, Parkinson's disease, Alzheimer's disease and delirium, is characterized by increased power of delta and theta frequencies with the degree of EEG slowing parallel to the frequency of hallucinations occurrence. According to the proposed model of hallucination, pathological low-frequency oscillations may interfere with information processing in two different ways leading to hallucinations. First, pathological low-frequency oscillations may be a source of signal noise, which is next transformed into emotionally charged signal. The filtering of the signal noise may depend on differences in synaptic weights between networks storing representations of emotionally charged and neutral objects. Initially filtered signal can be next reinforced by attention leading to hallucinations. Second, there is growing body of evidence that theta oscillations in distributed cortical networks participate in mechanism of working memory. Therefore, pathological low-frequency oscillations may interfere with the mechanisms of working memory leading to excessive activation of memory traces and to the intrusion of memories into consciousness. The assumption that neuronal representations of emotionally relevant and neutral objects differ in synaptic strengths is supported by studies showing enhanced memory for emotional stimuli. The model is also supported by experiments showing that perception of feared object is facilitated by stimulus-driven, involuntary and automatic recruitment of attention. Consistently with the model, hallucinations are most often unpleasant and emotionally irrelevant objects are rarely hallucinated in neurodegenerative disorders. Proposed disturbances of visual working memory are consistent with the high incidence of palinopsia (perseveration of previously viewed objects after a certain time) in demented patients. The significance of the model for understanding hallucinations that occur in schizophrenia is also discussed. The proposed model predicts that we should search for treatments decreasing the power of cortical delta and theta oscillations or reshaping synaptic plasticity in order to prevent the occurrence of hallucinations.