A hallucination is a perception in the absence of an external
stimulus
A stimulus is something that causes a physiological response. It may refer to:
*Stimulation
**Stimulus (physiology), something external that influences an activity
**Stimulus (psychology), a concept in behaviorism and perception
*Stimulus (economi ...
that has the qualities of a real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. Hallucination is a combination of 2
conscious
Consciousness, at its simplest, is sentience and awareness of internal and external existence. However, the lack of definitions has led to millennia of analyses, explanations and debates by philosophers, theologians, linguisticians, and scien ...
states of brain
wakefulness and
REM sleep. They are distinguishable from several related
phenomena, such as dreaming (
REM sleep), which does not involve wakefulness;
pseudohallucination
A pseudohallucination (from grc, ψευδής (pseudḗs) "false, lying" + "hallucination") is an involuntary sensory experience vivid enough to be regarded as a hallucination, but which is recognised by the person experiencing it as being subject ...
, which does not mimic real perception, and is accurately perceived as unreal;
illusion
An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people.
Illusions may oc ...
, which involves distorted or misinterpreted real perception; and
mental imagery
A mental image is an experience that, on most occasions, significantly resembles the experience of 'perceiving' some object, event, or scene, but occurs when the relevant object, event, or scene is not actually present to the senses. There are ...
, which does not mimic real perception, and is under voluntary control.
Hallucinations also differ from "
delusional perceptions", in which a correctly sensed and interpreted stimulus (i.e., a real perception) is given some additional significance. Many hallucinations happen also during
sleep paralyses
Sleep paralysis is a state, during waking up or falling asleep, in which one is conscious but is completely paralyzed. During an episode, one may hallucinate (hear, feel, or see things that are not there), which often results in fear. Episodes ...
.
Hallucinations can occur in any
sensory modality Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light ...
—
visual,
auditory,
olfactory,
gustatory
The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste (flavor). Taste is the perception produced or stimulated when a substance in the mouth reacts chemically with taste receptor ...
,
tactile,
proprioceptive
Proprioception ( ), also referred to as kinaesthesia (or kinesthesia), is the sense of self-movement, force, and body position. It is sometimes described as the "sixth sense".
Proprioception is mediated by proprioceptors, mechanosensory neurons ...
,
equilibrioceptive,
nociceptive
Nociception (also nocioception, from Latin ''nocere'' 'to harm or hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, c ...
,
thermoceptive and
chronoceptive. Hallucinations are referred to as multimodal if multiple sensory modalities occur.
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
Peripheral vision, or ''indirect vision'', is vision as it occurs outside the point of fixation, i.e. away from the center of gaze or, when viewed at large angles, in (or out of) the "corner of one's eye". The vast majority of the area in th ...
, or hearing faint noises or voices. Auditory hallucinations are very common in
schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
. They may be benevolent (telling the subject good things about themselves) or malicious, cursing the subject. 55% of auditory hallucinations are malicious in content, 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. Frequently, auditory hallucinations and their visual counterpart are experienced by the subject together.
Hypnagogic hallucination
Hypnagogia is the experience of the transitional state from wakefulness to sleep: the ''hypnagogic'' state of consciousness, during the onset of sleep. Its opposite state is described as the transitional state from sleep into wakefulness. Mental ...
s 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
deliriant
Deliriants are a subclass of hallucinogen. The term was coined in the early 1980s to distinguish these drugs from psychedelics and dissociatives such as LSD and ketamine, respectively, due to their primary effect of causing delirium, as oppose ...
s),
sleep deprivation,
psychosis,
neurological disorders, and
delirium tremens.
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
its objects erroneously".
Classification
Hallucinations may be manifested in a variety of forms. Various forms of hallucinations affect different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for those experiencing them.
Auditory
Auditory hallucinations (also known as ''paracusia'') 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. Elementary hallucinations are the perception of sounds such as hissing, whistling, an extended tone, and more. In many cases,
tinnitus
Tinnitus is the perception of sound when no corresponding external sound is present. Nearly everyone experiences a faint "normal tinnitus" in a completely quiet room; but it is of concern only if it is bothersome, interferes with normal hearin ...
is an elementary auditory hallucination. 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.
Complex hallucinations are those of voices, music, 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
Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavior t ...
disorders such as
schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
, and hold special significance in diagnosing these conditions.
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 disorder
Dissociative disorders (DD) are conditions that involve disruptions or breakdowns of memory, awareness, identity, or perception. People with dissociative disorders use dissociation as a defense mechanism, pathologically and involuntarily. The i ...
s is challenging due to many overlapping symptoms, especially
Schneiderian first rank symptoms such as hallucinations. However, many people who do not have a diagnosable
mental illness may sometimes hear voices as well. One important example to consider when forming a differential diagnosis for a patient with paracusia is lateral
temporal lobe epilepsy
Temporal lobe epilepsy (TLE) is a chronic disorder of the nervous system which is characterized by recurrent, unprovoked focal seizures that originate in the temporal lobe of the brain and last about one or two minutes. TLE is the most common ...
. 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
Wilson's disease is a genetic disorder in which excess copper builds up in the body. Symptoms are typically related to the brain and liver. Liver-related symptoms include vomiting, weakness, fluid build up in the abdomen, swelling of the legs, ...
, various
endocrine disease
Endocrine diseases are disorders of the endocrine system. The branch of medicine associated with endocrine disorders is known as endocrinology.
Types of disease
Broadly speaking, endocrine disorders may be subdivided into three groups:
# Endocrin ...
s, numerous
metabolic disturbances,
multiple sclerosis,
systemic lupus erythematosus,
porphyria
Porphyria is a group of liver disorders in which substances called porphyrins build up in the body, negatively affecting the skin or nervous system. The types that affect the nervous system are also known as acute porphyria, as symptoms are ...
,
sarcoidosis, and many others can present with psychosis.
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
Visual release hallucinations, also known as Charles Bonnet syndrome or CBS, are a type of psychophysical visual disturbance in which a person with partial or severe blindness experiences visual hallucinations.
First described by Charles Bonnet in ...
), lateral temporal lobe epilepsy, arteriovenous malformation,
stroke,
lesion,
abscess, or tumor.
The
Hearing Voices Movement
The Hearing Voices Movement (HVM) is the name used by organizations and individuals advocating the "hearing voices approach", an alternative way of understanding the experience of those people who "hear voices". In the medical professional literat ...
is a support and advocacy group for people who hallucinate voices, but do not otherwise show signs of mental illness or impairment.
High
caffeine
Caffeine is a central nervous system (CNS) stimulant of the methylxanthine class. It is mainly used recreationally as a cognitive enhancer, increasing alertness and attentional performance. Caffeine acts by blocking binding of adenosine to ...
consumption has been linked to an increase in likelihood of one experiencing auditory hallucinations. 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.
Visual
A ''visual hallucination'' is "the perception of an external visual stimulus where none exists". 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
phosphene
A phosphene is the phenomenon of seeing light without light entering the eye. The word ''phosphene'' comes from the Greek words ''phos'' (light) and ''phainein'' (to show). Phosphenes that are induced by movement or sound may be associated wi ...
s 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
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.
The contents of the hallucinations can range from the innocuous to commands to cause harm to the self or others.
Command hallucinations are often associated with
schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
. 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.
Command hallucinations are sometimes used to defend a crime that has been committed, often homicides. 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."
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?"
Olfactory
Phantosmia (olfactory hallucinations), smelling an odor that is not actually there,
[ and parosmia (olfactory illusions), inhaling a real odor but perceiving it as different scent than remembered,] are distortions to the sense of smell (olfactory system
The olfactory system, or sense of smell, is the sensory system used for smelling ( olfaction). Olfaction is one of the special senses, that have directly associated specific organs. Most mammals and reptiles have a main olfactory system and an ...
), and in most cases, are not caused by anything serious and will usually go away on their own in time. It can result from a range of conditions such as nasal infections, nasal polyp
Nasal polyps (NP) are noncancerous growths within the nose or sinuses. Symptoms include trouble breathing through the nose, loss of smell, decreased taste, post nasal drip, and a runny nose. The growths are sac-like, movable, and nontender, tho ...
s, dental problems, migraines, head injuries, seizures, strokes, or brain tumors. Environmental exposures can sometimes cause it as well, such as smoking, exposure to certain types of chemicals (e.g., insecticides or solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
s), or radiation treatment for head or neck cancer. It can also be a symptom of certain mental disorder
A mental disorder, also referred to as a mental illness or psychiatric disorder, is a behavioral or mental pattern that causes significant distress or impairment of personal functioning. Such features may be persistent, relapsing and remitt ...
s such as depression, bipolar disorder, intoxication, substance withdrawal, or psychotic disorder
Psychosis is a condition of the mind that results in difficulties determining what is real and what is not real. Symptoms may include delusions and hallucinations, among other features. Additional symptoms are incoherent speech and behavior ...
s (e.g., schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
).[ The perceived odors are usually unpleasant and commonly described as smelling burned, foul, spoiled, or rotten.]
Tactile
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
Cocaine (from , from , ultimately from Quechua: ''kúka'') is a central nervous system (CNS) stimulant mainly used recreationally for its euphoric effects. It is primarily obtained from the leaves of two Coca species native to South Ameri ...
use. However, formication may also be the result of normal hormonal changes such as menopause, or disorders such as peripheral neuropathy
Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or or ...
, high fevers, Lyme disease, skin cancer
Skin cancers are cancers that arise from the skin. They are due to the development of abnormal cells that have the ability to invade or spread to other parts of the body. There are three main types of skin cancers: basal-cell skin cancer (BCC) ...
, and more.
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
Temporal lobe epilepsy (TLE) is a chronic disorder of the nervous system which is characterized by recurrent, unprovoked focal seizures that originate in the temporal lobe of the brain and last about one or two minutes. TLE is the most common ...
. The regions of the brain responsible for gustatory hallucination in this case are the insula and the superior bank of the sylvian fissure
In neuroanatomy, the lateral sulcus (also called Sylvian fissure, after Franciscus Sylvius, or lateral fissure) is one of the most prominent features of the human brain. The lateral sulcus is a deep fissure in each hemisphere that separates th ...
.
General somatic sensations
General somatic sensations of a hallucinatory nature are experienced when an individual feels that their body is being mutilated, i.e. twisted, torn, or disemboweled. Other reported cases are invasion by animals in the person's internal organs, such as snakes in the stomach or frogs in the rectum. The general feeling that one's flesh is decomposing is also classified under this type of this hallucination.
Multimodal
A hallucination involving sensory modalities Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, ...
is called multimodal, analogous to unimodal hallucinations which have only one sensory modality. The multiple sensory modalities can occur at the same time (simultaneously) or with a delay (serial), be related or unrelated to each other, and be consistent with reality (congruent) or not (incongruent). For example, a person talking in a hallucination would be congruent with reality, but a cat talking would not be.
Multimodal hallucinations are correlated to poorer mental health outcomes, and are often experienced as feeling more real.
Cause
Hallucinations can be caused by a number of factors.
Hypnagogic hallucination
These hallucinations occur just before falling asleep and affect a high proportion of the population: in one survey 37% of the respondents experienced them twice a week. The hallucinations can last from seconds to minutes; all the while, the subject usually remains aware of the true nature of the images. These may be associated with narcolepsy
Narcolepsy is a long-term neurological disorder that involves a decreased ability to regulate sleep–wake cycles. Symptoms often include periods of excessive daytime sleepiness and brief involuntary sleep episodes. About 70% of those affect ...
. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare.
Peduncular hallucinosis
Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons
The pons (from Latin , "bridge") is part of the brainstem that in humans and other bipeds lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum.
The pons is also called the pons Varolii ("bridge of Va ...
on the brain stem
The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is co ...
. These hallucinations usually occur in the evenings, but not during drowsiness, as in the case of hypnagogic hallucination. The subject is usually fully conscious and then can interact with the hallucinatory characters for extended periods of time. As in the case of hypnagogic hallucination
Hypnagogia is the experience of the transitional state from wakefulness to sleep: the ''hypnagogic'' state of consciousness, during the onset of sleep. Its opposite state is described as the transitional state from sleep into wakefulness. Mental ...
s, insight into the nature of the images remains intact. The false images can occur in any part of the visual field, and are rarely polymodal.
Delirium tremens
One of the more enigmatic forms of visual hallucination is the highly variable, possibly polymodal delirium tremens. Individuals with delirium tremens may be 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 occurs for a shorter period of time, with rapid eye movement sleep
Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream ...
.
Parkinson's disease and Lewy body dementia
Parkinson's disease
Parkinson's disease (PD), or simply Parkinson's, is a long-term degenerative disorder of the central nervous system that mainly affects the motor system. The symptoms usually emerge slowly, and as the disease worsens, non-motor symptoms becom ...
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 hallucination may begin with illusions 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/inaccessible. 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 number of sites in the brain. Some places of noted degradation include the median raphe nuclei, the noradrenergic
Norepinephrine (NE), also called noradrenaline (NA) or noradrenalin, is an organic chemical in the catecholamine family that functions in the brain and body as both a hormone and neurotransmitter. The name "noradrenaline" (from Latin '' ad'', ...
parts of the locus coeruleus, and the cholinergic
Cholinergic agents are compounds which mimic the action of acetylcholine and/or butyrylcholine. In general, the word " choline" describes the various quaternary ammonium salts containing the ''N'',''N'',''N''-trimethylethanolammonium cati ...
neurons in the parabrachial area and pedunculopontine nuclei of the tegmentum.[
]
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.[
]
Charles Bonnet syndrome
Charles Bonnet syndrome
Visual release hallucinations, also known as Charles Bonnet syndrome or CBS, are a type of psychophysical visual disturbance in which a person with partial or severe blindness experiences visual hallucinations.
First described by Charles Bonnet in ...
is the name given to visual hallucinations experienced by a partially or severely sight impaired person. The hallucinations can occur at any time and can distress people of any age, as they may not initially be aware that they are hallucinating. They may fear for their own mental health initially, which may delay them sharing with carers until they start to understand it themselves. The hallucinations can frighten and disconcert as to what is real and what is not. The hallucinations can sometimes be dispersed by eye movements, or by reasoned logic such as, "I can see fire but there is no smoke and there is no heat from it" or perhaps, "We have an infestation of rats but they have pink ribbons with a bell tied on their necks." Over elapsed months and years, the hallucinations may become more or less frequent with changes in ability to see. The length of time that the sight impaired person can have these hallucinations varies according to the underlying speed of eye deterioration. A differential diagnosis are ophthalmopathic hallucinations.
Focal epilepsy
Visual hallucinations due to focal seizures differ depending on the region of the brain where the seizure occurs. For example, visual hallucinations during occipital lobe
The occipital lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The name derives from its position at the back of the head, from the Latin ''ob'', "behind", and ''caput'', "head".
The occipital lobe is the vi ...
seizures are typically visions of brightly colored, geometric shapes that may move across the visual field, multiply, or form concentric rings and generally persist from a few seconds to a few minutes. They are usually unilateral and localized to one part of the visual field on the contralateral side of the seizure focus, typically the temporal field. However, unilateral visions moving horizontally across the visual field begin on the contralateral side and move toward the ipsilateral side.
Temporal lobe seizures, on the other hand, can produce complex visual hallucinations of people, scenes, animals, and more as well as distortions of visual perception
Visual perception is the ability to interpret the surrounding Biophysical environment, environment through photopic vision (daytime vision), color vision, scotopic vision (night vision), and mesopic vision (twilight vision), using light in the ...
. Complex hallucinations may appear to be real or unreal, may or may not be distorted with respect to size, and may seem disturbing or affable, among other variables. One rare but notable type of hallucination is heautoscopy, a hallucination of a mirror image of one's self. These "other selves" may be perfectly still or performing complex tasks, may be an image of a younger self or the present self, and tend to be briefly present. Complex hallucinations are a relatively uncommon finding in temporal lobe epilepsy patients. Rarely, they may occur during occipital focal seizures or in parietal lobe
The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.
The parietal lobe integrates sensory informa ...
seizures.
Distortions in visual perception during a temporal lobe seizure may include size distortion (micropsia
Micropsia is a condition affecting human visual perception in which objects are perceived to be smaller than they actually are. Micropsia can be caused by optical factors (such as wearing glasses), by distortion of images in the eye (such as optica ...
or macropsia
Macropsia is a neurological condition affecting human visual perception, in which objects within an affected section of the visual field appear larger than normal, causing the person to feel smaller than they actually are. Macropsia, along with its ...
), distorted perception of movement (where moving objects may appear to be moving very slowly or to be perfectly still), a sense that surfaces such as ceilings and even entire horizons are moving farther away in a fashion similar to the dolly zoom effect, and other illusions. Even when consciousness is impaired, insight into the hallucination or illusion is typically preserved.
Drug-induced hallucination
Drug-induced hallucinations are caused by hallucinogen
Hallucinogens are a large, diverse class of psychoactive drugs that can produce altered states of consciousness characterized by major alterations in thought, mood, and perception as well as other changes. Most hallucinogens can be categorize ...
s, dissociative
Dissociatives, colloquially dissos, are a subclass of hallucinogens which distort perception of sight and sound and produce feelings of detachment – dissociation – from the environment and/or self. Although many kinds of drugs are capable of ...
s, and deliriants
Deliriants are a subclass of hallucinogen. The term was coined in the early 1980s to distinguish these drugs from psychedelics and dissociatives such as LSD and ketamine, respectively, due to their primary effect of causing delirium, as opposed t ...
, including many drugs with anticholinergic
Anticholinergics (anticholinergic agents) are substances that block the action of the neurotransmitter called acetylcholine (ACh) at synapses in the central and peripheral nervous system.
These agents inhibit the parasympathetic nervous sys ...
actions and certain stimulants, which are known to cause visual and auditory hallucinations. Some psychedelics such as lysergic acid diethylamide
Lysergic acid diethylamide (LSD), also known colloquially as acid, is a potent psychedelic drug. Effects typically include intensified thoughts, emotions, and sensory perception. At sufficiently high dosages LSD manifests primarily mental, vi ...
(LSD) and psilocybin
Psilocybin ( , ) is a naturally occurring psychedelic prodrug compound produced by more than 200 species of fungi. The most potent are members of the genus ''Psilocybe'', such as '' P. azurescens'', '' P. semilanceata'', and '' P.&nbs ...
can cause hallucinations that range in the spectrum of mild to intense.
Hallucinations, pseudohallucination
A pseudohallucination (from grc, ψευδής (pseudḗs) "false, lying" + "hallucination") is an involuntary sensory experience vivid enough to be regarded as a hallucination, but which is recognised by the person experiencing it as being subject ...
s, or intensification of pareidolia
Pareidolia (; ) is the tendency for perception to impose a meaningful interpretation on a nebulous stimulus, usually visual, so that one sees an object, pattern, or meaning where there is none.
Common examples are perceived images of animals, ...
, particularly auditory, are known side effects of opioids to different degrees—it may be associated with the absolute degree of agonism or antagonism of especially the kappa opioid receptor
Kappa (uppercase Κ, lowercase κ or cursive ; el, κάππα, ''káppa'') is the 10th letter of the Greek alphabet, representing the voiceless velar plosive sound in Ancient and Modern Greek. In the system of Greek numerals, has a value ...
, sigma receptor
Sigma receptors (σ-receptors) are protein cell surface receptors that bind ligands such as 4-PPBP (4-phenyl-1-(4-phenylbutyl) piperidine), SA 4503 (cutamesine), ditolylguanidine, dimethyltryptamine, and siramesine. There are two subtypes, ...
s, delta opioid receptor and the NMDA receptors or the overall receptor activation profile as synthetic opioids like those of the pentazocine, levorphanol, fentanyl, pethidine
Pethidine, also known as meperidine and sold under the brand name Demerol among others, is a synthetic opioid pain medication of the phenylpiperidine class. Synthesized in 1938 as a potential anticholinergic agent by the German chemist Otto Eis ...
, methadone
Methadone, sold under the brand names Dolophine and Methadose among others, is a synthetic opioid agonist used for chronic pain and also for opioid dependence. It is used to treat chronic pain, and it is also used to treat addiction to heroi ...
and some other families are more associated with this side effect than natural opioids like morphine and codeine and semi-synthetics like hydromorphone
Hydromorphone, also known as dihydromorphinone, and sold under the brand name Dilaudid among others, is an opioid used to treat moderate to severe pain. Typically, long-term use is only recommended for pain due to cancer. It may be used by mou ...
, amongst which there also appears to be a stronger correlation with the relative analgesic strength. Three opioids, Cyclazocine (a benzormorphan opioid/pentazocine relative) and two levorphanol-related morphinan opioids, Cyclorphan and Dextrorphan are classified as hallucinogens, and Dextromethorphan as a dissociative. These drugs also can induce sleep (relating to hypnagogic hallucinations) and especially the pethidines have atropine-like anticholinergic
Anticholinergics (anticholinergic agents) are substances that block the action of the neurotransmitter called acetylcholine (ACh) at synapses in the central and peripheral nervous system.
These agents inhibit the parasympathetic nervous sys ...
activity, which was possibly also a limiting factor in the use, the psychotomimetic side effects of potentiating morphine, oxycodone
Oxycodone, sold under various brand names such as Roxicodone and OxyContin (which is the extended release form), is a strong, semi-synthetic opioid used medically for treatment of moderate to severe pain. It is highly addictive and a commonly ...
, and other opioids with scopolamine (respectively in the Twilight Sleep technique and the combination drug Skophedal, which was eukodal (oxycodone), scopolamine and ephedrine, called the "wonder drug of the 1930s" after its invention in Germany in 1928, but only rarely specially compounded today) (q.q.v.).
Sensory deprivation hallucination
Hallucinations can be caused by sensory deprivation when it occurs for prolonged periods of time, and almost always occurs in the modality being deprived (visual for blindfolded/darkness, auditory for muffled conditions, etc.)
Experimentally-induced hallucinations
Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation.
The evidence for this statement has been accumulating for more than a century. Studies of benign hallucinatory experiences go back to 1886 and the early work of the Society for Psychical Research
The Society for Psychical Research (SPR) is a nonprofit organisation in the United Kingdom. Its stated purpose is to understand events and abilities commonly described as psychic or paranormal. It describes itself as the "first society to co ...
, which suggested approximately 10% of the population had experienced at least one hallucinatory episode in the course of their life. More recent studies have validated these findings; the precise incidence found varies with the nature of the episode and the criteria of "hallucination" adopted, but the basic finding is now well-supported.
Non-celiac gluten sensitivity
There is tentative evidence of a relationship with non-celiac gluten sensitivity
Non-celiac gluten sensitivity (NCGS) or gluten sensitivity is "a clinical entity induced by the ingestion of gluten leading to intestinal and/or extraintestinal symptoms that improve once the gluten-containing foodstuff is removed from the diet, a ...
, the so-called "gluten psychosis".
Pathophysiology
Dopaminergic and serotonergic hallucinations
It has been reported that in serotonergic hallucinations, the person maintains an awareness that they are hallucinating, unlike dopaminergic hallucinations.
Neuroanatomy
Hallucinations are associated with structural and functional abnormalities in primary and secondary sensory cortices. Reduced grey matter in regions of the superior temporal gyrus/middle temporal gyrus
Middle temporal gyrus is a gyrus in the brain on the temporal lobe. It is located between the superior temporal gyrus and inferior temporal gyrus. It corresponds largely to Brodmann area 21.
The middle temporal gyrus is bounded by:
* the superi ...
, including Broca's area, is associated with auditory hallucinations as a trait, while acute hallucinations are associated with increased activity in the same regions along with the hippocampus
The hippocampus (via Latin from Greek , ' seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic system, ...
, parahippocampus
The parahippocampal gyrus (or hippocampal gyrus') is a grey matter cortical region of the brain that surrounds the hippocampus and is part of the limbic system. The region plays an important role in memory encoding and retrieval. It has been inv ...
, and the right hemispheric homologue of Broca's area in the inferior frontal gyrus. Grey and white matter abnormalities in visual regions are associated with visual hallucinations in diseases such as Alzheimer's disease, further supporting the notion of dysfunction in sensory regions underlying hallucinations.
One proposed model of hallucinations posits that over-activity in sensory regions, which is normally attributed to internal sources via feedforward networks to the inferior frontal gyrus, is interpreted as originating externally due to abnormal connectivity or functionality of the feedforward network. This is supported by cognitive studies of those with hallucinations, who have demonstrated abnormal attribution of self generated stimuli.
Disruptions in thalamocortical circuitry may underlie the observed top down and bottom up dysfunction. Thalamocortical circuits, composed of projections between thalamic and cortical neurons and adjacent interneurons, underlie certain electrophysical characteristics ( gamma oscillations) that are underlie sensory processing. Cortical inputs to thalamic neurons enable attentional modulation of sensory neurons. Dysfunction in sensory afferents, and abnormal cortical input may result in pre-existing expectations modulating sensory experience, potentially resulting in the generation of hallucinations. Hallucinations are associated with less accurate sensory processing, and more intense stimuli with less interference are necessary for accurate processing and the appearance of gamma oscillations (called "gamma synchrony"). Hallucinations are also associated with the absence of reduction in P50 amplitude in response to the presentation of a second stimuli after an initial stimulus; this is thought to represent failure to gate sensory stimuli, and can be exacerbated by dopamine release agents.
Abnormal assignment of salience to stimuli may be one mechanism of hallucinations. Dysfunctional dopamine signaling may lead to abnormal top down regulation of sensory processing, allowing expectations to distort sensory input.
Treatments
There are few treatments for many types of hallucinations. However, for those hallucinations caused by mental disease, a psychologist or psychiatrist should be consulted, and treatment will be based on the observations of those doctors. Antipsychotic and atypical antipsychotic
The atypical antipsychotics (AAP), also known as second generation antipsychotics (SGAs) and serotonin–dopamine antagonists (SDAs), are a group of antipsychotic drugs (antipsychotic drugs in general are also known as major tranquilizers and ne ...
medication may also be utilized to treat the illness if the symptoms are severe and cause significant distress. For other causes of hallucinations there is no factual evidence to support any one treatment is scientifically tested and proven. However, abstaining from hallucinogen
Hallucinogens are a large, diverse class of psychoactive drugs that can produce altered states of consciousness characterized by major alterations in thought, mood, and perception as well as other changes. Most hallucinogens can be categorize ...
ic drugs, stimulant drugs, managing stress levels, living healthily, and getting plenty of sleep can help reduce the prevalence of hallucinations. In all cases of hallucinations, medical attention should be sought out and informed of one's specific symptoms. Meta-analyses
A meta-analysis is a statistical analysis that combines the results of multiple scientific studies. Meta-analyses can be performed when there are multiple scientific studies addressing the same question, with each individual study reporting m ...
show that cognitive behavioral therapy and metacognitive training can also reduce the severity of hallucinations.
Epidemiology
Prevalence of hallucinations varies depending on underlying medical conditions, which sensory modalities are affected, age and culture. As of 2022, auditory hallucinations are the most well studied and most common sensory modality of hallucinations, with an estimated lifetime prevalence of 9.6%. Children and adolescents have been found to experience similar rates (12.7% and 12.4% respectively) which occur mostly during late childhood and adolescence. This is compared with adults and those over 60 (with rates of 5.8% and 4.8% respectively). For those with schizophrenia, the lifetime prevalence of hallucinations is 80% and the estimated prevalence of visual hallucinations is 27%, compared to 79% for auditory hallucinations. A 2019 study suggested 16.2% of adults with hearing impairment experience hallucinations, with prevalence rising to 24% in the most hearing impaired group.
A risk factor for multimodal hallucinations is prior experience of unimodal hallucinations. In 90% cases of psychosis, a visual hallucination occurs in combination with another sensory modality, most often being auditory or somatic. In schizophrenia, multimodal hallucinations are twice as common as unimodal ones.
A 2015 review of 55 publications from 1962 to 2014 found 16–28.6% of those experiencing hallucinations report at least some religious content in them, along with 20-60% reporting some religious content in delusions. There is some evidence for delusions being a risk factor for religious hallucinations, with and 61.7% of people having experienced any delusion and 75.9% of those having experienced a religious delusion found to also experience hallucinations.
See also
* Phantosmia
* Pseudohallucination
A pseudohallucination (from grc, ψευδής (pseudḗs) "false, lying" + "hallucination") is an involuntary sensory experience vivid enough to be regarded as a hallucination, but which is recognised by the person experiencing it as being subject ...
* Closed-eye hallucination
Closed-eye hallucinations and closed-eye visualizations (CEV) are hallucinations that occur when one's eyes are closed or when one is in a darkened room. They can be a form of '' phosphene''. Some people report CEV under the influence of psyched ...
* Dimethyltryptamine
''N'',''N''-Dimethyltryptamine (DMT or ''N'',''N''-DMT, SPL026) is a substituted tryptamine that occurs in many plants and animals, including human beings, and which is both a derivative and a structural analog of tryptamine. It is used as a ...
* Folie à deux
* Ganzfeld effect
The Ganzfeld effect (from German for "complete field"), or perceptual deprivation, is a phenomenon of perception caused by exposure to an unstructured, uniform stimulation field. The effect is the result of the brain amplifying neural noise in or ...
* Hallucinogenic fish
Several species of fish are claimed to produce hallucinogenic effects when consumed. For example, ''Sarpa salpa'', a species of sea bream, is commonly claimed to be hallucinogenic. These widely distributed coastal fish are normally found in t ...
* Anomalous experiences
* Illusion
An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people.
Illusions may oc ...
* Hypnagogia
Hypnagogia is the experience of the transitional state from wakefulness to sleep: the ''hypnagogic'' state of consciousness, during the onset of sleep. Its opposite state is described as the transitional state from sleep into wakefulness. Mental ...
* Microwave auditory effect
The microwave auditory effect, also known as the microwave hearing effect or the Frey effect, consists of the human perception of audible clicks, or even speech, induced by pulsed or modulated radio frequencies. The communications are generated di ...
* Phantom eye syndrome
* Phosphene
A phosphene is the phenomenon of seeing light without light entering the eye. The word ''phosphene'' comes from the Greek words ''phos'' (light) and ''phainein'' (to show). Phosphenes that are induced by movement or sound may be associated wi ...
* Prisoner's cinema
Prisoner's cinema is the phenomenon of a "light show" of various colors that appear out of the darkness. The light has a form, but those that have seen it find it difficult to describe. Sometimes, the cinema lights resolve into human or other figur ...
* Psychedelic experience
* Hallucinogen persisting perception disorder
Hallucinogen persisting perception disorder (HPPD) is a non-psychotic disorder in which a person experiences apparent lasting or persistent visual hallucinations or perceptual distortions after a previous use of drugs, including but not limited t ...
HPPD
* Psychotic depression
Psychotic depression, also known as depressive psychosis, is a major depressive episode that is accompanied by psychotic symptoms.Hales E and Yudofsky JA, eds, The American Psychiatric Press Textbook of Psychiatry, Washington, DC: American Psych ...
* Simulated reality
* Vision (spirituality)
A vision is something seen in a dream, trance, or religious ecstasy, especially a supernatural appearance that usually conveys a revelation. Visions generally have more clarity than dreams, but traditionally fewer psychological connotations. ...
* Schizophrenia
Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social wit ...
* Bicameral mentality
Bicameral mentality is a hypothesis in psychology and neuroscience which argues that the human mind once operated in a state in which cognitive functions were divided between one part of the brain which appears to be "speaking", and a second part ...
* Apparitional experience
In parapsychology, an apparitional experience is an anomalous experience characterized by the apparent perception of either a living being or an inanimate object without there being any material stimulus for such a perception.
In academic discu ...
* Phantom limb
A phantom limb is the sensation that an amputated or missing limb is still attached. Approximately 80 to 100% of individuals with an amputation experience sensations in their amputated limb. However, only a small percentage will experience painf ...
References
Further reading
* Johnson FH (1978). ''The Anatomy of Hallucinations''. Chicago: Nelson-Hall Co. .
* Bentall RP, Slade PD (1988). Sensory Deception: A Scientific Analysis of Hallucination''. London: Croom Helm. .
* Aleman A, Larøi F (2008). ''Hallucinations: The Science of Idiosyncratic Perception''. American Psychological Association (APA). .
* Sacks O (2012). ''Hallucinations''. New York: Alfred A. Knopf.
External links
Hearing Voices Network
*
"The voice inside: A practical guide to coping with hearing voices"
Psychology Terms
Hallucination: A Normal Phenomenon?
Fasting-induced hallucination
Geometric visual hallucinations, Euclidean symmetry and the functional architecture of striate cortex
{{Authority control
Medical signs
Symptoms and signs of mental disorders