![]() The French researcher, Alfred Maury, 1 noted a continuum of form and cause between dreams and hallucinations, and the English neurologist, John Hughlings Jackson, 2 argued that strong “sensory discharges” were likely a common mechanism of dreams and hallucinations. The similarities to “daytime” hallucinations received much scrutiny over the centuries. Philosophers and scientists have long been fascinated by perceptual phenomena occurring around and during sleep, such as the hypnagogic and hypnopompic hallucinations on the borders of sleep and the dreams and parasomnias of sleep. In summary, both phenomena are non-veridical perceptions that share some phenomenological and neural similarities, but insufficient evidence exists to fully support the notion that the majority of hallucinations depend on REM processes or REM intrusions into waking consciousness. Key differences remain however: (1) Sleep-related perceptions are immersive and largely cut off from reality, whereas hallucinations are discrete and overlaid on veridical perceptions and (2) Sleep-related perceptions involve only a subset of neural networks implicated in hallucinations, reflecting perceptual signals processed in a functionally and cognitively closed-loop circuit. Findings show that sleep-related experiences share considerable overlap with hallucinations at the level of subjective descriptions and underlying brain mechanisms. In the current article, we make detailed comparisons between sleep-related experiences and hallucinations in Parkinson’s disease, schizophrenia and eye disease, at the levels of phenomenology (content, sensory modalities involved, perceptual attributes) and of brain function (brain activations, resting-state networks, neurotransmitter action). With our recent understanding of hallucinations in different population groups and at the neurobiological, cognitive and interpersonal levels, it is now possible to draw comparisons between the 2 sets of experiences as never before. These observations have prompted researchers to suggest a common aetiology for these phenomena based on the neurobiology of rapid eye movement (REM) sleep. Yet similarities to sleep-related experiences such as hypnagogic and hypnopompic hallucinations, dreams and parasomnias, have been noted since antiquity. By definition, hallucinations occur only in the full waking state.
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