Hallucination
An
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
proprioception (sense of balance and position in space).
Studies have shown that hallucinatory experiences take place across the population as a whole. Previous studies, one as early as 1894
[Sidgewick, H., Johnson, A, Myers, FWH et al (1894) Report on the census of hallucinations. Proceedings of the Society for Psychical Research, 34, 25-394.]
, have reported that approximately 10% of the population experience hallucinations. A recent survey of over 13,000 people
[Ohayon MM. (2000) Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Research, 97(2-3), 153-64.]
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. This does not mean that the experience of 'hearing voices' is necessarily a sign of
mental illness, and many people may have these or similar hallucinations without ever becoming impaired or distressed in any way.
Florid hallucinations are usually associated with
drug use (particularly
hallucinogenic drugs),
sleep deprivation, psychosis or
neurological illness.
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 and desires. 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
[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.]
.
Psychological research has argued that hallucinations may result from biases in what are known as
metacognitive abilities
[Bentall RP. (1990) The illusion of reality: a review and integration of psychological research on hallucinations. Psychological Bulletin, 107(1), 82-95.]
. 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.
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. This may be associated with
narcolepsy, but can also affect non-narcoleptics. Hypnagogic hallucinations are usually associated with brainstem damage of some sort.
[ http://brain.oxfordjournals.org/cgi/content/abstract/121/10/1819 Manford and Andermann (1998)] Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
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 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 in tact. The false images can occur in any part of the visual field, and are rarely
polymodal.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
Delirium Tremens
One of the most enigmatic forms of visual hallucinations are the highly variable, possibly polymodal
Delirium Tremens. 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 short, with
REM overflow.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
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
[Mark Derr (2006) Marilyn and Me, "The New York Times" Feb. 14th, 2006 ]
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/inaccessable. 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 handful of other 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.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
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.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
Charles Bonnet Syndrome
Charles Bonnet Syndrome is the name given to visual hallucinations experienced by blind patients. The hallucinations can usually be dispursed by opening or closing the eyelids until the visual images dissapear. 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.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
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.
[Manford and Andermann (1998) Complex visual hallucinations. Clinical and Neurobiological insightsBrain, 121(10), 1819-1840.]
.
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.
*
Delusion*
Illusion*
Hallucinogenic drug*
LSD*
Salvia Divinorum*
Psychosis*
Hearing Voices Movement*
Charles Bonnet Syndrome*
Phantom eye syndrome*
Folie à deux*
Closed-eye hallucination*
Form constant*
Hearing Voices Network*
Visual hallucinations during migraineThe Anatomy of Hallucinations by Fred H. Johnson, Nelson-Hall, 1978
