Importantly, they are often laden with affect, and believed to originate from external (nonself) sources. Hallucinations are often mistaken for veridical perceptions and interpreted as symbolic and personally meaningful. Visual hallucinations in the psychosis spectrum and comparative information from neurodegenerative disorders and eye disease. ![]() In schizophrenia, most hallucinations are auditory (“voices”) although hallucinations in other modalities also occur (Waters F, Collerton D, Jardri R, et al. For example, hallucinations are prominent in people with schizophrenia spectrum disorders, Parkinson’s disease (PD), and eye disease (in which they are known as Charles Bonnet Syndrome ), each showing a distinct character. The auditory and visual modalities of hallucinations are the most commonly enquired about and reported, but have a varied presentation depending on the population group in which they are studied. Here, sensory perceptions overlap with illusions and voluntary internal images, hallucinations, and dreams. There are no clear boundaries between perceptions. ![]() We marshal empirical and theoretical work to address the questions:įuzzy forms of visual experience. This task will better distinguish the characteristics and properties specific to daytime hallucinations from those that are general to sleep perceptions, and draw upon information regarding underlying mechanisms. 8 The main objective of this article is to revisit the status of perceptual experiences that occur during sleep. 5–7ĭespite these suggestions of continuity at some level, a strict dichotomy between sleep-related perceptual phenomena and hallucinations is still central to definitions of hallucinations. Lhermitte 3 elaborated further by designating the midbrain structures associated with peduncular hallucinations as the brain’s “dream centre.” Observations of sleep disturbances in clinical disorders associated with hallucinations (eg, schizophrenia and Lewy body disorders) also prompted suggestions of a common aetiology for both dreams and hallucinations, 4, 5 and the notion that hallucinations may be the results of “rapid eye movement (REM) intrusions” of visual imagery into wakefulness. 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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