hallucination

More about Hallucinations: Causes and symptoms Diagnosis Treatment Prognosis Resources

Definition

Hallucinations are false or distorted sensory experiences that appear to be real perceptions. These sensory impressions are generated by the mind rather than by any external stimuli, and may be seen, heard, felt, and even smelled or tasted.

Description

A hallucination occurs when environmental, emotional, or physical factors such as stress, medication, extreme fatigue, or mental illness cause the mechanism within the brain that helps to distinguish conscious perceptions from internal, memory-based perceptions to misfire. As a result, hallucinations occur during periods of consciousness. They can appear in the form of visions, voices or sounds, tactile feelings (known as haptic hallucinations), smells, or tastes.

Patients suffering from dementia and psychotic disorders such as schizophrenia frequently experience hallucinations. Hallucinations can also occur in patients who are not mentally ill as a result of stress overload or exhaustion, or may be intentionally induced through the use of drugs, meditation, or sensory deprivation. A 1996 report, published in the British Journal of Psychiatry, noted that 37% of 4,972 people surveyed experienced hypnagogic hallucinations (hallucinations that occur as a person is falling to sleep). Hypnopomic hallucinations (hallucinations that occur just upon waking) were reported by 12% of the sample.

— Paula Anne Ford-Martin

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n.

1. 1. Perception of visual, auditory, tactile, olfactory, or gustatory experiences without an external stimulus and with a compelling sense of their reality, usually resulting from a mental disorder or as a response to a drug.
   2. The objects or events so perceived.
2. A false or mistaken idea; a delusion.

hallucinational hal·lu'ci·na'tion·al or hal·lu'ci·na'tive adj.

Definitions

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.

Description

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.… her armor had cracked, and all the night winds of the world went howling through." In contrast to those who take hallucinogens, however, a majority of patients with narcolepsy, alcoholic hallucinosis, or post-traumatic disorders finds their hallucinations frightening.

Demographics

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

In 2000, one of the few studies of hallucinations in a general Western population reported the following statistics:

• Of a total sample of 13,000 adults, 38.7% reported hallucinations: 6.4% had hallucinations once a month, 2.7% once a week, and 2.4% more than once a week.
• Of the subjects, 27% reported having hallucinations in the daytime. In this group, visual (3.2%) and auditory (0.6%) hallucinations were closely associated with diagnoses of psychotic or anxiety disorders.
• Of the subjects, 3.1% reported haptic (tactile) hallucinations; most of these subjects were current drug users.

There is currently 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. Hallucinogens are the third most frequently abused class of drugs (after alcohol and marijuana) among high school and college students. Various surveys report that about 7% of people in the United States over the age of 12 have taken LSD at least once; that 5% of high school seniors admit to using MDMA (Ecstasy); and that 20–24% of college students use MDMA. The highest rate of hallucinogen abuse is found in Caucasian 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 into 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. For example, the man who killed a Swedish politician in September 2003 told the police that voices in his head told him "to attack."
• 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.

Causes

The neurologic causes of hallucinations are not currently completely understood, although researchers have identified some factors in the context of specific disorders, and have proposed various hypotheses to explain hallucinations in others. There does not appear to be a single causal factor that accounts for hallucinations in all people who experience them.

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 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 neuro-transmitter 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 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.

Schizophrenia

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 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.

Diagnosis

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.

Treatment

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.

Prognosis

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

Resources

BOOKS

Altman, Lawrence K., MD. Who Goes First? The Story of Self-Experimentation in Medicine. Berkeley, CA: University of California Press, 1998.

American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders, 4th edition, text revision. Washington, DC: American Psychiatric Association, 2000.

Beers, Mark H., MD. "Behavior Disorders in Dementia." The Merck Manual of Geriatrics, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

"Drug Use and Dependence." The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

Matthiessen, Peter. The Snow Leopard. New York: Penguin Books USA, 1987.

"Psychiatric Emergencies." The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

"Schizophrenia and Related Disorders." Section 15, Chapter 193 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 2002.

PERIODICALS

Braun, Claude M. J., Mathieu Dumont, Julie Duval, et al. "Brain Modules of Hallucination: An Analysis of Multiple Patients with Brain Lesions." Journal of Psychiatry and Neuroscience 28 (November 2003): 432–439.

Cameron, Scott, MD, and Michael Richards, MD. "Hallucinogens." eMedicine. Cited January 9, 2004 (March 23, 2004). http://www.emedicine.com/med/topic3407.htm.

Chuang, Linda, MD, and Nancy Forman, MD. "Mental Disorders Secondary to General Medical Conditions." eMedicine. Cited January 30, 2003 (March 23, 2004). http://www.emedicine.com/med/topic3447.htm.

Cowell, Alan. "Swedish Foreign Minister's Killer Blames 'Voices' in His Head." New York Times. Cited January 15, 2004.

Ford, J. M., and D. H. Mathalon. "Electrophysiological Evidence of Corollary Discharge Dysfunction in Schizophrenia During Talking and Thinking." Journal of Psychiatric Research 38 (January-February 2004): 37–46.

Gaser, C., I. Nenadic, H. P. Volz, et al. "Neuroanatomy of 'Hearing Voices': A Frontotemporal Brain Structural Abnormality Associated with Auditory Hallucinations in Schizophrenia." Cerebral Cortex 14 (January 2004): 91–96.

Gleason, Ondria C., MD. "Delirium." American Family Physician 67 (March 1, 2003): 1027–1034.

Ohayon, M. M. "Prevalence of Hallucinations and Their Pathological Associations in the General Population." Psychiatry Research 97 (December 27, 2000): 153–164.

Pelak, V. S., and G. T. Liu. "Visual Hallucinations." Current Treatment Options in Neurology 6 (January 2004): 75–83.

Rovner, Barry R., MD. "The Charles Bonnet Syndrome: Visual Hallucinations Caused by Vision Impairment." Geriatrics 57 (June 2002): 45–46.

Schneider, L. S., and K. S. Dagerman. "Psychosis of Alzheimer's Disease: Clinical Characteristics and History." Journal of Psychiatric Research 38 (January-February 2004): 105–111.

Tsai, M. J., Y. B. Huang, and P. C. Wu. "A Novel Clinical Pattern of Visual Hallucination After Zolpidem Use." Journal of Toxicology: Clinical Toxicology 41 (June 2003): 869–872.

OTHER

National Institute of Mental Health (NIMH). Schizophrenia. NIH Publication No. 02-3517. Bethesda, MD: NIMH, 2002. (March 23, 2004). http://www.nimh.nih.gov/publicat/schizoph.cfm.

National Institute on Drug Abuse (NIDA). Research Report: Hallucinogens and Dissociative Drugs. NIH Publication No. 01-4209. Bethesda, MD: NIDA, 2001.

ORGANIZATIONS

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

American Association for Geriatric Psychiatry. 7910 Woodmont Avenue, Suite 1050, Bethesda, MD 20814-3004. (301) 654-7850; Fax: (301) 654-4137. main@aagponline.org. http://www.aagponline.org.

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

National Institute of Mental Health (NIMH) Office of Communications. 6001 Executive Boulevard, Room 8184, MSC 9663, Bethesda, MD 20892-9663. (301) 443-4513 or (866) 615-NIMH; Fax: (301) 443-5158. nimhinfo@nih.gov. http://www.nimh.nih.gov.

National Schizophrenia Foundation. 403 Seymour Avenue, Suite 202, Lansing, MI 48933. (517) 485-7168 or (800) 482-9534; Fax: (517) 485-7180. inquiries@nsfoundation.org. http://www.nsfoundation.org.

National Sleep Foundation (NSF). 1522 K Street NW, Suite 500, Washington, DC 20005. (202) 347-3471; Fax: (202) 347-3472. nsf@sleepfoundation.org. http://www.sleepfoundation.org.

Rebecca Frey, PhD

A perceptual experience in the absence of external stimulation. Hallucinations differ from illusions, which are changes in the perception of a real object. Hallucinations tend to fade with fixation or with attention to the content. Except for afterimages, which lie like a film over objects, hallucinations replace objects and object space. A hallucination is not objectlike in its realness. The conviction of reality is due to the loss of an object for comparison and the inability to disprove the image through other sensory modalities.

Hallucinations that are recognized as such by the experiencer include those resulting from sensory deprivation, drug use, and the phantom limb state. See also Schizophrenia.

Hallucinations may occur in a range of neurologic and psychiatric conditions, although they are usually considered hallmarks of schizophrenia. Delusional misidentification syndromes are subtype of hallucinations and may also occur in neurological and psychiatric disease. For example, Capgras syndrome, which is commonly seen in schizophrenia, causes the individual to replacea familiar person (usually the spouse) with an imposter with the same or similar physical appearance. Frégoli syndrome is the delusional confusion of an individual as a familiar person in disguise.

Neurotransmitters are directly involved in the regulation of drug-induced and schizophrenic hallucinations, with many accounts pointing to the involvement of serotonin and dopamine. Therefore, it is possible to treat individuals with antipsychotic drugs that stabilize the chemical systems involved.

With localized damage to the brain, hallucinations are usually brief and intermittent, though in some cases, especially neurologic damage involving the brainstem, hallucinations can be chronic and sustained.

Physical input to the eyes and ears constrains and guides the construction of mental images,but the final result—the perception of an object or sound as a meaningful event occurringin the external world—also reflects very complex physiological processes. They begin in the brainstem, pass to the limbic system of the brain, and finally involve the temporal, parietal, and occipital areas of the cerebral cortex. Various types of hallucination are caused by disruptions that occur at different levels along that sequence of brain processes. See also Cognition.

At its earliest phase, damage to the upper brainstem produces peduncular (crepuscular) hallucinations of faces, torsos, and occasionally geometric patterns or landscapes near the viewer at the close of day. The images may be static and immobile or may change in content and affective tonality while being viewed. A smiling young boy, for example, may change into a scowling oldwoman. The hallucinations are often vivid and chromatic, and tend to be multimodal: they are seen, heard, and even touched, and occur over the entire visual field. Olfactory and gustatory images have also been described. Peduncular hallucinations are similar to the hypnagogic hallucinations that are experienced when falling asleep. See also Sleep and dreaming.

Neurologic damage involving limbic and temporal-lobe structures yields hallucinations of faces or formed scenes laden with meaning and affect. Changes in size (micropsia, macropsia) and shape (metamorphopsia) may occur. Déjà vu, derealization, and dreamy states are common. Auditory hallucinations are usually of speech or music. Microscopic (Lilliputian) and autoscopic (out-of-the-body) hallucinations also occur with temporal-lobe lesions. Exposure to a wide range of drugs and many psychiatric disorders, especially schizophrenia, can lead to hallucinations whose form suggests dysfunction involving limbic or temporal-lobe structures. See also Psychotomimetic drug.

Damage to the parietal lobe leads to illusory distortions of shape, size, and motion, whereas occipital lesions or stimulation—or migraine—gives elementary hallucinations of sparks, flames, lines, or simple patterns. These hallucinations share features with afterimages.Palinopsia, the hallucinatory persistence of an object after the viewer has turned away, is a form of pathological afterimagery. See also Perception.

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Hallucination may be simply defined as the perception of an external object in the absence of a corresponding stimulus, yet such a simple definition obscures a whole series of conceptual difficulties which surround the medical and psychiatric use of the term. The range of conditions subsumed under this category is massive, and includes such varied phenomena as religious visions, phantom limbs, tinnitus, psychedelic ‘trips’, schizophrenic inner voices, the personal experience of doppelgangers, and a sceptical apprehension of the unreality of the outer world.

Such variety has naturally frustrated any attempt at providing a clear classification of the phenomenon. Attempts to distinguish the various forms of hallucination according to their origins, their content, their intensity, and the condition of their hosts have been largely unsuccessful. Most psychiatrists in Europe and North America have now adopted a fairly broad definition of the phenomenon, simply relying upon the distinction between illusion, which resulted from the misinterpretation of an existing external object, and hallucination, in which the false perception is generated without any reference to the outside world. Even this definition, which was introduced by the French psychiatrist J. E. D. Esquirol in the early nineteenth century, fails to account for such borderline phenomena as synaesthesia in which the sensations provoked by an object become confused, so that the subject may taste colours or see sounds.

Alongside this ongoing contest over the definition and classification of hallucination there exists a more fundamental struggle over the meaning and significance of the phenomenon. Artists and mystics have long criticized the modern medicalization of hallucinations, portraying the process as a secularizing attempt to pathologize religious or spiritual experience. Certainly popular attitudes to hallucination have been transformed across the last thousand years. In the Platonic tradition of classical philosophy, the subjective vision was celebrated as a form of privileged insight beyond the phenomenal experience of the external world. Likewise in the Christian and Jewish religions the objective quality of the inner hallucination had long been regarded as a proof of its spiritual reality, although its origin could have been either demonic or divine.

These Platonic and Christian traditions were united in the work of the Primitive Church fathers. Their writing held up the visionary experience as a charism, a gift from God which allowed individuals to perceive some object which was normally invisible to men. This conception was further refined by St Augustine, who divided visions into three classes: the corporeal, in which an apparition of an object was presented before the individual's eyes through either natural or spiritual means; the imaginative, in which an image was supernaturally created in the host's mind; and the intellectual, in which sense of personal assurance was created directly by God, without recourse to implanted words or images.

This framework for interpreting the hallucinatory experience persisted into the nineteenth century. Many romantic writers, such as Coleridge and Wordsworth, complained that normal vision enslaved the mind to the mundane world of material object. In contrast, they proposed a ‘Spiritual Optics’ (to borrow Thomas Carlyle's phrase) in which the inner eye would be awakened to the creative inspiration of the spirit. Such a programme sat unhappily with contemporary medical investigations in this field. In the late eighteenth and early nineteenth centuries, many writers commented upon the correlation between hallucination, injury, and disease. This correlation suggested that the hallucination had a somatic basis, originating in either the disordered operations of the peripheral nerves or an aberrant psychological process in the brain.

This interpretation of hallucination as a symptom of organic nervous disorder persisted throughout the nineteenth century. In 1881 the Italian psychiatrist, August Tamburini, presented a coherent neurological model for the experience, arguing that hallucination was produced through a pathological excitement or epilepsy in the higher sensory centres of the brain. This materialist account did little to diminish the mystical celebration of hallucination. Writers influenced by spiritualism and the Swedish mystic Emanuel Swedenborg accepted the scientific identification of hallucination with organic disturbance, arguing that this identity provided strong evidence for the objective reality of visions.

The mystical assessment of the significance of hallucinations was undermined by a series of psychological surveys at the end of the nineteenth century. During the 1880s the statistician, Francis Galton, circulated questionnaires on mental imagery to schools and acquaintances. From the responses he was able to demonstrate a gradation between hallucination and the familiar acts of visualization which occurred in everyday life. Galton suggested that hallucination was not a distinct experience, but rather that it represented an extreme point on two axes representing the strength of the mental image and its resistance to conscious control. This statistical erosion of the boundary between normal visualization and pathological hallucination was reinforced in a more wide-scale survey published by the Society for Psychical Research (SPR) in 1892. The SPR's ‘Census of Hallucinations’ discovered 1684 cases of waking hallucination amongst 17 000 respondents. Further analysis suggested that hallucination was most prevalent amongst women, children, and the insane, although the experience could occur in almost any individual.

In the twentieth century the hallucinatory experience seems to have lost its spiritual significance. The popular use of hallucinogenic drugs, such as LSD and psilocybin, and increased understanding of the chemical mechanisms of their actions, has encouraged a more instrumental attitude towards the visionary experience. Hallucination is no longer seen as a gratuitous event except in pathological cases such as fever or schizophrenia. Rather it is a state which can be induced directly through chemical, electrical, or mechanical means. As the neurosurgeon Wilder Penfield demonstrated, intense mental images may be created through the electrical stimulation of a subject's brain. Likewise hallucinations of movement (see proprioception) can be induced at a particular joint through the mechanical vibration of the muscles attached to it. Through such technical advances the meaning and cultural significance of hallucination has been transformed. The vision, which once revealed the mind of God to men, is now seen as a symptom revealing the disordered mind of man to others.

— Rhodri Hayward

Bibliography

• Berrios, G. E. (1995). The history of mental symptoms. Cambridge University Press, Cambridge.
• Critchley, M. (1987). Hallucinations and their impact upon art. Carnegie Press, Preston

See also illusions.

noun

1. An erroneous perception of reality: delusion, ignis fatuus, illusion, mirage, phantasm, phantasma, will-o'-the-wisp. See real/imaginary.
2. An illusion of perceiving something that does not really exist: phantasmagoria, phantasmagory. Slang trip. See real/imaginary.

n

Definition: dream, delusion
Antonyms: experience, fact, reality, truth

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(həl-ōō′sin-ā′shən)
n

An artificial sensory experience without the presence of an external cause.

For more information on hallucination, visit Britannica.com.

The occurrence of an experience in itself indistinguishable from a perception of something, but without an appropriate external cause. Hallucination is sometimes distinguished from pseudohallucination, in which the experience occurs but is not mistaken for the perception of an external object. Both are distinct from illusion, in which there is an external source, but its nature is mistaken (a mirage is thus an illusion, but not a hallucination). The possibility of hallucination is a frequent starting-point for the distinction between appearance and reality. See also illusion, argument from.

A state of mind whereby a person senses something that in reality does not exist; a perception of an object having no reality. 90 S.E. 2d 593, 596. Any of the senses may be involved, although sight or hearing are most commonly affected. The state of hallucination most often results from mental illness or from ingesting drugs designed to create these perceptions. See controlled substance [hallucinogens].

A false perception of sensory vividness arising without the stimulus of a corresponding sense impression. In this it differs from illusion, which is merely the misinterpretation of an actual sense perception. Visual and auditory hallucinations are the most common, but hallucinations of the other senses may also be experienced. Human figures and voices most frequently form the subject of a hallucination, but in certain types other classes of objects may be seen, as, for instance, the rats and insects of delirium tremens.

Although hallucination is often associated with various mental and physical diseases, it may nevertheless occur spontaneously while the agent shows no departure from full vigor of body and mind. It may also be induced (i.e., in hypnotism) in a high percentage of subjects. The essential difference between sane and insane hallucinations is that in the former case the agent can, by reflection, recognize the subjective nature of the impression, even when it has every appearance of objectivity, whereas in the latter case the patient cannot be made to understand that the vision is not real.

Until the early twentieth century, hallucinatory percepts were regarded merely as intensified memory images; however, the most intense of ordinary representations do not possess the sensory vividness of the smallest sensation received from the external world. It follows that other conditions must be present besides the excitement of the brain, which is the correlate of representation. The seat of excitement is the same in actual sense perceptions and in memory images, but in the former the stimulus is peripherally originated in the sensory nerve, whereas in the latter it originates in the brain itself.

When a neural system becomes highly excited—a state which may be brought about by emotion, ill health, drugs, or a number of other causes—it may serve to divert from their proper paths any set of impulses arising from the sense organs. Because any impulse ascending through the sensory nerves produces an effect of sensory vividness—normally, a true perception—the impulses thus diverted gives to the memory image an appearance of actuality not distinguishable from that produced by a corresponding sense impression—a hallucination.

In hypnosis a state of cerebral dissociation is induced, whereby a neural system may be abnormally excited and hallucination thus readily engendered. Drugs, especially hallucinogens, which excite the brain, also induce hallucinations.

In 1901 the British physician Sir Henry Head demonstrated that certain visceral disorders produce hallucinations, such as the appearance of a shrouded human figure. The question of whether there is any relationship between the hallucination and the person it represents is, and has long been, a vexing one. Countless well-authenticated stories of apparitions coinciding with a death or some other crisis are on record and would seem to establish some causal connection between them. In former times apparitions were considered to be the doubles or "ethereal bodies" of real persons, and Spiritualists believe that they are the spirits of the dead (or, in some instances, of the living) temporarily forsaking the physical body.

The dress and appearance of the apparition does not necessarily correspond with the actual dress and appearance of the person it represents. Thus a man at the point of death, in bed and wasted by disease, may appear to a friend miles away as if in ordinary health and wearing familiar clothing. Nevertheless, there are notable instances where some remarkable detail of dress is reproduced in the apparition. It seems clear, however, that it is the agent's general personality that is, as a rule, conveyed to the percipient, and not, except in special cases, his or her actual appearance.

It has been suggested that those images that do not arise in the subliminal consciousness of the agent may be telepathically received by him or her from other minds. A similar explanation has been offered for the hallucinatory images that many people can induce by crystal gazing or staring into a pool of water, a drop of ink, or a magic mirror in search of information about scenes or people they know nothing about.

Collective hallucination is a term applied to hallucinations shared by a number of people. There is no firm evidence, however, of the operation of any agency other than suggestion or telepathy.

Hallucination and Psychical Research

One of the most succinct definitions of hallucination occurs in Phantasms of the Living (2 vols., 1886), by Edmund Gurney, F. W. H. Myers, and Frank Podmore: "percepts which lack, but which can only by a distinct reflection be recognised as lacking, the objective basis which they suggest." If the sensory perception coincides with an objective occurrence or counterpart, the hallucination is called veridical, (truth-telling), as in the phantasm of the dying. If the apparition is seen by several people at the same time, the case is called collective veridical hallucination.

In the years following the foundation of the Society for Psychical Research (SPR), London, the hallucination theory of psychic phenomena was in great vogue. If no other explanation was available the person who had had a supernormal experience was told it was a hallucination, and if several people testified to the same occurrence it was said that the hallucination of one was communicated to the others. Sir William Crookes counters that idea in his Researches in the Phenomena of Spiritualism (1870): "The supposition that there is a sort of mania or delusion which suddenly attacks a whole roomful of intelligent persons who are quite sane elsewhere, and that they all concur, to the minutest particulars, in the details of the occurrences of which they suppose themselves to be witnesses, seems to my mind more incredible than even the facts which they attest."

Charles Richet, in Thirty Years of Psychical Research (1923), omits hallucination completely in his discussion of metapsychical phenomena (a term for paranormal). He believed that hallucination should be reserved to describe a morbid state when a mental image is exteriorized without any exterior reality. According to Richet, "It is extremely rare that a person who is neither ill, nor drunk, nor hypnotised should, in the walking state, have an auditory, visual, or tactile illusion of things that in no way exist. The opinion of alienists that hallucination is the chief sign of mental derangement, and the infallible characteristic of insanity seems to me well grounded. With certain exceptions (for every rule there are exceptions) a normal healthy individual when fully awake does not have hallucinations. If he see[s] apparitions these correspond to some external reality or other. In the absence of any external reality there are no hallucinations but those of the insane and of alcoholics."

An instance recounted by Sir John Herschel did not conform to Richet's idea. He had been watching with some anxiety the demolition of a familiar building. On the following evening, in good light, he passed the spot where the building had stood. "Great was my amazement to see it," he wrote, "as if still standing, projected against the dull sky. I walked on, and the perspective of the form and disposition of the parts appeared to change as they would have done if real."

In the case of hauntings where a ghost is seen, Gurney suggests that a person thinking of a given place that is at the time actually experienced in sense perception by others may be imparting into the consciousness of the others a thought existing in his own.

Of course, data provided by a registering apparatus or photography may rule out the hallucination theory as applied to hauntings, provided that there is some proper scientific control. Similarly, if objects are displaced, as in poltergeist cases, the theory of hallucination is no longer tenable. As Andrew Lang writes in Cock Lane and Common Sense (1896), "Hallucinations cannot draw curtains, or open doors, or pick up books, or tuck in bedclothes or cause thumps."

The things seen during a psychic experience of an otherwise normal person should also be distinguished from the hallucinations of the mentally deranged, of the sick, drunk, or drugged. The latter are not veridical, nor telepathic, nor collective. In the "Census of Hallucinations," published in the Proceedings of the SPR (1894), the committee excluded, as far as possible, all pathological subjects. J. G. Piddington (see Proceedings, vol. 19), in testing this census for cases that would show the same nature as hallucinations arising from visceral diseases, concluded that there was not a single case in the census report that fell into line with the visceral type.

In hypnotic hallucinations the hypnotized subject may see apparitions if so suggested and may not see ordinary people who are in the same room. But the subject may hear the noises they make, see the movement of objects they touch, and be frightened by what appears to be poltergeist phenomena. If the suggestion is posthypnotic the subject may also see a phantom shape when given a signal or at a prescribed time.

The visions seen by some people on the verge of sleep were called " hypnagogic hallucinations" by F. W. H. Myers. The afterimages on waking from sleep he named "hypnopompic hallucinations." A comprehensive study of both classes of phenomena was published by G. E. Leaning in the Proceedings of the SPR, (vol. 35, 1926).

The difference between hallucination and illusion is that there is an objective basis for the illusion, which is falsely interpreted. In hallucination, although more than one sense may be affected, there is no external basis for the perception.

Sources:

Besterman, Theodore. Crystal-Gazing. London, 1924. Reprint, New Hyde Park, NY: University Books, 1965.

Bramwell, J. M. Hypnotism: Its History, Practice, and Theory. London, 1903.

Gurney, Edmund, F. W. H. Myers, and Frank Podmore. Phantasms of the Living. 2 vols. London: Trubner, 1886. Reprint, Gainesville, FL: Scholars Facsimiles Reprints, 1970.

Huxley, Aldous. The Doors of Perception. London, 1954. Johnson, Fred H. The Anatomy of Hallucinations. Chicago: Nelson Hall, 1978.

MacKenzie, Andrew. Apparitions and Ghosts. London: Barker, 1971. Reprint, New York: Popular Library, 1972.

——. Hauntings and Apparitions. London: Heinemann, 1982.

Myers, F. W. H. Human Personality and Its Survival of Bodily Death. 2 vols. London: Longmans Green, 1903. Reprint, New York: Arno Press, 1975.

Podmore, Frank. Apparitions and Thought Transference. London, 1894.

Reed, Graham. The Psychology of Anomalous Experience. Boston: Houghton Mifflin, 1974.

Richet, Charles. Thirty Years of Psychical Research. London: W. Collins, 1923. Reprint, New York: Arno Press, 1975.

Rogo, D. Scott. Mind Beyond the Body: The Mystery of ESP Projection. New York: Penguin, 1978.

Samuels, Mike. Seeing With the Mind's Eye: The History, Techniques, and Uses of Visualization. New York: Bookworks; Random House, 1975.

Tyrrell, G. N. M. Apparitions. London: Duckworth, 1953. Reprint, London: Society for Psychical Research, 1973.

A false perception that appears to be real, as when, for example, a man dying of thirst in a desert thinks that he sees a lake. (See also delusion.)

Briefly defined as sensory perception in the absence of external stimuli, hallucination has three characteristics: thoughts or memory images, perhaps when they are as vivid and immediate as perceptions, are experienced as if they were perceptions; they are externalized, or projected, being experienced as if they came from outside the person; and the mistaking of imagery for perception is not corrected in the light of the other information available. The term pseudohallucination has been used to describe imagery as vivid and immediate as perception but not mistaken as such. Pseudohallucinations are more likely to be perceived in response to isolation or an intense emotional need: for example, shipwrecked sailors may visualize boats coming to their rescue. The fanciful elaboration of perception of external stimuli — for example, faces seen in the fire — is illusion. The imagery of a vision is experienced as if it came from outside, although not from ordinary reality as perception does.

Young children often fail to distinguish between imagery and perception and suppose that what they imagine is external and perceptible to others. Adults sometimes fail to make the distinction, especially at a time of high expectation or arousal. A widow mourning her husband may see him or hear his voice or footsteps repeatedly after his death, resulting in a 'sense of presence' which fades with the passage of time. In a wood at night, dark shadows are seen as lurking beasts. Waking from a frightening dream, a person feels that what he has experienced has happened in reality.

Mistakes like these are corrected when the person recognizes that they conflict with other information or the views of others. Normally imagery is continually reappraised in the light of further information becoming available, and further information is sought by testing reality. Hearing a noise, a person makes a small head movement and tests whether the change in the strength and character of the noise conforms to his expectation. Perceiving someone in a crowd as an acquaintance, a person looks again or asks a companion for confirmation. Macbeth in Shakespeare's play, while planning to murder Duncan, hallucinates a dagger, and asks: 'Art thou not, fatal vision, sensible to feeling as to sight? Or art thou but a dagger of the mind, a false creation, proceeding from the heat-oppressed brain?'

After a long period of wakefulness or busyness, attention tends to be withdrawn from the outside world, and the testing of reality to be impaired and reduced. Hallucination is relatively common under these conditions and remains uncorrected for longer. Sufficient information is available but is not used. On the other hand, the subjects of experiments on the effects of sensory deprivation, who are put into a darkened and soundproofed room, do not get sufficient information to enable them to test reality and to reappraise their hallucinatory experiences. (See isolation experiments.) Also, a person on his own is less able to test reality and to reappraise what he has experienced.

Hallucinations tend to be disowned, the person feeling that he has no control over the imagery, which he feels is imposed on him by an outside agency. They are often reported as distressing, threatening, or tormenting, only occasionally, for example by a widow, as reassuring. There are other distressing phenomena that are not hallucinations, although akin to them in some respects. Thus, some recurring images obtrude and cannot be stopped, but are accepted as belonging to the individual. Such images are termed obsessions. Ringing in the ear ('tinnitus'), resulting, for instance, from disease of the ear, is sometimes described by a fanciful simile, e.g. as being like sea flowing over shingle, or as if there were nearby a machine crushing stones. What is being described may be thought mistakenly to be hallucination if the explicit comparison of the 'like' or the 'as if' fails to be noted.

Hallucination is common in patients who have suffered damage to the brain as a result of trauma, infection, or intoxication by drugs or alcohol. The association of hallucination, fearfulness, and agitation in these cases may be described as delirium. A patient who suffers from delirium tremens as a result of alcoholism may see such frightening things as red spiders or pink elephants, or he may feel that lice are crawling over his skin, because hallucination although usually visual may be experienced through any of the senses. Indeed, hallucinations in functional psychoses are more often auditory than visual. Schizophrenic patients may hear the voices of their persecutors, conversations about themselves between third parties, or their own thoughts spoken aloud (echo de pensée). Severely depressed patients may hear voices making derogatory remarks or threatening them with punishment or torture. Some schizophrenic patients even experience tactile hallucinations which give rise to delusional beliefs that they are being sexually assaulted. Olfactory hallucinations are sometimes perceived by severely melancholic patients who come to believe that they are giving off revolting odours from their bodies causing people to avoid them. Patients mistake hallucinations of all these kinds for perceptions coming from outside themselves, and attribute to others what they experience, usually without any testing of reality.

Explanations of hallucination refer to several processes. In delirium there tends to be a high level of arousal and at the same time a lowering of vigilance, impairment of perception, and impairment and reduction of reality testing. Enhancement of imagery as a direct effect of drugs or toxins on nervous tissue is similar to that of electrical stimulation of the temporal lobes of the brain when it produces, in a conscious patient whose brain has been exposed during surgery, intense visual, auditory, or other imagery as 'strips' of experience. Poisoning by drugs may also, more importantly, increase the random activity of nervous tissue. Sensations then become blurred, to produce background noise, which is then elaborated into illusion. A person poisoned by LSD may see visual patterns like lace curtains, usually coloured. In some illnesses in which there is hallucination, the functioning of peripheral nerves is affected by neuritis, and as a result the patient may experience numbness, pins and needles, or itching, which is elaborated into the illusion of lice. Similarly, the result of neuritis of the retina may be spiders dangling in front of the eyes, brain-elaborations of phosphenes. In schizophrenia, the patient has typically disengaged from social activities, and the testing of reality is reduced as a result, but this does not account for his disowning of what he experiences. It has to be supposed that thoughts and feelings have been dissociated as a psychological defence in order to reduce the anxiety which would otherwise arise. The patient positively resists any reappraisal of what he has experienced.

(Published 1987)

— Derek Russell Davis

Bibliography
• Galton, F. (1907). Inquiries into Human Faculty and its Development.
• Harris, J. P., and Gregory, R. L. (1981). 'Tests of hallucinations of Ruth'. Perception, 10.
• Siegel, R. K. (1993). Fire in the Brain: Clinical Tales of Hallucination.
• — —  and West, J. L. (eds., 1975). Hallucinations.

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IN BRIEF: Something that is not real but of which someone is aware.

Someone who travels in the desert might have a hallucination of a swimming pool.

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Dansk (Danish)
n. - hallucination, sansebedrag

Nederlands (Dutch)
hallucinatie, waanvoorstelling

Français (French)
n. - hallucination

Deutsch (German)
n. - Halluzination

Ελληνική (Greek)
n. - παραίσθηση, ψευδαίσθηση, φαντασιοπληξία

Italiano (Italian)
allucinazione

Português (Portuguese)
n. - alucinação (f), (m) desvario

Русский (Russian)
галлюцинации

Español (Spanish)
n. - alucinación

Svenska (Swedish)
n. - hallucination

中文（简体）(Chinese (Simplified))
幻觉, 幻想

中文（繁體）(Chinese (Traditional))
n. - 幻覺, 幻想

한국어 (Korean)
n. - 환각, 착각, 망각

日本語 (Japanese)
n. - 幻覚, 妄想, 錯覚, 幻想

العربيه (Arabic)
‏(الاسم) الهلوسه, الهذيان‏

עברית (Hebrew)
n. - ‮הזיה‬

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