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Compelling perceptual experiences which may be visual, tactile, olfactory, or auditory, but which lack a physical stimulus.

Although hallucinations are false perceptions, they carry the force of reality and are a definitive sign of mental illness. Hallucinations may be caused by organic deterioration or functional disorders, and can occur in normal people while asleep or awake, or as a result of sensory deprivation. Generally not positive experiences, hallucinations are often described as frightening and distressing. A person under a hallucinatory state may be either alert and intelligent or incoherent, depending on the type and degree of the disturbance.

One psychological condition commonly characterized by hallucinations is schizophrenia. In schizophrenia, the hallucinations are usually auditory, involving one or more voices. The voices may issue commands, comment on or seem to narrate the person's actions, or sound like an overheard conversation, and can be analyzed for greater insight into the patient's emotional state. Auditory hallucinations can also occur in severe depression and mania; seriously depressed persons may hear voices making derogatory remarks about them or threatening them with bodily harm. Visual hallucinations, on the other hand, are more likely to characterize organic neurological disturbances, such as epilepsy, and may occur prior to an epileptic seizure. Hallucinations involving the senses of smell and touch are less frequent than visual or auditory ones; however, tactile hallucinations have proven useful in the study and diagnosis of schizophrenia. Together with fearfulness and agitation, hallucinations are also a component of delirium tremens, which can afflict persons suffering from alcohol dependence.

Hallucinations can also be induced by ingesting drugs that alter the chemistry of the brain. (The technical name used for drug-induced hallucinations is hallucinosis.) The most widely known hallucinogens, or mind-altering drugs, are LSD, psilocybin, peyote, and mescaline, which act on the brain to produce perceptual, sensory, and cognitive experiences that are not occurring in reality. Effects vary from user to user and also individually from one experience to the next. Hallucinations produced by LSD are usually visual in nature. On an LSD "trip," for example, hallucinations can last eight to ten hours while those produced by mescaline average six to eight hours. Two illegal drugs manufactured to produce psychoactive effects, PCP (phencyclidine) and MDMA (Ecstasy), are not true hallucinogens, but both produce hallucinations of body image as well as psychoses. A person may also experience hallucinations while attempting to withdraw from a drug, such as "pink elephants" and other visual hallucinations from alcohol withdrawal. Withdrawal symptoms from cocaine are associated with the hallucinatory tactile sensation of something crawling under one's skin, often termed "the cocaine bug."

Other causes of hallucinations are hypnosis, lack of sleep, stress, illness, and fatigue, which can produce a rare and unique hallucination known as "the doppelganger." A person who has this experience sees his or her mirror image facing him or her three or four feet away, appearing as a transparent projection on a glassy surface. The hypnagogic hallucinations that occur in the zone between sleep and waking are both visual and auditory, and are strikingly detailed to those who can remember them. Sensory deprivation in subjects of laboratory experiments over a period of time has also been shown to produce hallucinations, as has electrical stimulation of the brain. Experiences called pseudohallucinations involve the perception of vivid images without the sense that they are actually located in external space—the perceiver recognizes that they are not real. Associated with isolation and emotional distress, they include such examples as shipwrecked sailors visualizing rescue boats or travelers stranded in the desert visualizing an oasis. Pseudo-hallucinations do not have the same psychiatric significance as true hallucinations.

People suffering from hallucinations may try to conceal them from others because of their negative connotations, and may receive more drastic forms of treatment or inadequate prognoses because of them. In contrast to mainstream cultural opinion, however, users of hallucinogens in the United States view hallucinations as positive and potentially enlightening, and in other cultures they are regarded for their healing faculties. In the Moche culture of coastal Peru, for example, traditional healers may ingest mescaline as part of a healing ritual in the belief that the hallucinations produced by it offer insight into the patient's condition and thus aid in the healing process.

Further Reading

Andrews, Barbara. Dreams and Waking Visions: A Journal. New York: St. Martin's Press, 1989.

Guiley, Rosemary. The Encyclopedia of Dreams: Symbols and Interpretations. New York: Crossroad, 1993.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Kenneth John William Craik Biography to Jami (Mulla Nuruddin ʼAbdurrahman ibn-Ahmad Biography