A sensory perception occurring without any stimulation of the sense organ. In its true form the individual is fully awake and the perception is located out of the body. Hallucinations indicate a loss of contact with reality, but they can be a normal phenomenon, such as during grief. This term was introduced in its current form by the French physician Jean Etienne Esquirol (17721840).
Hallucinations may occur in any sensory modality - visual, auditory, olfactory, gustatory, tactile, or proprioception (sense of balance and position in space).Prevalence and types of hallucinatory experience
Studies have shown hallucinatory experiences take place across the population as a whole. Previous studies, one as early as 1894, have reported that approximately 10% of the population experience hallucinations. A recent survey of over 13,000 people 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 despite the fact that 'hearing voices' is not a sign of mental illness.
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. 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.
Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities.
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. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare. These hallucinations occur most often in the evenings, but not during drowsiness as in the case above. As in the case of hypnagogic hallucinations, insight into the nature of the images remains intact.
Delirium Tremens
One of the most enigmatic forms of visual hallucinations are the highly variable, possibly polymodal Delirium Tremens.
Parkinson's disease and Lewy body Dementia
Parkinson's disease is linked with Lewy body Dementia for their similar hallucinatory symptoms. The segue into hallucinations may start with illusions where sensory perception is greatly distorted, but no novel sensory information is present. Insight into these hallucinations is usually preserved and REM sleep is usually reduced.
Migraine Coma
This type of hallucination is usually experienced during the recovery from a comatose state. The hallucinations occur during states of full consciousness and insight into the hallucinatory nature of the images is preserved.
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.
Focal Epilepsy
The visual hallucinations from focal epilepsy are characterized by being brief, and stereotyped.
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.
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