Hallucinations are compelling perceptual experiences that may be visual, tactile, olfactory, or auditory, but that lack a physical stimulus.
Hallucinations are false perceptions, but they carry the force of reality. Generally not positive experiences, hallucinations are often described as frightening and distressing.
Causes of hallucinations vary. 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. Professionals can analyze the hallucinations to obtain a greater insight into the patient's emotional state.
Auditory hallucinations can also occur in patients who experience severe depression and mania. Persons with severe depression may hear voices making derogatory remarks about them or threatening them with bodily harm. Visual hallucinations, by contrast, are more likely to characterize organic neurological disturbances (those with a physical or physiological cause). Epilepsy is an example, and hallucinations may occur prior to an epileptic seizure.
Together with fearfulness, disorientation, tremors, and agitation, hallucinations are a component of delirium tremens, also called DT. Delirium tremens occurs when persons with chronic alcoholism stop drinking alcohol.
Hallucinations involving the senses of smell and touch are less frequent than visual or auditory ones, but they do occur. For instance, when individuals attempt to withdraw from cocaine, they may experience the hallucinatory tactile sensation of something crawling under their skin. This sensation is sometimes termed the cocaine bug.
Other causes of hallucinations include the following:
Fatigue can produce a rare and unique hallucination known as the doppelganger. Persons who have this experience see their mirror image facing them from three or four feet away, appearing as a transparent projection on a glassy surface.
So-called hypnagogic hallucinations occur in the zone between sleep and waking. These hallucinations are both visual and auditory and, to those who can remember them, are strikingly detailed. In addition, subjects of sensory-deprivation experiments and of experiments involving electrical stimulation of the brain, may experience hallucinations.
Hallucinations can also result when individuals ingest drugs that alter the chemistry of the brain. (The technical term used for drug-induced hallucinations is hallucinosis.) The most widely known hallucinogens, or mind-altering drugs, are LSD (lysergic acid diethylamide), psilocybin (found in so-called psychedelic mushrooms), and mescaline (a psychoactive compound found in the cactus peyote). These drugs act on the brain to produce perceptual, sensory, and cognitive experiences that are not reality-based. Effects vary from user to user and also individually from one experience to the next. Hallucinations produced by LSD are usually visual in nature and may last eight to 10 hours. Mescalineinduced hallucinations last six to eight hours.
Phencyclidine (PCP) and ecstasy (3,4-methylenedioxy-methamphetamine [MDMA]) are illegal drugs manufactured to produce psychoactive effects. They are not true hallucinogens, but both produce hallucinations of body image, as well as psychoses (mental disorders in which users experience disconnection from reality).
Users of hallucinogens often view hallucinations as positive and potentially enlightening, and some cultures hold them in high regard for their perceived healing faculties. For example, some traditional healers may ingest mescaline as part of healing rituals with the belief that the induced hallucinations offer insight into the patient's condition and thus aid in the healing process.
Experiences called pseudohallucinations involve the perception of vivid images, but the individual realizes that they are not real. Associated with isolation and emotional distress, these hallucinations include such examples as shipwrecked sailors visualizing rescue boats or travelers stranded in the desert visualizing an oasis. Pseudohallucinations are not true hallucinations, however, and do not have the same psychiatric significance.
See also Hallucinogens .
Collerton, Daniel, et al., eds. The Neuroscience of Visual Hallucinations. Hoboken, NJ: Wiley-Blackwell, 2015.
Macpherson, Fiona, and Dimitris Platchias, eds. Hallucina-tion: Philosophy and Psychology. Cambridge, MA: MIT Press, 2013.
Sacks, Oliver. Hallucinations. New York: Vintage Books, 2013.
Woods, Angela, et al. “Interdisciplinary Approaches to the Phenomenology of Auditory Verbal Hallucinations.” Schizophrenia Bulletin 40 (June 2014): S246–S254.
MedlinePlus. “Hallucinations.” http://www.nlm.nih.gov/medlineplus/ency/article/003258.htm (accessed August 6, 2015).
National Institute of Mental Health, 6001 Executive Blvd., Rm. 6200, MSC 9663, Bethesda, MD, 20892-9663, (866) 615-6464, firstname.lastname@example.org, http://www.nimh.nih.gov .
Schizophrenia and Related Disorders Alliance of America, PO Box 941222, Houston, TX, 77094-8222, (240) 4239432, (800) 493-2094, email@example.com, http://www.sardaa.org .