Descriptions of hallucinatory phenomena have figured prominently since the beginning of recorded history. Jean Etienne Esquirol (1772-1840) is usually credited for having introduced the term in 1817, differentiating between hallucinations and illusions. Both are wrong perceptions, but in illusions, an external stimulus is always present whereas hallucinations are perceptions that occur in the absence of corresponding sensory stimuli. They occur in a variety of conditions but more often in the mentally ill, especially in schizophrenia where hallucinations, particularly auditory hallucinations represent for many, such as Henri Ey one of the cardinal features. This chapter, however, deals with visual hallucinations as found in individuals who are not necessarily mentally ill: the Charles Bonnet syndrome and autoscopy.
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