The processes responsible for dreams can be activated outside sleep by, for instance, drugs and by sensory, food or sleep deprivation, as well as in psychiatric and neurological disorders. In these situations sensory processing is disordered, and leads to unusual perceptions. These may be primarily visual, as in peduncular hallucinosis due to midline lesions of the midbrain, or in the Charles Bonnet syndrome in which visual hallucinations occur despite blindness. Degenerative disorders such as fatal familial insomnia, Parkinson's disease and Lewy body disease are also associated with hallucinations during wakefulness which are analogous to dreaming in sleep.
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