Answer:
Hallucinations are perceptions in the absence of an external stimulus and are accompanied by a compelling sense of their reality. They are a diagnostic feature of schizophrenia, occurring in an estimated 60%–70% of people with this disorder, with auditory hallucinations being the most common. However, hallucinations are not only associated with illness but can also occur in healthy individuals. For example, data from 6 community survey studies in various countries indicate that 7%–30% of children and adolescents report experiencing hallucinations.1 In the context of grief after the death of a spouse, one-third to one-half of bereaved spouses report hallucinations of the deceased.2,3 Transcultural influences may also affect the distinction between reality and imagination as well as the normalcy of visualizing images and ideas.4 In healthy people, pseudohallucinations can even be generated at will by mild sensory deprivation; for example, vivid dreamlike visual imagery can be induced in some individuals by placing 2 halves of a ping pong ball over the eyes and playing a recording of the sounds of a waterfall for several minutes.5 The neurobiological basis of hallucinations has most frequently been investigated in patients with schizophrenia, although studies examining hallucinatory phenomena in healthy individuals may also be informative. It should be noted, however, that it is unknown at present whether hallucinations are generated by similar mechanisms in patients and in healthy people.