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A hallucination is a perception in the absence of a stimulus that the person may or may not believe is real. Hallucinations may occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive. Hallucinations are different from illusions. In an illusory experience, a genuine sensation is attributed to an incorrect cause, misinterpreting a coat hanging on a door to be an intruder or thinking there is water on a hot road, due to the heat rising from the road. A delusional perception is where a genuine perception (ie. correctly sensed and interpreted) is given some additional (and typically bizarre) significance. Hypnagogic hallucinations and hypnopompic hallucinations are considered normal phenomena. Hypnagogic hallucinations can occur as one is falling asleep and hypnopompic hallucinations occur when one is waking up. Hallucinations may also be associated with drug use (particularly hallucinogenic drugs), sleep deprivation, psychosis or neurological illness.



Studies have now shown hallucinatory experiences take place across the world. Previous studies, one as early as 1894,[1] have reported that approximately 10% of the population experience hallucinations. A recent survey of over 9,000 people[2] reported a much higher figure with almost 39% of people reported hallucinatory experiences, 27% of which reported daytime hallucinations, mostly outside the context of illness or drug use. From this survey, olfactory (smell) and gustatory (taste) hallucinations seem the most common in the general population.

Auditory hallucinations

Auditory hallucinations, particularly of one or more talking voices, are particularly associated with psychotic disorders such as schizophrenia. and the presence may have significance in diagnosing these conditions, People not suffering from diagnosable mental illness may sometimes hear voices as well.[3] The Hearing Voices Movement is a support and advocacy group for people who hallucinate voices, but do not otherwise show signs of mental illness or impairment. Other types of auditory hallucinations include musical hallucinations, where people will hear music playing in their mind, usually songs they are familiar with. This can be caused by lesions on the brain stem, occurring most often from strokes, but also tumors, sencephalitis, or abscesses.[4] Recent reports have also mentioned that it is possible to get musical hallucinations from listening to music for long periods of time.

Visual hallucinations

Hypnagogic hallucination

These hallucinations occur just before falling asleep and affect a surprising number of people in the population. The hallucinations can last from seconds to minutes, all the while the subject usually remains aware of the true nature of the images. These are usually associated with narcolepsy, but can also affect normal minds. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare.[5]

Peduncular hallucinosis

Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons on the brain stem. These hallucinations occur most often in the evenings, but not during drowsiness as in the case of hypnagogic hallucination. The subject is usually fully conscious and can interact with the hallucinatory characters for extended periods of time. As in the case of hypnagogic hallucinations, insight into the nature of the images remains intact. The false images can occur in any part of the visual field, and are rarely polymodal.[5]

Delirium tremens

Main article: Delerium tremens

One of the most enigmatic forms of visual hallucinations are the highly variable, possibly polymodal delirium tremens. Individuals suffering from delerium tremens may be agitated and confused, especially in the later stages of this disease. Insight is gradually reduced with the progression of this disorder. Sleep is disturbed and occurs for a shorter period of time, with REM overflow.[5]

Parkinson's disease and Lewy body dementia

Parkinson's disease is linked with Lewy body dementia for their similar hallucinatory symptoms. The symptoms strike during the evening in any part of the visual field and are rarely polymodal. The segue into hallucinations may start with illusions[6] where sensory perception is greatly distorted, but no novel sensory information is present. These typically last for several minutes, during which time the subject may be either conscious and normal or drowsy/inaccessible. Insight into these hallucinations is usually preserved and REM sleep is usually reduced. Parkinson's disease is usually associated with a degraded substantia nigra pars compacta, but recent evidence suggests that PD affects a number of sites in the brain. Some places of noted degradation include the median raphe nuclei, the noradrenergic parts of the locus coeruleus and the cholinergic neurons in the parabrachial and pedunculopontine nuclei of the tegmentum.[5]

Migraine coma

This type of hallucination is usually experienced during the recovery from a comatose state. The migraine coma can last for up to two days and a state of depression is sometimes comorbid. The hallucinations occur during states of full consciousness and insight into the hallucinatory nature of the images is preserved. It has been noted that ataxic lesions accompany the migraine coma.[5]

Charles Bonnet syndrome

Charles Bonnet syndrome is the name given to visual hallucinations experienced by blind patients. The hallucinations can usually be dispersed by opening or closing the eyelids until the visual images disappear. The hallucinations usually occur during the morning or evening, but are not dependent on low light conditions. These prolonged hallucinations usually do not disturb the patients very much as they are aware that they are hallucinating.[5]

Focal epilepsy

The visual hallucinations from focal epilepsy are characterized by being brief, and stereotyped. They are usually localized to one part of the visual field and last only a few seconds. Other epileptic features may present themselves between visual episodes. Consciousness is usually impaired in some way, but nevertheless insight into the hallucination is preserved. Usually this type of focal epilepsy is caused by a lesion in the posterior temporoparietal.[5]

Tactile hallucinations

Can be associated with substance use, such as someone who feels bugs crawling on them after a prolonged period of cocaine use.

Scientific explanations

Various theories have been put forward to explain the occurrence of hallucinations. When psychodynamic (Freudian) theories were popular in psychiatry, hallucinations were seen as a projection of unconscious wishes, thoughts and wants. As biological theories have become orthodox, hallucinations are more often thought of (by psychiatrists at least) as being caused by functional deficits in the brain. With reference to mental illness, the function (or dysfunction) of the neurotransmitter dopamine is thought to be particularly important.[7] Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities.[8] These are abilities that allow us to monitor or draw inferences from our own internal psychological states (such as intentions, memories, beliefs and thoughts). The ability to discriminate between self-generated and external sources of information is considered to be an important metacognitive skill and one which may break down to cause hallucinatory experiences. Projection of an internal state or a person's own reaction to another may arise in the form of hallucinations, especially auditory hallucinations. A few scientists have argued that such hallucinations may be the result of other conscious thoughts.

Stages of a hallucination

  1. Emergence of surprising or warded-off memory or fantasy images [9]
  2. Frequent reality checks [9]
  3. Last vestige of insight as hallucinations become “real” [9]
  4. Fantasy and distortion elaborated upon and confused with actual perception [9]
  5. Internal-external boundaries destroyed and possible pantheistic experience [9]

In the media

Occasionally television programs and movies let the viewer see hallucinations experienced by one of the characters. For example, an episode of Casualty showed a patient's delirium tremens hallucination, live-acted by a tarantula. On the ABC show LOST, John Locke sends his protégé Boone on a vision quest via a compound induced hallucination. Also in Pirates of the Caribbean: At World's End, Jack Sparrow has hallucinations, seeing a crew made up entirely out of copies of himself. Another case occurs in Melrose Place, when Dr Kimberely Shaw saw visions of a non-existent personality constantly. This hallucination was caused by a tumor pressing against her brain. On Ally McBeal, the main character frequently has a hallucination of a dancing baby, due to the fact that as she gets older, her biological clock ticks faster. In the movie A Scanner Darkly, the characters experience a large amount of drug induced hallucinations. One larger example is the book and movie Fight Club, where the entire plot line is based on a hallucination of the main character, due to depression, sleep deprivation, and possibly insanity. These are several examples out of many as hallucinations can add an interesting twist to a movie or show.

See also

Further reading

  • Johnson, Fred H. (1978). The Anatomy of Hallucinations. Nelson-Hall.
  • Slade, P.D. and Bentall, R.P. (1988). Sensory Deception: a scientific analysis of hallucination. London: Croom Helm.


  1. ^ Sidgwick, H., Johnson, A, Myers, FWH et al (1894) Report on the census of hallucinations. Proceedings of the Society for Psychical Research, 34, 25-394.
  2. ^ Ohayon MM. (2000) Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Research, 97(2-3), 153-64.
  3. ^ Thompson, Andrea (September 15, 2006). Hearing Voices: Some People Like It. Retrieved on 2006-11-25.
  4. ^ Rare Hallucinations Make Music In The Mind. (August 9, 2000). Retrieved on 2006-12-31.
  5. ^ a b c d e f g Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.
  6. ^ Mark Derr (2006) Marilyn and Me, "The New York Times" Feb. 14th, 2006
  7. ^ Kapur S. (2003) Psychosis as a state of aberrant salience: a framework linking biology, phenomenology, and pharmacology in schizophrenia. American Journal of Psychiatry, 160(1), 13-23.
  8. ^ Bentall RP. (1990) The illusion of reality: a review and integration of psychological research on hallucinations. Psychological Bulletin, 107(1), 82-95.
  9. ^ a b c d e Horowitz, Mardi J., M.D. “Hallucinations: An Information Processing Approach." In Siegel, Ronald K., Ph. D. and L. J. West, eds. Hallucinations: Behavior, Experience and Theory (1975). New York: John Wiley.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hallucination". A list of authors is available in Wikipedia.
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