Back to Psychiatry Diseases
An 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 hallucinari or
allucinari, 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
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 survery, olfactory
(smell) and gustatory (taste) hallucinations seem the most common in the
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
Are you a doctor or a nurse?
Do you want to join the Doctors Lounge online medical community?
Participate in editorial activities (publish, peer review, edit) and
give a helping hand to the largest online community of patients.
Click on the link below to see the requirements:
Doctors Lounge Membership