Perception of objects, sounds, or sensations having no demonstrable reality, usually arising from a disorder of the nervous system or in response to certain drugs (see hallucinogen). Hallucinations are in many ways similar to dreams: they derive their content from perceptions known to memory, though these can be greatly transformed. Hallucinations can result when attention collapses from intense arousal due to extreme anxiety, fatigue, excitement, or other causes. They figure prominently in the diagnosis of schizophrenia.
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