Hallucination

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A hallucination in the broadest sense is a perception in the absence of a stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli and that have qualities of real perception in that they are vivid, substantial, and located in external objective space. These definitions distinguish hallucinations from the related phenomena of dreaming (no consciousness), illusion (distorted or misinterpreted real perception), imagery (does not mimick real perception and is under voluntary control), and pseudohallucination (does not mimick real perception, but is not under voluntary control).^[1] Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted genuine perception is given some additional (and typically bizarre) significance.

Hallucinations may occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, and thermoceptive.

A mild form of hallucination is known as a disturbance, and can occur in any of the senses above. These may be things like seeing movement in peripheral vision, or hearing faint noises and voices.

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 deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.

Contents 1 Prevalence 2 Auditory hallucinations 2.1 Hypnagogic hallucination 2.2 Peduncular hallucinosis 2.3 Delirium tremens 2.4 Parkinson's disease and Lewy body dementia 2.5 Migraine coma 2.6 Charles Bonnet syndrome 2.7 Focal epilepsy 3 Tactile hallucinations 4 Scientific explanations 5 Stages of a hallucination 6 In the media 7 See also 8 External links 9 Further reading 10 References

[edit] Prevalence

Studies have now shown hallucinatory experiences take place across the world. Previous studies, one as early as 1894,^[2] have reported that approximately 10% of the population experience hallucinations. A recent survey of over 13,000 people^[3] 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.

[edit] Auditory hallucinations

Main article: Auditory hallucinations

Auditory hallucinations, particularly of one or more talking voices, are particularly associated with psychotic disorders such as schizophrenia, and hold special significance in diagnosing these conditions, although many people not suffering from diagnosable mental illness may sometimes hear voices as well.^[4] 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. One reason this can be caused is by lesions on the brain stem, occurring most often from strokes, but also tumors, encephalitis, or abscesses.^[5] Other reasons include hearing loss and epileptic activity. Recent reports have also mentioned that it is also possible to get musical hallucinations from listening to music for long periods of time.^[6]

[edit] Hypnagogic hallucination

Main article: Hypnagogia

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.^[7]

[edit] 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.^[7]

[edit] 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.^[7]

[edit] 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^[8] 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.^[7]

[edit] 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.^[7]

[edit] 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.^[7]

[edit] 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.^[7]

[edit] Tactile hallucinations

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

[edit] 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.^[9] Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities.^[10] 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. A recent hypothesis that is gaining acceptance concerns the role of overactive top-down processing, or strong perceptual expectations, that can generate spontaneous perceptual output (that is, hallucination).^[11]

[edit] Stages of a hallucination

1. Emergence of surprising or warded-off memory or fantasy images ^[12]
2. Frequent reality checks ^[12]
3. Last vestige of insight as hallucinations become “real” ^[12]
4. Fantasy and distortion elaborated upon and confused with actual perception ^[12]
5. Internal-external boundaries destroyed and possible pantheistic experience ^[12]

[edit] In the media

Occasionally television programs and movies let the viewer see hallucinations experienced by one of the characters.

1. an episode of Casualty showed a patient's delirium tremens hallucination, live-acted by a tarantula.
2. On the ABC show LOST, John Locke sends his protégé Boone on a vision quest via a compound induced hallucination.
3. In Pirates of the Caribbean: At World's End, Jack Sparrow has hallucinations, seeing a crew made up entirely out of copies of himself.
4. In Melrose Place, Dr Kimberely Shaw saw visions of a non-existent personality constantly. This hallucination was caused by a tumor pressing against her brain.
5. 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.
6. In the TV show Scrubs, hallucinations of hypothetical ridiculous situations are commonplace as gags, such as JD's head blowing up.
7. In the movie A Scanner Darkly, the characters experience a large amount of drug-induced hallucinations.
8. In the movie Dead Man's Shoes the main character spends all his time with a man who is discovered to be a hallucination of his dead brother. These are several examples out of many as hallucinations can add an interesting twist to a movie or show.
9. 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.

[edit] See also

• Apparitional experience
• Closed-eye hallucination
• Delusion
• Dimethyltryptamine
• Dream
• Focal seizures
• Folie à deux
• Form constant
• Hallucinations in the sane
• Illusion
• Imaginary friend
• Phantom eye syndrome
• Hallucinogenic drug
• Psychosis
• Simulated reality

[edit] External links

• INTERVOICE Online, website of the international hearing voices movement
• Hearing Voices Network
• Visual hallucinations during migraine
• "Anthropology and Hallucinations", chapter from The Making of Religion
• "The voice inside: A practical guide to coping with hearing voices"

[edit] 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.
• Aleman, A. and Larøi, F. (2008).Hallucinations: the science of idiosyncratic perception. Washington, DC: APA Books. http://books.apa.org/books.cfm?id=4318044

[edit] References