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Author: Rebecca Frey
Editor: Kristin Key
Date: 2012
Publisher: Gale, a Cengage Company
Document Type: Topic overview
Length: 2,762 words

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A hallucination is a sensory perception without a source in the external world. The English word “hallucination” comes from the Latin verb hallucinari, which means “to wander in the mind.” Hallucinations can affect any of the senses, although certain diseases or disorders are associated with specific types of hallucinations.

It is important to distinguish between hallucinations and illusions or delusions, as the terms are often confused in conversation and popular journalism. A hallucination is a distorted sensory experience that appears to be a perception of something real even though it is not caused by an external stimulus. For example, some elderly people who have been recently bereaved may have hallucinations in which they “see” the dead loved one. An illusion, by contrast, is a mistaken or false interpretation of a real sensory experience, as when a traveler in the desert sees what looks like a pool of water, but in fact is a mirage caused by the refraction of light as it passes through layers of air of different densities. The bluish-colored light is a real sensory stimulus, but mistaking it for water is an illusion. A delusion is a false belief that a person maintains in spite of evidence to the contrary and in spite of proof that other members of their culture do not share the belief. For example, some people insist that they have seen flying saucers or unidentified flying objects (UFOs) even though the objects they have filmed or photographed can be shown to be ordinary aircraft, weather balloons, satellites, etc.


The demographics of hallucinations vary depending on their cause; however, many researchers think that they are underreported for several reasons:

  • Fear of being thought “crazy” or mentally ill.
  • Gaps in research. For example, some types of hallucinations are associated with disorders that primarily affect the elderly, who are often underrepresented in health surveys.
  • Fear of being reported to law enforcement for illegal drug use.

There is no evidence that hallucinations occur more frequently in some racial or ethnic groups than in others. In addition, gender does not appear to make a difference. The demographics of hallucinations associated with some specific age groups, conditions, or disorders are as follows:

  • Children. Hallucinations are rare in children below the age of eight. About 40% of children diagnosed with schizophrenia, however, have visual or auditory hallucinations.
  • Eye disorders. About 14% of patients treated in eye clinics for glaucoma or age-related macular degeneration report visual hallucinations.
  • Alzheimer's disease (AD). About 40%–50% of patients diagnosed with AD develop hallucinations in the later stages of the disease.
  • Drug use. According to the 2010 National Survey on Drug Use and Health, 14.8% of persons living in the United States have used hallucinogens in their lifetime, with LSD being the most popular (9.2%), followed by MDMA, or Ecstasy (6.3%). The highest rate of hallucinogen abuse is found in males between the ages of 18 and 25.
  • Normal sleep/wake cycles. Sleep researchers in Great Britain and the United States have reported that 30%–37% of adults experience hypnagogic hallucinations, which occur during the passage from wakefulness Page 728  |  Top of Articleinto sleep, while about 10%–12% report hypnopompic hallucinations, which occur as a person awakens. Hallucinations related to ordinary sleeping and waking are not considered an indication of a mental or physical disorder.
  • Migraine headaches. About 10% of patients diagnosed with migraine headaches experience visual hallucinations prior to the onset of an acute attack.
  • Adult-onset schizophrenia. According to the National Institute of Mental Health (NIMH), about 75% of adults diagnosed with schizophrenia experience hallucinations, most commonly auditory or visual. The auditory hallucinations may be command hallucinations, in which the person hears voices ordering him or her to do something.
  • Temporal lobe epilepsy (TLE). About 80% of patients diagnosed with TLE report gustatory and olfactory hallucinations as well as auditory and visual hallucinations.
  • Narcolepsy. Frequent hypnagogic hallucinations are considered one of four classic symptoms of narcolepsy and are experienced by 60% of patients diagnosed with the disorder.
  • Post-traumatic stress disorder (PTSD). Studies of combat veterans diagnosed with PTSD have found that 50%–65% have experienced auditory hallucinations. Visual, olfactory, and haptic hallucinations have been reported by survivors of rape and childhood sexual abuse.


It would be difficult to describe a “typical” hallucination, as these experiences vary considerably in length of time, quality, and sense or senses affected. Some hallucinations last only a few seconds; however, some people diagnosed with Charles Bonnet syndrome (CBS) have reported visual hallucinations lasting over several days, while people who have taken certain drugs have experienced hallucinations involving colors, sounds, and smells lasting for hours. Albert Hoffman, the Swiss chemist who first synthesized lysergic acid diethylamide (LSD), experienced nine hours of hallucinations after taking a small amount of the drug in 1943. In 1896, the American neurologist S. Weir Mitchell published an account of the six hours of hallucinations that followed his experimental swallowing of peyote buttons.

There is not always a close connection between the cause of a person's hallucinations and the emotional response to them. One study of patients diagnosed with CBS found that 30% of the patients were upset by their hallucinations, while 13% found them amusing or pleasant. The environment in which LSD and other hallucinogens are taken may affect an individual's psychological constitution and personal reactions. The writer Peter Matthiessen, for example, noted that his 1960s experiences with LSD “were magic shows, mysterious, enthralling,” while his wife “… freaked out; that is the drug term, and there is no better. … [H]er armor had cracked, and all the night winds of the world went howling through.” Indeed, like Mathiessen's wife, and in contrast to those for whom hallucinogens prove amusing, most patients with narcolepsy, alcoholic hallucinosis, or post-traumatic disorders find their hallucinations frightening.


Though there are many known causes of hallucinations, the neurological mechanism behind their origin is not yet known. Researchers have identified some factors in the context of specific disorders, and have proposed various hypotheses to explain hallucinations in others, but there does not appear to be a single causal factor that accounts for hallucinations in all people who experience them.

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Sleep deprivation

Research subjects who have undergone sleep deprivation experiments typically begin to hallucinate after 72–96 hours without sleep. It is thought that these hallucinations result from the malfunctioning of nerve cells within the prefrontal cortex of the brain. This area of the brain is associated with judgment, impulse control, attention, and visual association, and is refreshed during the early stages of sleep. When a person is sleep-deprived, the nerve cells in the prefrontal cortex must work harder than usual without an opportunity to recover. The hallucinations that develop on the third day of wakefulness are thought to be hypnagogic hallucinations that occur during “microsleeps,” or short periods of light sleep lasting about one to ten seconds.

Post-traumatic memory formation

Hallucinations in trauma survivors are caused by abnormal patterns of memory formation during the traumatic experience. In normal situations, memories are formed from sensory data, organized in a part of the brain known as the hippocampus, and integrated with previous memories in the frontal cortex. People then “make sense” of their memories through the use of language, which helps them to describe their experiences to others and to themselves. In traumatic situations, however, bits and pieces of memory are

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stored in the amygdala, an almond-shaped structure in the brain that ordinarily attaches emotional significance to memories, without being integrated by the hippocampus and interpreted in the frontal cortex. In addition, the region of the brain that governs speech (Broca's area) often shuts down under extreme stress. The result is that memories of the traumatic event remain in the amygdala as a chaotic wordless jumble of physical sensations or sensory images that can re-emerge as hallucinations during stressful situations at later points in the patient's life.

Irritative hallucinations

In 1973, a British researcher named Cogan categorized hallucinations into two major groups that he called “irritative” and “release” hallucinations. Irritative hallucinations result from abnormal electrical discharges in the brain, and are associated with such disorders as migraine headaches and epilepsy. Brain tumors and traumatic damage to the brain are other possible causes of abnormal electrical activity manifesting as visual hallucinations.

Hallucinations have also been reported with a number of infectious diseases that affect the brain, including bacterial meningitis, rabies, herpes virus infections, Lyme disease, HIV infection, toxoplasmosis, Jakob-Creuzfeldt disease, and late-stage syphilis.

Release hallucinations

Release hallucinations are most common in people with impaired eyesight or hearing. They are produced by the spontaneous activity of nerve cells in the visual or auditory cortex of the brain in the absence of actual sensory data from the eyes or ears. These experiences differ from the hallucinations of schizophrenia in that those patients experiencing release hallucinations are often able to recognize them as unreal. Release hallucinations are also more elaborate and usually longer in duration than irritative hallucinations. The visual hallucinations of patients with CBS are an example of release hallucinations.

Neurotransmitter imbalances

Neurotransmitters are chemicals produced by the body that carry electrical impulses across the gaps (synapses) between adjoining nerve cells. Some neurotransmitters inhibit the transmission of nerve impulses, while others excite or intensify them. Hallucinations in some conditions or disorders result from imbalances among these various chemicals.

NARCOLEPSY. Narcolepsy is a disorder characterized by uncontrollable brief episodes of sleep, frequent hypnagogic or hypnopompic hallucinations, and sleep paralysis. Between 1999 and 2000, researchers discovered that people with narcolepsy have a much lower than normal number of hypocretin neurons, which are nerve cells in the hypothalamus that secrete a neurotransmitter known as hypocretin. Low levels of this chemical are thought to be responsible for the daytime sleepiness and hallucinations of narcolepsy.

PRESCRIPTION MEDICATIONS. Hallucinations have been reported as side effects of such drugs as ketamine (Ketalar), which is sometimes used as an anesthetic but has also been used illegally to commit date rape; paroxetine (Paxil), an SSRI antidepressant; mirtazapine (Remeron), a serotonin-specific antidepressant; and zolpidem (Ambien), a sleep medication. Ketamine prevents brain cells from taking up glutamate, a neurotransmitter that governs perception of pain and of one's relationship to the environment. Paroxetine alters the balance between the neurotransmitters serotonin and acetylcholine.

Hallucinations in patients with Alzheimer's disease are thought to be a side effect of treatment with neuroleptics (antipsychotic medications), although they may also result from inadequate blood flow in certain regions of the brain. The antiretroviral drugs used to treat HIV infection may also produce hallucinations in some patients.

HALLUCINOGENS AND DRUGS OF ABUSE. Like the hallucinations caused by prescription drugs, hallucinations caused by drugs of abuse result from disruption of the normal balance of neurotransmitters in the brain. Hallucinations in cocaine and amphetamine users, for example, are associated with the overproduction of dopamine, a neurotransmitter associated with arousal and motor excitability. LSD appears to produce hallucinations by blocking the action of the neurotransmitters serotonin (particularly serotonin-2) and norepinephrine. Phencyclidine (PCP) acts like ketamine in producing hallucinations by blocking the reception of glutamate.

People who have used LSD sometimes experience flashbacks, which are spontaneous recurrences of the hallucinations and other distorted perceptions caused by the drug. Some doctors refer to this condition as hallucinogen persisting perception disorder, or HPPD.

There are two types of alcohol withdrawal syndromes characterized by hallucinations. Alcoholic hallucinosis typically occurs after abrupt withdrawal from alcohol after a long period of excessive drinking. The patient hears threatening or accusing voices rather than “seeing things,” and his or her consciousness is otherwise normal. Delirium tremens (DTs), on the other hand, is a

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—An almond-shaped brain structure in the limbic system that is activated in stressful situations to trigger the emotion of fear. Hallucinations related to post-traumatic stress are thought to be caused by the activation of memory traces in the amygdala that have not been integrated and modified by other parts of the brain.
—Pertaining to the sense of hearing.
Charles Bonnet syndrome (CBS)
—A disorder characterized by visual hallucinations following a sudden age-related deterioration in a person's vision, most commonly glaucoma or macular degeneration. CBS is named for a Swiss doctor who first described it in his visually impaired grandfather in 1780.
Command hallucination
—A type of auditory hallucination in which the person hears voices ordering him or her to perform a specific act.
Corollary discharge
—A mechanism in the brain that allows one to distinguish between self-generated and external stimuli or perceptions.
—A false belief that a person maintains in spite of obvious proof or evidence to the contrary.
—A vivid sensory or emotional experience that happens independently of the initial event or experience. Flashbacks resulting from the use of LSD are sometimes referred to as hallucinogen persisting perception disorder, or HPPD.
—Pertaining to the sense of taste.
—A drug or other substance that induces hallucinations.
—Pertaining to the sense of touch; sometimes called tactile hallucinations.
—A part of the brain that is involved in memory formation and learning. The hippocampus is shaped like a curved ridge and belongs to an organ system called the limbic system.
—Pertaining to drowsiness; refers to hallucinations that occur as a person falls asleep.
—Persisting after sleep; refers to hallucinations that occur as a person awakens.
—A false interpretation of a real sensory image or impression.
Irritative hallucinations
—Hallucinations caused by abnormal electrical activity in the brain.
Lysergic acid diethylamide (LSD)
—The first synthetic hallucinogen, discovered in 1938.
—Another name for an antipsychotic medication.
—Chemicals that carry nerve impulses from one nerve cell to another.
—Pertaining to the sense of smell.
—A severe mental disorder characterized by loss of contact with reality. Hallucinations are associated with such psychotic disorders as schizophrenia and brief psychotic disorder.
Release hallucinations
—Hallucinations that develop after partial loss of sight or hearing, and represent images or sounds formed from memory traces rather than present sensory input. They are called “release” hallucinations because they would ordinarily be blocked by incoming sensory data.

withdrawal syndrome that begins several days after drinking stops. A patient with the DTs is disoriented, confused, depressed, feverish, and sweating heavily as well as hallucinating, and the hallucinations are usually visual.

MOOD DISORDERS. Visual hallucinations occasionally occur in patients diagnosed with depression, particularly the elderly. These hallucinations are thought to result from low levels of the neurotransmitter serotonin. The hallucinations that occur in patients with Parkinson's disease appear to result from a combination of medication side effects, depressed mood, and impaired eyesight.


The auditory hallucinations associated with schizophrenia may be the end result of a combination of factors. These hallucinations have sometimes been attributed to unusually high levels of the neurotransmitter dopamine in the patient's brain. Other researchers have noted abnormal patterns of brain activity in patients with schizophrenia. In particular, these patients suffer from dysfunction of a mechanism known as corollary discharge, which allows people to distinguish between stimuli outside the self and internal intentions and thoughts. Electroencephalograms (EEGs) of patients with schizophrenia that were taken while the patients were

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talking showed that corollary discharges from the frontal cortex of the brain (where thoughts are produced) failed to inform the auditory cortex (where sounds are interpreted) that the talking was self-generated. This failure would lead the patients to interpret internal speech as coming from external sources, thus producing auditory hallucinations. In addition, the brains of patients with schizophrenia appear to suffer tissue loss in certain regions. In early 2004, some German researchers reported a direct correlation between the severity of auditory hallucinations in patients with schizophrenia and the amount of brain tissue that had been lost from the primary auditory cortex.


The differential diagnosis of hallucinations can be complicated, but in most cases taking the patient's medical history will help the doctor narrow the list of possible diagnoses. If the patient has been taken to a hospital emergency room, the doctor may ask those who accompanied the patient for information. The doctor may also need to perform a medical evaluation before a psychiatric assessment of the hallucinations can be made. The medical evaluation may include laboratory tests and imaging studies as well as a physical examination, depending on the patient's other symptoms. If it is suspected that the patient is suffering from delirium, dementia, or a psychotic disorder, the doctor may assess the patient's mental status by using a standard instrument known as the mini-mental status examination (MMSE) or the Folstein (after the clinician who devised it). The MMSE yields a total score based on the patient's appearance, mood, cognitive skills, thought content, judgment, and speech patterns. A score of 20 or lower usually indicates delirium, dementia, schizophrenia, or severe depression.

Hallucinations in elderly patients may require specialized evaluation because of the possibility of overlapping causes. The American Association for Geriatric Psychiatry lists hallucinations as an indication for consulting a geriatric psychiatrist. In addition, elderly patients should be routinely screened for visual or hearing impairments.


Hallucinations are treated with regard to the underlying disorder. Depending on the disorder, treatment may involve antipsychotic, anticonvulsant, or antidepressant medications; psychotherapy; brain or ear surgery; or therapy for drug dependence. Hallucinations related to normal sleeping and waking are not a cause for concern.


The prognosis of hallucinations depends on the underlying cause or disorder.



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American Academy of Neurology, 1080 Montreal Ave., St. Paul, MN, 55116, (651) 695-2717, (800) 879-1960, Fax: (651) 879-2791, memberservices@aan.com, http://www.aan.com .

American Association for Geriatric Psychiatry, 7910 Woodmont Ave., Ste. 1050, Bethesda, MD, 20814-3004, (301) 654-7850, Fax: (301) 654-4137, http://www.AAGPonline.org .

American Psychiatric Association, 1000 Wilson Blvd., Ste. 1825, Arlington, VA, 22209-3901, (703) 907-7300, apa@psych.org, http://www.psych.org .

National Institute of Mental Health, 6001 Executive Blvd., Room 8184, MSC 9663, Bethesda, MD, 20892-9663, (301) 433-4513; TTY: (301) 443-8431 Fax: (301) 443-4279, (866) 615-6464; TTY: (866) 415-8051, nimhinfo@nih.gov, http://www.nimh.nih.gov .

National Schizophrenia Foundation, 403 Seymour Ave., Ste. 202, Lansing, MI, 48933, (517) 485-7168, Fax: (517) 485-7180, (800) 482-9534, inquiries@nsfoundation.org, http://www.schizophrenia.com .

National Sleep Foundation, 1010 N Glebe Rd., Ste. 310, Arlington, VA, 22201, (703) 243-1697, nsf@sleepfoundation.org, http://www.sleepfoundation.org .

Rebecca Frey, PhD

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

Source Citation

Source Citation   (MLA 8th Edition)
Frey, Rebecca. "Hallucination." The Gale Encyclopedia of Mental Health, edited by Kristin Key, 3rd ed., vol. 1, Gale, 2012, pp. 727-732. Gale Health and Wellness, https%3A%2F%2Flink.gale.com%2Fapps%2Fdoc%2FCX4013200219%2FHWRC%3Fu%3Dmnkanokahs%26sid%3DHWRC%26xid%3Dc05e919a. Accessed 13 Dec. 2019.

Gale Document Number: GALE|CX4013200219