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A hallucination is a perception in the absence of an external context
stimulus that has the compelling sense of reality.[6] They are
distinguishable from several related phenomena, such as dreaming (REM
sleep), which does not involve wakefulness; pseudohallucination, which
does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception;
and mental imagery, which does not mimic real perception, and is under voluntary control.[7] Hallucinations also differ from "delusional
perceptions", in which a correctly sensed and interpreted stimulus
(i.e., a real perception) is given some additional significance.[8]

Hallucinations can occur in any sensory modalityrCovisual, auditory,
olfactory, gustatory, tactile, proprioceptive, equilibrioceptive,
nociceptive, thermoceptive and chronoceptive. Hallucinations are
referred to as multimodal if multiple sensory modalities occur.[9][10]

A mild form of hallucination is known as a disturbance, and can occur
in most of the senses above. These may be things like seeing movement
in peripheral vision, or hearing faint noises or voices. Auditory hallucinations are very common in schizophrenia. They may be
benevolent (telling the subject good things about themselves) or
malicious (cursing the subject). 55% of auditory hallucinations are
malicious in content,[11] for example, people talking about the
subject, not speaking to them directly. Like auditory hallucinations,
the source of the visual counterpart can also be behind the subject.
This can produce a feeling of being looked or stared at, usually with
malicious intent.[12][13] Frequently, auditory hallucinations and
their visual counterpart are experienced by the subject together.[14]

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 can be associated with drug use
(particularly deliriants), sleep deprivation, psychosis (including stress-related psychosis[15]), neurological disorders, and delirium
tremens. Many hallucinations happen also during sleep paralysis.[16]

The word "hallucination" itself was introduced into the English
language by the 17th-century physician Sir Thomas Browne in 1646 from
the derivation of the Latin word alucinari meaning to wander in the
mind. For Browne, hallucination means a sort of vision that is
"depraved and receive[s] its objects erroneously".[17]

Classification
Hallucinations may be manifested in a variety of forms.[18] Various
forms of hallucinations affect different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for those
experiencing them.[9]

Auditory
Main article: Auditory hallucination
Auditory hallucinations (also known as paracusia)[19] are the
perception of sound without outside stimulus. Auditory hallucinations
can be divided into elementary and complex, along with verbal and
nonverbal. These hallucinations are the most common type of
hallucination, with auditory verbal hallucinations being more common
than nonverbal.[20][21] Elementary hallucinations are the perception
of sounds such as hissing, whistling, an extended tone, and more.[22]
In many cases, tinnitus is an elementary auditory hallucination.[21]
However, some people who experience certain types of tinnitus,
especially pulsatile tinnitus, are actually hearing the blood rushing
through vessels near the ear. Because the auditory stimulus is present
in this situation, it does not qualify it as a hallucination.[23]

Complex hallucinations are those of voices, music,[21] or other sounds
that may or may not be clear, may or may not be familiar, and may be
friendly, aggressive, or among other possibilities. A hallucination of
a single individual person of one or more talking voices is
particularly associated with psychotic disorders such as
schizophrenia, and hold special significance in diagnosing these conditions.[24]

In schizophrenia, voices are normally perceived coming from outside
the person, but in dissociative disorders they are perceived as
originating from within the person, commenting in their head instead
of behind their back. Differential diagnosis between schizophrenia and dissociative disorders is challenging due to many overlapping
symptoms, especially Schneiderian first rank symptoms such as hallucinations.[25] However, many people who do not have a diagnosable
mental illness may sometimes hear voices as well.[26] One important
example to consider when forming a differential diagnosis for a
patient with paracusia is lateral temporal lobe epilepsy. Despite the
tendency to associate hearing voices, or otherwise hallucinating, and
psychosis with schizophrenia or other psychiatric illnesses, it is
crucial to take into consideration that, even if a person does exhibit psychotic features, they do not necessarily have a psychiatric
disorder on its own. Disorders such as Wilson's disease, various
endocrine diseases, numerous metabolic disturbances, multiple
sclerosis, systemic lupus erythematosus, porphyria, sarcoidosis, and
many others can present with psychosis.[27]

Musical hallucinations are also relatively common in terms of complex
auditory hallucinations and may be the result of a wide range of
causes ranging from hearing-loss (such as in musical ear syndrome, the
auditory version of Charles Bonnet syndrome), lateral temporal lobe epilepsy,[28] arteriovenous malformation,[29] stroke, lesion, abscess,
or tumor.[30]

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.[31]

High caffeine consumption has been linked to an increase in likelihood
of one experiencing auditory hallucinations.[32] A study conducted by
the La Trobe University School of Psychological Sciences revealed that
as few as five cups of coffee a day (approximately 500 mg of caffeine)
could trigger the phenomenon.[33]

Visual
Main article: Visual hallucination
See also: Pareidolia and Palinopsia
A visual hallucination is "the perception of an external visual
stimulus where none exists".[34] A separate but related phenomenon is
a visual illusion, which is a distortion of a real external stimulus.
Visual hallucinations are classified as simple or complex:

Simple visual hallucinations (SVH) are also referred to as non-formed
visual hallucinations and elementary visual hallucinations. These
terms refer to lights, colors, geometric shapes, and indiscrete
objects. These can be further subdivided into phosphenes which are SVH
without structure, and photopsias which are SVH with geometric
structures.
Complex visual hallucinations (CVH) are also referred to as formed
visual hallucinations. CVHs are clear, lifelike images or scenes such
as people, animals, objects, places, etc.
For example, one may report hallucinating a giraffe. A simple visual hallucination is an amorphous figure that may have a similar shape or
color to a giraffe (looks like a giraffe), while a complex visual
hallucination is a discrete, lifelike image that is, unmistakably, a
giraffe.

Command
See also: Bicameral mentality
Command hallucinations are hallucinations in the form of commands;
they appear to be from an external source, or can appear coming from
the subject's head.[35] The contents of the hallucinations can range
from the innocuous to commands to cause harm to the self or
others.[35] Command hallucinations are often associated with
schizophrenia. People experiencing command hallucinations may or may
not comply with the hallucinated commands, depending on the
circumstances. Compliance is more common for non-violent commands.[36]

Command hallucinations are sometimes used to defend a crime that has
been committed, often homicides.[37] In essence, it is a voice that
one hears and it tells the listener what to do. Sometimes the commands
are quite benign directives such as "Stand up" or "Shut the door."[38]
Whether it is a command for something simple or something that is a
threat, it is still considered a "command hallucination." Some helpful questions that can assist one in determining if they may have this
includes: "What are the voices telling you to do?", "When did your
voices first start telling you to do things?", "Do you recognize the
person who is telling you to harm yourself (or others)?", "Do you
think you can resist doing what the voices are telling you to do?"[38]

Olfactory
Main articles: Phantosmia and Parosmia
Phantosmia (olfactory hallucinations), smelling an odor that is not
actually there,[39] and parosmia (olfactory illusions), inhaling a
real odor but perceiving it as different scent than remembered,[40]
are distortions to the sense of smell (olfactory system), and in most
cases, are not caused by anything serious and will usually go away on
their own in time.[39] It can result from a range of conditions such
as nasal infections, nasal polyps, dental problems, migraines, head
injuries, seizures, strokes, or brain tumors.[39][41] Environmental
exposures can sometimes cause it as well, such as smoking, exposure to
certain types of chemicals (e.g., insecticides or solvents), or
radiation treatment for head or neck cancer.[39] It can also be a
symptom of certain mental disorders such as depression, bipolar
disorder, intoxication, substance withdrawal, or psychotic disorders
(e.g., schizophrenia).[41] The perceived odors are usually unpleasant
and commonly described as smelling burned, foul, spoiled, or
rotten.[39]

Tactile
Main article: Tactile hallucination
Tactile hallucinations are the illusion of tactile sensory input,
simulating various types of pressure to the skin or other organs. One
subtype of tactile hallucination, formication, is the sensation of
insects crawling underneath the skin and is frequently associated with prolonged cocaine use.[42] However, formication may also be the result
of normal hormonal changes such as menopause, or disorders such as
peripheral neuropathy, high fevers, Lyme disease, skin cancer, and
more.[42]

Gustatory
This type of hallucination is the perception of taste without a
stimulus. These hallucinations, which are typically strange or
unpleasant, are relatively common among individuals who have certain
types of focal epilepsy, especially temporal lobe epilepsy. The
regions of the brain responsible for gustatory hallucination in this
case are the insula and the superior bank of the sylvian
fissure.[43][44]

Sexual
Sexual hallucinations are the perception of erogenous or orgasmic
stimuli. They may be unimodal or multimodal in nature and frequently
involve sensation in the genital region, though it is not
exclusive.[45] Frequent examples of sexual hallucinations include the
sensation of being penetrated, experiencing orgasm, feeling as if one
is being touched in an erogenous zone, sensing stimulation in the
genitals, feeling the fondling of one's breasts or buttocks and tastes
or smells related to sexual activity.[46] Visualizations of sexual
content and auditory voices making sexually violent remarks may
sometimes be included in this classification. While it features
components of other classifications, sexual hallucinations are
distinct due to the orgasmic component and unique presentation.[47]

The regions of the brain responsible differ by the subsection of
sexual hallucination. In orgasmic auras, the mesial temporal lobe,
right amygdala and hippocampus are involved.[48][49] In males, genital
specific sensations are related to the postcentral gyrus and arousal
and ejaculation are linked to stimulation in the posterior frontal lobe.[50][51] In females, however, the hippocampus and amygdala are connected.[51][52] Limited studies have been done to understand the
mechanism of action behind sexual hallucinations in epilepsy,
substance use, and post-traumatic stress disorder etiologies.[47]
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