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A hallucination is a false sensory perception 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 mixed.
The word 'hallucinatory' has its roots in the Latin hallucinere or allucinere, meaning 'to wander in mind'. The first usage of the word 'hallucination' in the English language is recorded as by the English physician Sir Thomas Browne in 1642. However, it was first used in its current sense by psychiatrist Jean-Etienne Esquirol in 1837.
Florid hallucinations are usually associated with drug use (particularly hallucinogenic drugs), sleep deprivation, psychosis or neurological illness.
However, studies have shown that hallucinatory experiences are common across the population as a whole. Previous studies, one as early as 18941, have reported that approximately 10% of the population experience hallucinations. A recent survey of over 13,000 people2 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.
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.
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 important3.
Psychological research has argued that hallucinations may result from biases in what are known as metacognitive abilities4. 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.
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.
1Sidgewick, H., Johnson, A, Myers, FWH et al (1894) Report on the census of hallucinations. Proceedings of the Society for Psychical Research, 34, 25-394.
2Ohayon MM. (2000) Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Research, 97(2-3), 153-64.
3Kapur 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.
4Bentall RP. (1990) The illusion of reality: a review and integration of psychological research on hallucinations. Psychological Bulletin, 107(1), 82-95.