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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 which 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, which does not involve consciousness; illusion, which involves distorted or misinterpreted real perception; imagery, which does not mimic real perception and is under voluntary control; and pseudohallucination, which does not mimic real perception, but is not under voluntary control. 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.


Studies have shown that hallucinatory experiences take place worldwide. One study from as early as 1894 reported that approximately 10% of the population experienced hallucinations. A 1996-1999 survey of over 13,000 people reported a much higher figure, with almost 39% of people reporting hallucinatory experiences, 27% of which were 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.

Types of hallucinations

Hallucinations may be manifested in a variety of forms. Various forms of hallucinations affect the different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for the patient.

Auditory hallucinations

Auditory hallucinations (also known as Paracusia), 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. 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 (often resulting from strokes); also, tumors, encephalitis, or abscesses. 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.

Olfactory hallucinations

Phantosmia is the phenomenon of smelling odors that aren't really present. The most common odors are unpleasant smells such as rotting flesh, vomit, urine, feces, smoke, etc. Phantosmia often results from damage to the nervous tissue in the olfactory system. The damage can be caused by viral infection, brain tumor, trauma, surgery, and possibly exposure to toxins or drugs. Phantosmia can also be induced by epilepsy affecting the olfactory cortex and is also thought to possibly have psychiatric origins. Phantosmia is different from parosmia, in which a smell is actually present, but perceived differently from its usual smell.

Hypnagogic hallucination

These hallucinations occur just before falling asleep, and affect a surprisingly high proportion of 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.

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 usually occur 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.

Delirium tremens

One of the more enigmatic forms of visual hallucination is the highly variable, possibly polymodal delirium tremens. Individuals suffering from delirium 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.

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 hallucination may begin with illusions 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.

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.

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. A differential diagnosis are opthalmopathic hallucinations .

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.

Tactile hallucinations

Other types of hallucinations create the sensation of tactile sensory input, simulating various types of pressure to the skin or other organs. This type of hallucination is often associated with substance use, such as someone who feels bugs crawling on them (known as formication) 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 psychologists 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. Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities. 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 internal (self-generated) and external (stimuli) sources of information is considered to be an important metacognitive skill, but one which may break down to cause hallucinatory experiences. Projection of an internal state (or a person's own reaction to another's) may arise in the form of hallucinations, especially auditory hallucinations. 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).

Stages of a hallucination

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

In the media

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

  1. An episode of Casualty showed a patient's delirium tremens hallucination, acted out 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 Kimberly 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, because as she gets older, her biological clock ticks faster.
  6. In the TV show Scrubs, hallucinations of ludicrous hypothetical situations are commonplace as gags, such as JD's head blowing up.
  7. In the book and movie A Scanner Darkly, the characters experience a large number 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.
  9. In the book and movie Fight Club, the entire plot is based on a hallucination of the main character, due to depression, sleep deprivation, and possibly insanity.
  10. Numerous examples of hallucination have appeared in the TV series House.
  11. In the movie A Beautiful Mind, mathematician John Nash has hallucinations due to schizophrenia.
  12. In the Avatar: The Last Airbender episode, The Fire Lord, Zuko hallucinates after becoming ill; and in the finale, Azula hallucinates and sees her mother in a mirror.
  13. In the film "Hallucination", directed by Morteza Halimi, edited by Ahmad Halimi, a very common type of hallucination is portrayed.
  14. In the film "Jacob's Ladder", the character of Jacob Singer (played by Tim Robbins) undergoes a series of disturbing hallucinations, supposedly due to his exposure to mind-altering chemicals in the Vietnam War.
  15. A Russian rock band is named Smyslovye Gallyutsinatsii, which is translated as "hallucinations of meaning" or "semantic hallucinations".

See also

External links

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.


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