HALLUCINATION

the experience of perceiving objects or events that do not have an external source. For example, a person may hear his 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. Traditional psychiatric sources define hallucinations as perceptions without corresponding stimuli from without (Swiss psychiatrist E. Bleuler), or as the apparent perception of an external object when no such object is present (L.E. Hinsie and J. Shatzky, Psychiatric Dictionary, 1940). A historical survey of the subject of hallucinations clearly 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. A.J.F. Brierre de Boismont, a French physician, 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. Perhaps the most simple and yet enduring conceptions were those evolved by Sir Francis Galton in the 1880s, and English neurologist John Hughlings Jackson's formulation of the hallucination as being released or triggered by the nervous system was a milestone along the way. During the first three decades of the 20th century, a spirited interest in hallucinations continued. 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 recently been disputed, the regression hypothesis (i.e., that hallucinating is a regression, or return, to infantile ways) is still widely 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. The medical and scientific literature has continued to contain many references to hallucinatory phenomena, though for 20 years after 1932 there was a surprising decrease of interest. Attention has been revived by the recent upsurge of work on hallucinogenic drugs. experience of perceiving objects or events that are not actually present to the senses. Hallucinations are akin to illusions (see illusion) in that both involve false perceptions; but an illusion, unlike a hallucination, is stimulated by an actual object. Interest in hallucinations has grown in recent years, owing largely to research into hallucinogenic drugs. Two assumptions underlie the modern general theory of hallucination. One is that life experiences leave enduring physical changes (variously called neural traces, templates, or engrams) that function in memory, thought, and imagination. The other is that a constantly shifting balance exists between internal and external environmental forces so that both physiological and cultural experiential factors are determinants of the content and meaning of hallucinations. It is believed by many that beneath consciousness there is a steady stream of unconscious thinking, consisting largely of memories (especially from early life) and heavily charged with emotions. At the same time, there is a steady stream of information presented to the brain by the senses. In order to function at all, the brain must select what information it will respond to; the rest is temporarily held to the side, or ignored outright. This process of selection from sensory data is performed by the mechanism of attention (q.v.). As long as the attention is able to cope with incoming data, so that the brain is smoothly presented with relevant information, the stream of unconscious thoughts are kept out of consciousness. A hallucination can result if the balance described above is disturbed. Two conditions must obtain. First, the unconscious stream must be aroused, i.e., must have a sufficient level of intensity. Second, the attention mechanism must fail to protect consciousness from the intrusion of unconscious thoughts. This can happen if the incoming flow of information from the senses is drastically reduced. In such cases, with the screen provided by attention removed, sufficiently intense unconscious thoughts can enter consciousness, sometimes as a hallucination. Generally, hallucinations derive their content from images stored in memory, although these can be greatly transformed. Hallucinations are in many ways similar to dreams. As a person falls asleep, the amount of incoming sensory information, and hence the level of attention, begins to subside. Unconscious thoughts impinge upon one's consciousness. This effect of sleep can be replicated even in waking life, where the amount of mental stimulation is dramatically reduced. Hallucinations can result when attention does not gently subside but instead collapses from intense arousal due to extreme anxiety or extended periods of wakefulness. Hallucinations can also be induced fairly directly by various interventions into the brain. During brain surgery, for example, if cortical (of the cortex, or surface) brain cells are electrically stimulated, the patient can experience strong hallucinations based on deep-seated memories. But hallucinations are much more commonly induced by drugs, which may provoke unsettling or even terrifying experiences. Some hallucinogens, such as sodium amybarbital, the truth serum, suppress sensory input in a way that encourages the vivid recall of recent memories. Others work the opposite way: they increase sensory input to the extent that the attention is overwhelmed, and hallucinations can emerge. See also dream; sleep. Additional reading Works on the psychology of hallucination include A. Brierre de Boismont, Hallucinations: Or, The Rational History of Apparitions, Dreams, Ecstasy, Magnetism, and Somnambulism (1853, reprinted 1976; originally published in French, 1852), an early classic; William Grey Walter, The Neurophysiological Aspects of Hallucinations and Illusory Experience (1960), visual phenomena viewed by a brain researcher; Louis Jolyon West (ed.), Hallucinations (1962), an analysis of the subject by contributors from several scientific disciplines; and Peter D. Slade and Richard P. Bentall, Sensory Deception: A Scientific Analysis of Hallucination (1988), which discusses therapeutic approaches. Louis Jolyon West The Editors of the Encyclopdia Britannica