Hallucination

hallucination, the experience of perceiving objects or events that do not have an external source, such as hearing one’s name called by a voice that no one else seems to hear. A hallucination is distinguished from an illusion, which is a misinterpretation of an actual stimulus.

A historical survey of the study of hallucinations reflects the development of scientific thought in psychiatry, psychology, and neurobiology. By 1838 the significant relationship between the content of dreams and of hallucinations had been pointed out. In the 1840s the occurrence of hallucinations under a wide variety of conditions (including psychological and physical stress) as well as their genesis through the effects of such drugs as stramonium and hashish had been described.

French physician Alexandre-Jacques-François Brierre de Boismont in 1845 described many instances of hallucinations associated with intense concentration, or with musing, or simply occurring in the course of psychiatric disorder. In the last half of the 19th century, studies of hallucinations continued. Investigators in France were particularly oriented toward abnormal psychological function, and from this came descriptions of hallucinosis during sleepwalking and related reactions. In the 1880s English neurologist John Hughlings Jackson described hallucination as being released or triggered by the nervous system.

Other definitions of the term emerged later. Swiss psychiatrist Eugen Bleuler (1857–1939) defined hallucinations as “perceptions without corresponding stimuli from without,” while the Psychiatric Dictionary in 1940 referred to hallucination as the “apparent perception of an external object when no such object is present.” A spirited interest in hallucinations continued well into the 20th century. Sigmund Freud’s concepts of conscious and unconscious activities added new significance to the content of dreams and hallucinations. It was theorized that infants normally hallucinate the objects and processes that give them gratification. Although the notion has since been disputed, this “regression” hypothesis (i.e., that hallucinating is a regression, or return, to infantile ways) is still employed, especially by those who find it clinically useful. During the same period, others put forth theories that were more broadly biological than Freud’s but that had more points in common with Freud than with each other.