Hallucination

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A hallucination is a sensory perception experienced in the absence of an external stimulus, as distinct from an illusion, which is a misperception of an external stimulus. Hallucinations may occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, or proprioceptive (sense of balance and position in space).

1 Prevalence and types of hallucinatory experience
2 Scientific explanations
3 Visual Hallucination Subtypes
- 3.1 Hypnagogic Hallucination
- 3.2 Peduncular Hallucinosis
- 3.3 Delirium Tremens
- 3.4 Parkinson's disease and Lewy body Dementia
- 3.5 Migraine Coma
- 3.6 Charles Bonnet Syndrome
- 3.7 Focal Epilepsy
4 Paranormal theories
5 On television and movies
6 See also
7 External links
8 Further reading
9 References

Prevalence and types of hallucinatory experience

Studies have now shown hallucinatory experiences take place across the population as a whole. 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.

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.

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.^[3] The Hearing Voices Movement is a response to the Psychiatric interpretation of auditory hallucination. 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 too much music.[1] Florid hallucinations are usually associated with drug use (particularly hallucinogenic drugs), sleep deprivation, psychosis or neurological illness.

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^[5].

Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities^[6]. 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. In a 2005 study of pubescent males, it was discovered that 94 out of 100 males who claimed to be sexually attracted to a member of the opposite sex experienced hallucinations in which they sustained erections and inserted their penises in their partners mouths, vaginas, or anuses. Furthermore, in another study, males who claimed to be drug addicts or alcoholics reported hallucinations of drugs and alcohol use.

Visual Hallucination Subtypes

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 non-narcoleptics. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare. ^[7].

Peduncular Hallucinosis

Peduncular means pertaining to the peduncle, which is a name given to a neural tract running to and from the pons. These hallucinations occur most often in the evenings, but not during drowsiness as in the case of hypnagogic hallucination as described above. 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. ^[8].

Delirium Tremens

One of the most enigmatic forms of visual hallucinations are the highly variable, possibly polymodal Delirium Tremens, which is a form of withdrawal from alcohol in people with late-stage alcoholism. As the name suggests, the subject is usually 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. ^[9].

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 ^[10] 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 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. ^[11].

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.^[12].

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. ^[13].

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. ^[14].

Paranormal theories

A rarely expressed but persistent alternate explanation of hallucinations, espoused by non-materialists, is that people prone to hallucinations can sometimes perceive non-physical phenomena such as angels, visions or the voices of departed spirits or demons. For this reason, a hallucination may also be classified as an anomalous phenomenon, when no suitable scientific explanation is verified.

On television and movies

Occasionally television programs and movies let the viewer see hallucinations experienced by one of the characters. For example, an episode of Casualty (TV series) showed a patient's delirium tremens hallucination, live-acted by a tarantula.