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Hallucination
Hallucination
Hallucination
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Hallucination
My eyes at the moment of the apparitions by August Natterer, a German artist who created many drawings of his hallucinations
SpecialtyPsychiatry
CausesHypnagogia, Peduncular hallucinosis, Delirium tremens, Parkinson's disease, Delusion, Lewy body dementia, Charles Bonnet syndrome, hallucinogens, sensory deprivation, schizophrenia, psychedelics, sleep paralysis, drug intoxication or withdrawal, sleep deprivation, epilepsy, psychological stress, non-celiac gluten sensitivity, fever,[1] covert weaponry[2][3]
TreatmentCognitive behavioral therapy[4] and metacognitive training[5]
MedicationAntipsychotic, AAP

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 modality—visual, 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

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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

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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

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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

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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

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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

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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

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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

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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 explicit 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]

Somatic

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Somatic hallucinations refer to an interoceptive sensory experience in the absence of stimulus. Somatic hallucinations can be broken down into further subcategories: general, algesic, kinesthetic, and cenesthopathic.[45][47]

  • Cenesthopathic- Effecting the cenesthetic sensory modality, cenesthopathic hallucinations are a pathological alteration in the sense of bodily existence, caused by aberrant bodily sensations. Most often, cenesthopathic hallucinations will refer to sensation in the visceral organs. Therefore, it is also known as visceral hallucinations.[53][47] Manifestations are often subjective, hard to describe and unique to the sufferer. Common manifestations include pressure, burning, tickling, or tightening in various body systems.[54] While these hallucinations can be experienced by a variety of psychiatric and neurological disorder, cenesthopathic schizophrenia is recognized by the ICD as a subtype of schizophrenia marked by primarily cenesthopathic hallucinations and other body image aberrations.[55][47]
  • Kinesthetic- Kinesthetic hallucinations, effecting the sensory modality of the same name, are the sensation of movement of the limbs or other body parts without actual movement.[56][47][54][53]
  • Algesic- Algesic hallucinations, effecting the algesic sensory modality, refers to a perceived perception of pain.[47][54][53]
  • General- General somatic hallucination refers to somatic hallucinations not otherwise categorized by the above subsections. Common examples include 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.[47]

Multimodal

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A hallucination involving sensory modalities 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).[9][10] 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.[9]

Cause

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See also: Psychosis (causes)

Hallucinations can be caused by a number of factors.[3]

Hypnagogic hallucination

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Main article: Hypnagogia

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.[57] 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. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare.[58]

Peduncular hallucinosis

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Main article: Peduncular hallucinosis

Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons on the brain stem. 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 hallucinations, 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.[58]

Delirium tremens

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Main article: Delirium tremens

One of the more enigmatic forms of visual hallucination is the highly variable, possibly polymodal delirium tremens. It is associated with withdrawal in alcohol use disorder. Individuals with delirium tremens may be agitated and confused, especially in the later stages of this disease.[59] 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.[60]

Parkinson's disease and Lewy body dementia

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Parkinson's disease is linked with Lewy body dementia for their similar hallucinatory symptoms. Presence hallucinations can be an early indicator of cognitive decline in Parkinson's Disease.[61] 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[62] 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 parts of the locus coeruleus, and the cholinergic neurons in the parabrachial area and pedunculopontine nuclei of the tegmentum.[58]

Migraine coma

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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.[58]

Migraine attacks

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See also: Migraine aura and Scintillating scotoma

Migraine attacks may result in visual hallucinations including auras and in rarer cases, auditory hallucinations.[63]

Charles Bonnet syndrome

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Charles Bonnet syndrome 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.[64]

Focal epilepsy

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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 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.[43][65]

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. 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 seizures.[43]

Distortions in visual perception during a temporal lobe seizure may include size distortion (micropsia or macropsia), 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.[66] Even when consciousness is impaired, insight into the hallucination or illusion is typically preserved.[67]

Drug-induced hallucination

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Main article: Hallucinogen

Drug-induced hallucinations are caused by hallucinogens, dissociatives, and deliriants, including many drugs with anticholinergic actions and certain stimulants, which are known to cause visual and auditory hallucinations. Some psychedelics such as lysergic acid diethylamide (LSD) and psilocybin can cause hallucinations that range in the spectrum of mild to intense.[citation needed]

Hallucinations, pseudohallucinations, or intensification of pareidolia, 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, sigma receptors, delta opioid receptor and the NMDA receptors or the overall receptor activation profile as synthetic opioids like those of the pentazocine, levorphanol, fentanyl, pethidine, methadone and some other families are more associated with this side effect than natural opioids like morphine and codeine and semi-synthetics like hydromorphone, 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.[68][69][70] These drugs also can induce sleep (relating to hypnagogic hallucinations) and especially the pethidines have atropine-like anticholinergic activity, which was possibly also a limiting factor in the use, the psychotomimetic side effects of potentiating morphine, oxycodone, 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.).[71]

Sensory deprivation hallucination

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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.)[72]

Experimentally-induced hallucinations

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Main article: Hallucinations in the sane

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,[73][74] 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.[75]

Non-celiac gluten sensitivity

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There is tentative evidence of a relationship with non-celiac gluten sensitivity, the so-called "gluten psychosis".[76]

Pathophysiology

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Dopaminergic and serotonergic hallucinations

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It has been reported that in serotonergic hallucinations, the person maintains an awareness that they are hallucinating, unlike dopaminergic hallucinations.[16]

Neuroanatomy

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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, 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, parahippocampus, and the right hemispheric homologue of Broca's area in the inferior frontal gyrus.[77] Grey and white matter abnormalities in visual regions are associated with hallucinations in diseases such as Alzheimer's disease, further supporting the notion of dysfunction in sensory regions underlying hallucinations.[78]

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.[77] This is supported by cognitive studies of those with hallucinations, who have demonstrated abnormal attribution of self generated stimuli.[79]

Disruptions in thalamocortical circuitry may underlie the observed top down and bottom up dysfunction.[80] Thalamocortical circuits, composed of projections between thalamic and cortical neurons and adjacent interneurons, underlie certain electrophysical characteristics (gamma oscillations) that are associated with 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.[81]

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.[82]

Treatments

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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 medication may also be utilized to treat the illness if the symptoms are severe and cause significant distress.[83] For other causes of hallucinations there is no factual evidence to support any one treatment is scientifically tested and proven. However, abstaining from hallucinogenic 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 show that cognitive behavioral therapy[4] and metacognitive training[5] can also reduce the severity of hallucinations. Furthermore, there are recovery movements all around the world that advocate for individuals with schizophrenia or voice-hearers (individuals that hear voices). The Hearing Voices Movement,[84][circular reference] starting in Europe, aims to[neutrality is disputed] utilize knowledge and experience of voice hearers combined with experts in disorders such as schizophrenia, such as psychiatrists.

Epidemiology

[edit]

Prevalence of hallucinations varies depending on underlying medical conditions,[85][9] which sensory modalities are affected,[10] age[86][85] and culture.[87] 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%.[86] Children and adolescents have been found to experience similar rates (12.7% and 12.4% respectively) which occur mostly during late childhood and adolescence. In this group, hallucinations are not necessarily indicative of later psychopathology and are recognized to occur on a continuum which includes normal, transient hallucinatory phenomena.[88] However, hallucinations become increasingly associated with psychopathology in late adolescence.[88]

The prevalence of hallucinations in adults and those over 60 is comparatively lower (with rates of 5.8% and 4.8% respectively).[86][85] For those with schizophrenia, the lifetime prevalence of hallucinations is 80%[9] and the estimated prevalence of visual hallucinations is 27%, compared to 79% for auditory hallucinations.[9] A 2019 study suggested 16.2% of adults with hearing impairment experience hallucinations, with prevalence rising to 24% in the most hearing impaired group.[89]

A risk factor for multimodal hallucinations is prior experience of unimodal hallucinations.[9] In 90% cases of psychosis, a visual hallucination occurs in combination with another sensory modality, most often being auditory or somatic.[9] In schizophrenia, multimodal hallucinations are twice as common as unimodal ones.[9]

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,[90]: 415  along with 20–60% reporting some religious content in delusions.[90]: 415  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.[90]: 421 

See also

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References

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Further reading

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hallucination

View on Grokipedia
from Grokipedia
A hallucination is a sensory that occurs in the absence of an external stimulus, often accompanied by a vivid of reality as if the experience were genuinely occurring. These perceptions can manifest across various sensory modalities, including visual (seeing images or figures), auditory (hearing voices or sounds), tactile (feeling touches or sensations on the skin), olfactory (smelling odors), or gustatory (tasting flavors), and are generated internally by the rather than from real environmental input. Unlike illusions, which distort actual stimuli, hallucinations involve no corresponding external trigger, distinguishing them as a form of perceptual anomaly. Hallucinations occur in approximately 6–15% of the general over their lifetime, with varying by type and ; for instance, auditory verbal hallucinations are reported by 5–15% of individuals without associated , often transiently during stress, bereavement, or transitions. In clinical contexts, they are a hallmark symptom of psychiatric disorders such as , affecting 60–70% of patients primarily through auditory forms like hearing critical or commanding voices, and occur in during manic or depressive episodes. Neurological conditions also contribute, including (visual hallucinations in 20–50% of cases), epilepsy (often focal seizures triggering sensory experiences), and migraines, while substance use—such as hallucinogens like or withdrawal from alcohol—can induce acute episodes. Other precipitants include , extreme fatigue, fever, or injuries, highlighting hallucinations as a multifaceted influenced by neurobiological, psychological, and environmental factors. Management of hallucinations depends on the underlying cause and severity, with treatment aimed at addressing the root condition. For schizophrenia-spectrum disorders, medications are first-line and effective in reducing symptoms in approximately 70–80% of cases, supplemented by (CBT) to enhance coping strategies. In neurological etiologies, interventions may target the specific condition; non-pharmacological approaches like show promise for refractory cases. may increase with age, with reports of up to 37% in older adults, often linked to or , underscoring the need for comprehensive assessment.

Introduction

Definition

A hallucination is defined as a perception of an object, event, or phenomenon in the absence of a corresponding external sensory stimulus, which nonetheless feels vividly real to the experiencing individual. These perceptions can involve any of the sensory modalities, including sight, sound, , touch, or , though they most commonly manifest as visual or auditory experiences. Key characteristics of hallucinations include their vividness, involuntariness, and the subjective of held by the , distinguishing them from mere thoughts or imaginings. These experiences occur while the individual is awake and alert, without external provocation, and often carry a of authenticity that can be distressing or neutral depending on . Unlike illusions, which distort actual external stimuli, hallucinations arise entirely from internal processes. The term "hallucination" derives from the Latin hallucinatio, rooted in the verb alucinari (or hallucinari), meaning "to wander in the mind" or "to be distraught." It entered the English language in the mid-17th century, with the earliest recorded medical usage appearing in 1646 in the writings of physician Sir Thomas Browne, though contemporaries like contributed to its application in neurological and psychiatric contexts during this period. Hallucinations are broadly classified into elementary (or simple) forms, consisting of basic sensory elements such as flashes of light, geometric patterns, dots, lines, or unformed sounds, and organized (or complex) forms, which involve structured scenes, recognizable objects, people, animals, or coherent voices. This distinction highlights the spectrum of hallucinatory complexity, from abstract percepts to elaborate narratives, without implying specific underlying causes. Hallucinations are often distinguished from illusions, which involve the misinterpretation or distortion of actual external sensory stimuli, whereas hallucinations occur in the complete absence of such stimuli. For instance, an , such as the where parallel lines appear bent due to contextual arrows, relies on a real visual input that is perceptually altered, but the perceiver recognizes the distortion upon closer inspection or measurement. In contrast, a hallucination generates a sensory without any corresponding external object or event, leading to a that feels as vivid and real as veridical sensation but lacks an objective basis. Delusions, another related phenomenon, differ fundamentally from as they represent fixed, false beliefs about reality rather than sensory perceptions. A might involve the unshakeable conviction that one is being persecuted by unseen forces, despite contradictory evidence, whereas a hallucination entails experiencing sensory content, such as hearing voices or seeing images, that is not externally present. Although both can coexist in psychiatric conditions like , where auditory hallucinations may reinforce persecutory delusions, the distinction lies in delusions being cognitive misinterpretations of reality and hallucinations being perceptual ones. Pseudohallucinations further blur the boundary but are differentiated by the individual's insight that the experience originates internally and is not externally real, lacking the compelling sense of objective reality characteristic of true hallucinations. The concept of pseudohallucinations remains controversial, with some researchers questioning its validity and distinctiveness from true hallucinations due to inconsistent definitions and low construct validity. For example, a person might vividly "hear" an internal voice commenting on their thoughts and explicitly recognize it as a mental construct, stating, "I know this is just in my mind," which contrasts with the external projection and conviction in genuine hallucinations. This awareness prevents pseudohallucinations from fully mimicking the perceptual authenticity of hallucinations, though they can still cause distress, particularly in psychiatric contexts. Hypnagogic and hypnopompic hallucinations, while classified as true hallucinations due to their perceptual nature without external triggers, are specifically delimited by their timing in relation to cycles. Hypnagogic hallucinations emerge during the transition to (sleep onset), often involving fleeting visual or that feels dream-like yet awake, whereas hypnopompic hallucinations occur upon awakening, sometimes extending dream content into brief , such as seeing shadowy figures in the bedroom. Both lack voluntary control and external stimuli but are typically benign and short-lived, distinguishing them from more persistent pathological hallucinations through their association with normal boundaries. In certain cultural or spiritual contexts, experiences like visions during or near-death encounters may resemble hallucinations but are often differentiated by elements of voluntary induction, positive emotional valence, and interpretive frameworks that emphasize or transcendence rather than . For instance, meditative visions might arise under intentional focus and be regarded as internally generated , contrasting with the involuntary and distressing quality of clinical hallucinations, while near-death visions frequently report enhanced serenity and purpose, lacking the disorientation typical of hallucinatory episodes. These distinctions highlight how cultural attribution influences perception, though overlap can occur without implying equivalence.

Classification

Auditory Hallucinations

Auditory hallucinations involve the perception of sounds, such as , music, or noises, in the absence of an external stimulus. These experiences are the most prevalent form of hallucination in , affecting approximately 70-75% of patients. Subtypes of auditory hallucinations include command hallucinations, where issue direct orders, such as instructions to oneself or others; third-person commentary, involving that discuss or argue about the individual as if observing them; and simpler forms like unformed noises or incoherent sounds. Command hallucinations often carry an imperative tone, while third-person variants may involve multiple debating the person's actions or thoughts. Simple noises, such as buzzing or murmuring, lack verbal content and are less complex. These hallucinations are frequently repetitive, featuring emotional tones that range from threatening and derogatory to neutral, and they commonly evoke significant distress, though levels of insight into their unreality can vary among individuals. The repetitive nature often amplifies emotional impact, with negative content leading to heightened anxiety or fear in many cases. Representative examples include hearing accusatory voices that criticize or condemn the person's , or perceiving the voice of a deceased loved one offering comfort or guidance, particularly in contexts like bereavement where such experiences may occur in up to 50% of grievers. Clinically, command hallucinations are associated with elevated risks of or toward others due to potential compliance with the directives, necessitating careful and intervention. Unlike , which involves distorted perceptions of actual internal auditory signals like ringing or buzzing, auditory hallucinations generate entirely false sensory experiences without such physiological basis.

Visual Hallucinations

Visual hallucinations refer to the of visual stimuli, such as lights, patterns, or complex scenes, in the absence of corresponding external sensory input. These experiences occur while the individual is awake and alert, distinguishing them from dreams or hypnagogic . They are broadly categorized into simple (or elementary) forms, which include unformed geometric shapes, flashes of , or indistinct colors, and complex (or formed) forms, which involve recognizable objects like , animals, or narrative scenes. The prevalence of visual hallucinations varies by population but is notable in clinical settings. In patients with psychotic disorders, such as , they occur in 25-50% of cases, with a weighted mean of about 27% across studies. Among elderly individuals with significant vision loss, the rate rises to 10-15%, particularly in conditions like Charles Bonnet syndrome. Characteristics of visual hallucinations often include vivid, colorful imagery that appears dynamic and projected into the external space, giving the impression of reality. In peduncular hallucinosis, associated with lesions, these perceptions frequently feature Lilliputian figures—tiny, detailed representations of people or objects—adding a distinctive, miniature quality to the experience. Representative examples illustrate their diversity. In syndrome, patients with may see brief flashes of light or recurring patterns, typically without emotional distress. In , more elaborate scenes, such as crowds or fantastical landscapes, can emerge, often accompanying cognitive fluctuations. Culturally, visual hallucinations manifest in bereavement as sightings of deceased loved ones, reported by up to 30% of grieving individuals in some studies; these are generally viewed as comforting and non-pathological, differing from clinical hallucinations or those induced by cultural rituals.

Other Sensory Modalities

Olfactory hallucinations, also known as , involve the perception of smells in the absence of external stimuli, often described as unpleasant odors such as burning rubber, rot, or smoke. These experiences are relatively uncommon, occurring in approximately 3.9% of pediatric cases and up to 6% of general episodes. They are frequently associated with , where they may manifest as brief auras known as uncinate fits. Gustatory hallucinations entail the sensation of tastes without any ingested substance, typically simple and unformed, such as a metallic or bitter flavor. These are rarer than olfactory hallucinations, with an incidence estimated at about half that of olfactory types in disorders. They commonly arise during seizures, where a metallic serves as a characteristic aura. Tactile hallucinations produce perceptions of touch or skin sensations without physical contact, including —the feeling of insects crawling on or under the skin. This phenomenon is notably prevalent in substance-related contexts, such as , where it contributes to intense distress and compulsive behaviors like skin-picking. Similar tactile experiences, including sensations of bugs or vermin, can emerge during alcohol withdrawal as part of . Somatic hallucinations involve illusory internal bodily sensations, such as unexplained , pressure, movement, or within the body. Examples include the of a lump in the , animals moving inside the body, or twisting sensations in limbs without objective cause. These are less frequent than auditory or visual types in , with past-month prevalence rates around 4-7% for related tactile-somatic experiences. Sexual hallucinations are rare and encompass erotic sensations, genital , or visions of sexual acts without external triggers. They often present as orgasmic auras or bodily feelings of sexual pleasure, predominantly linked to where limbic structures are involved. Such experiences occur in a minority of cases, with reports emphasizing their episodic and intense nature in neurological contexts. Multimodal hallucinations integrate perceptions across multiple sensory modalities, such as a voice accompanied by a visual figure or tactile presence, which may signal greater clinical severity. In , these are more prevalent than unimodal hallucinations, affecting approximately 53% of individuals with any hallucinatory experiences over their lifetime. Command hallucinations, typically auditory directives to perform actions, can extend multimodally, incorporating visual or somatic elements to heighten their imperative quality.

Causes

Hallucinations can arise from normal physiological processes, particularly during transitions between and , without indicating underlying . These experiences, often linked to the intrusion of rapid eye movement (REM) sleep elements into conscious awareness, are common in the general population and typically benign. Hypnagogic hallucinations occur at the onset of and involve vivid sensory perceptions, such as visual images of shapes or faces, auditory sounds like voices, or tactile sensations of falling. They affect up to 70% of individuals at some point, representing a frequent normal variant rather than a disorder. These phenomena are associated with REM sleep intrusion, where dream-like activity overlaps with during the hypnagogic state. Hypnopompic hallucinations, analogous to hypnagogic ones, emerge upon awakening from sleep and feature similar multisensory content, though they are less prevalent, reported by approximately 12.5% of people. Like their counterparts, they stem from incomplete separation between REM sleep and wake states, often resolving spontaneously without intervention. Sleep paralysis frequently accompanies these hallucinations, characterized by temporary immobility during sleep-wake transitions, with an estimated lifetime prevalence of 7.6% in the general population. Up to 75% of episodes include hallucinatory elements, such as terrifying visions of shadowy figures or intruders, heightening distress but remaining non-pathological in isolation. In , a involving disrupted REM regulation, hypnagogic or hypnopompic hallucinations occur in about 50% of cases, often alongside —sudden muscle weakness triggered by emotions. These hallucinations intensify the disorder's impact on daily functioning but are directly tied to the condition's core pathophysiology of REM instability. Brief hallucinations also manifest as normal variants in states of extreme fatigue or , where sensory perceptions like hearing a deceased loved one's voice provide temporary emotional relief without signifying mental illness. Such experiences, termed bereavement hallucinations, are reported by a significant portion of mourners and typically fade as adjustment progresses.

Neurological and Medical Conditions

Hallucinations can arise from various neurological and medical conditions, often linked to disruptions in , function, or systemic processes. These manifestations differ from those in other categories by their association with underlying diseases such as lesions, degenerative disorders, or immune-mediated responses, rather than transient physiological states or external triggers. Common features include visual, tactile, olfactory, or gustatory experiences that may accompany motor, cognitive, or sensory symptoms, providing diagnostic clues for the underlying . Peduncular hallucinosis is a rare characterized by vivid, complex visual hallucinations, often featuring colorful, Lilliputian scenes of , animals, or landscapes, resulting from lesions in the , particularly the or . These hallucinations typically emerge following vascular events, infections, or tumors compressing the peripeduncular region, and they are usually non-distressing, occurring in clear alongside sleep disturbances or oculomotor abnormalities. The condition was first described in 1922 and remains distinct due to its anatomical specificity in the rostral . Delirium tremens represents a severe form of alcohol withdrawal delirium, marked by tactile and visual hallucinations such as sensations of crawling on the skin or vivid scenes of threats, accompanied by tremors, autonomic hyperactivity, and potential seizures. It typically onset 48-72 hours after cessation of heavy alcohol use in dependent individuals, reflecting excitotoxic neuronal changes in the . This affects a subset of withdrawal cases, with hallucinations contributing to profound confusion and agitation. In , visual hallucinations occur in 20-40% of patients over the disease course, often depicting people, animals, or familiar figures in realistic but recurring scenarios, fluctuating in intensity alongside motor symptoms like bradykinesia and rigidity. These are linked to dopaminergic dysregulation and pathology in visual processing areas. In dementia, the prevalence rises to 60-80%, with similar complex visual content that may recur nightly and correlate with cognitive decline, distinguishing it from other s. Migraine-associated hallucinations include elementary visual auras like scintillating scotomas during attacks, as well as more complex perceptual distortions in syndromes such as , where objects or body parts appear distorted in size, shape, or distance. These episodes, often lasting minutes to hours, stem from in the occipital and parietal lobes, and in rare cases, may persist in migrainous coma. , first linked to migraine in 1955, exemplifies these transient, episodic phenomena without loss of reality testing. Charles Bonnet syndrome involves recurrent, elaborate visual hallucinations in elderly individuals with significant vision loss from conditions like or , serving as a compensatory response to deafferentation in the . Patients typically recognize these images—such as landscapes, figures, or patterns—as unreal and non-threatening, with episodes triggered by low light or fatigue. The syndrome, named after an 18th-century observer, affects up to 30% of those with profound and lacks the delusions seen in psychiatric disorders. Focal , particularly seizures, can produce brief, stereotyped olfactory or gustatory hallucinations, such as unpleasant burning smells or metallic tastes, arising from hyperexcitability in the or insula. These auras last seconds to minutes, often preceding impaired , and are ictal phenomena rather than interictal distortions. Such sensory seizures highlight the role of limbic structures in . rarely manifests with auditory or visual hallucinations as part of a broader neuropsychiatric , potentially driven by immune-mediated crossing the blood-brain barrier. Case reports describe resolution of such symptoms, including persecutory voices or fleeting images, upon elimination in individuals without celiac or . This association, emerging in literature since the 2010s, underscores 's role in atypical immune responses affecting the . Hallucinations have also been reported in association with and its long-term effects (), particularly in severe cases involving , hypoxia, or . These can include visual and auditory hallucinations, with prevalence of psychotic symptoms estimated at 1-3% in patients as of 2024, and up to 21.6% during inpatient rehabilitation following acute . Such episodes often resolve with treatment of the underlying condition but may persist in rare cases without prior .

Substance-Induced Causes

Substance-induced hallucinations arise from the , intoxication, or withdrawal of various psychoactive substances, disrupting normal and through alterations in systems such as serotonin, dopamine, and glutamate. These hallucinations can manifest across sensory modalities, often resolving upon cessation of exposure but sometimes persisting or recurring. They are distinct from endogenous causes, as they are directly linked to exogenous chemical triggers, with clinical presentations varying by substance class and dosage. Classic hallucinogens like lysergic acid diethylamide (LSD) and primarily induce vivid visual hallucinations, including geometric patterns, color distortions, and synesthetic experiences, mediated by agonism at 5-HT2A serotonin receptors. These effects typically onset within 30-90 minutes of ingestion and last 6-12 hours, with users reporting enhanced perceptual depth rather than full detachment from reality at moderate doses. Dissociative anesthetics such as , an , produce out-of-body experiences, depersonalization, and immersive visual or auditory hallucinations, often accompanied by a sense of ego dissolution; these emerge at sub-anesthetic doses and contribute to states mimicking near-death phenomena. Stimulants including and amphetamines trigger hallucinations through excessive release, commonly featuring paranoid auditory perceptions (e.g., voices accusing the user) and tactile sensations known as , where individuals feel crawling under their —a hallmark of cocaine-induced . Visual hallucinations may also occur, with prevalence rates of psychotic symptoms reaching 60-86% among chronic cocaine users and 17-37% for amphetamines, often escalating with binge use. Alcohol withdrawal, particularly in severe cases of , leads to multimodal hallucinations (visual, auditory, and tactile) peaking 48-72 hours after cessation, involving terrifying imagery such as small animals or ; these affect up to 12% of hospitalized alcohol-dependent individuals and are compounded by autonomic hyperactivity. Anticholinergic agents like atropine cause with predominantly visual hallucinations, such as fragmented scenes or animate objects, due to muscarinic receptor disrupting signaling in the ; these are common in from therapeutic or accidental overdose, presenting alongside and agitation. Cannabis, via CB1 receptor partial agonism, occasionally induces mild auditory hallucinations or perceptual distortions, particularly in high-potency strains or vulnerable users, with psychotic symptoms reported in 0.8-10% of regular consumers. Opioid-related hallucinations are rare, occurring in less than 2% of users during intoxication or withdrawal, but when present, they are typically visual (e.g., vivid scenes) and may be precipitated by agents like or during naloxone-induced withdrawal, often in contexts. Among hallucinogen users, 4.2% experience (HPPD), characterized by recurrent visual disturbances like trails or geometric patterns long after substance clearance, potentially lasting months to years and linked to prior or exposure.

Environmental and Experimental Causes

, often induced in controlled environments like isolation tanks filled with saltwater, can lead to hallucinations as the brain compensates for the lack of external stimuli by generating internal perceptual experiences. Studies have shown that visual and auditory hallucinations may emerge after prolonged exposure, typically within 24 to 48 hours, due to the brain's tendency to fill sensory gaps with spontaneous neural activity. For instance, early experiments demonstrated that participants in sensory isolation reported vivid and auditory perceptions, highlighting the role of reduced input in triggering these phenomena. Extreme stress or trauma, such as bereavement, can also provoke hallucinations, particularly in the form of sensing the presence or voice of a deceased loved one. A of bereavement hallucinations indicates that approximately 56.6% (95% CI: 49.9–63.2%) of grievers experience such episodes, often visual or auditory encounters with the departed, which are typically short-term and non-pathological. These experiences are attributed to heightened emotional states disrupting normal perceptual processing, providing temporary comfort without indicating underlying disorder. In experimental settings, substances like and have been used to induce hallucinations under controlled conditions, revealing dose-dependent effects on visual perceptions. The (1960–1963), led by and Richard Alpert, administered these psychedelics to participants and documented progressive intensification of visual distortions and geometric patterns with increasing doses, contributing to early understandings of hallucinogenic mechanisms. Modern dose-response studies confirm that higher levels correlate with stronger perceptual alterations, including vivid hallucinations, underscoring the reproducibility of these effects in . High-altitude exposure and associated hypoxia can trigger auditory and visual hallucinations among mountaineers, stemming from oxygen deprivation affecting brain function. Research on extreme-altitude climbers above 8,500 meters without supplemental oxygen found that 88% reported hallucinatory experiences, such as seeing companions or hearing voices, often linked to rather than alone. These episodes typically resolve upon descent and , illustrating the environmental impact on . Electrostimulation techniques, particularly (TMS) applied to the occipital cortex, can experimentally induce —perceived flashes of light resembling simple visual hallucinations. Seminal studies have established that TMS pulses generate these phosphenes by directly exciting visual cortical neurons, with thresholds varying by coil type and intensity, providing a non-invasive model for probing hallucinatory pathways. This method has been instrumental in distinguishing phosphene induction from broader hallucinatory states, aiding neuroscientific investigations.

Pathophysiology

Neurochemical Mechanisms

Hallucinations arise from disruptions in systems that regulate , , and . Key mechanisms involve imbalances in , serotonin, , glutamate, and endogenous opioids, each contributing to specific types of hallucinatory experiences across various conditions. The dopaminergic hypothesis posits that hyperactivity in the mesolimbic dopamine pathway, particularly excessive stimulation of D2 receptors, underlies positive symptoms of , including auditory hallucinations. This excess release in subcortical regions like the enhances salience attribution to internal stimuli, leading to perceptual distortions interpreted as external voices or sounds. studies confirm elevated synthesis and release in the of individuals prone to psychotic symptoms, supporting this pathway's role in hallucination generation. Serotonergic pathways are critically involved in visual hallucinations, with agonism of 5-HT2A receptors by psychedelics such as inducing profound perceptual alterations. reduces parieto-occipital alpha oscillations and N170 visual-evoked potentials, shifting cortical processing toward internal and causing vivid visual distortions that correlate strongly with subjective hallucinatory intensity. These effects are selectively blocked by 5-HT2A antagonists like , confirming receptor specificity. Inversely, serotonergic hypoactivity, as implicated in depression, may facilitate hallucinatory phenomena by diminishing inhibitory control over sensory networks, though such occurrences are less common and often tied to severe mood dysregulation. Cholinergic deficiency, characterized by reduced signaling in cortical areas, contributes to visual hallucinations in , where degeneration of cholinergic neurons impairs attention and . This hypoactivity disrupts the integration of visual information, leading to misperceptions of environmental stimuli as hallucinatory elements. The model exemplifies this mechanism, as muscarinic receptor blockade induces delirium-like states with prominent visual and tactile hallucinations, mimicking cholinergic loss without neuronal damage and highlighting acetylcholine's role in maintaining perceptual stability. Glutamatergic dysfunction, particularly hypofunction, produces hallucinations resembling those in when induced by . As an uncompetitive , disrupts excitatory-inhibitory balance in cortical circuits, increasing glutamate release in prefrontal and temporal regions while impairing gamma oscillations essential for sensory binding, resulting in and perceptual symptoms like vivid illusions and . This model demonstrates dose-dependent escalation from mild perceptual changes to full psychotic episodes, underscoring glutamate's modulatory influence on hallucinatory states.

Neuroanatomical Basis

Hallucinations arise from disruptions in specific regions and that process sensory information and internal representations, often involving aberrant activation or connectivity in cortical and subcortical structures. studies have identified key areas implicated in the generation of hallucinatory experiences across modalities, highlighting the role of both localized hyperactivity and distributed network dysfunction. Auditory hallucinations, particularly auditory verbal hallucinations in , are associated with hyperactivity in the , including Heschl's gyrus, which processes auditory stimuli. This region shows increased blood oxygen level-dependent signals during hallucination episodes, as captured by (fMRI). Additionally, in the is involved in the language-related aspects of perceived voices, with decoding studies demonstrating predictive activity in this area prior to hallucination onset. Visual hallucinations engage the occipital and temporal lobes, where the ventral visual stream processes complex and scene formation. In Charles Bonnet syndrome, which occurs in individuals with , lesions or deafferentation in the occipital cortex lead to vivid, formed visual percepts, often involving activation of visual association areas in the . These findings indicate that reduced sensory input can trigger spontaneous activity in the "what" pathway of visual processing. Multimodal hallucinations, involving combined sensory modalities, implicate the as a critical for integrating sensory inputs and the for cross-modal synthesis. Thalamic nuclei facilitate the gating of sensory signals to cortical areas, and disruptions here contribute to the blending of sensory experiences in psychotic states. The , particularly the , supports spatial and attentional integration, with aberrant activity linked to hallucinatory perceptions across senses. Subcortical structures, such as the , play a role in peduncular hallucinosis, where lesions in the or disrupt ascending reticular activating pathways that modulate and sensory filtering. These lesions, often vascular or compressive, lead to vivid, Lilliputian visual hallucinations without insight impairment, underscoring the brainstem's influence on thalamocortical transmission. Network-level models emphasize dysregulation of the (DMN), which includes the medial , posterior cingulate, and , as a mechanism for generating involuntary sensory . In conditions like and , hyperconnectivity or reduced anticorrelations within the DMN allow internal mentation to intrude into perceptual awareness, bypassing external sensory checks. This triple-network imbalance, involving the DMN, , and executive network, further propagates hallucinatory states.

Diagnosis

Clinical Assessment

Clinical assessment of hallucinations begins with a thorough history-taking to characterize the experience. Clinicians inquire about the sensory modality of the hallucinations, which may include auditory, visual, tactile, olfactory, or gustatory perceptions, as well as their frequency, duration, content, and associated distress levels. For auditory hallucinations, a such as the Psychotic Symptom Rating Scales (PSYRATS) auditory hallucinations subscale is commonly used; this tool rates dimensions including frequency (e.g., daily occurrence), duration (e.g., lasting minutes to hours), intensity, emotional impact, and degree of control over the experience, providing a multidimensional profile to gauge severity and impact. The mental status examination (MSE) further evaluates the patient's perceptual disturbances and related cognitive features. Key components include assessing —whether the patient recognizes the hallucinations as unreal or believes them to be veridical—and observing the patient's affect during recall of the experiences, such as anxiety, , or indifference, which can indicate emotional burden. This examination helps distinguish hallucinations from other perceptual anomalies and informs the functional implications. Screening questionnaires aid in quantifying hallucination proneness, particularly in non-acute settings or for purposes. The Launay-Slade Hallucination Scale (LSHS), a 12-item self-report measure, assesses predisposition to hallucinatory experiences across modalities by rating the frequency of phenomena like intrusive thoughts or vivid on a , with higher scores indicating greater proneness; it is validated for use in both clinical and general populations. Collateral information from family members or caregivers is essential to contextualize the patient's report, providing details on the onset, progression, and situational triggers of hallucinations, as well as any observable behavioral changes. Basic laboratory investigations are performed to exclude metabolic or toxic etiologies. These typically include a , (to assess imbalances, glucose levels, and renal/hepatic function), , , and toxicology screening for substances.

Differential Diagnosis

Differential diagnosis of hallucinations involves distinguishing them from other perceptual disturbances, perceptual misinterpretations, and related psychiatric or neurological phenomena to guide appropriate clinical management. Hallucinations are defined as sensory perceptions occurring in the absence of corresponding external stimuli, and they can arise from diverse etiologies including psychiatric, neurological, and substance-related causes. Accurate differentiation requires careful history-taking, assessment of , sensory modality, content, duration, and associated symptoms, often necessitating , laboratory tests, or psychiatric evaluation to rule out underlying organic pathology. A primary distinction lies between hallucinations, illusions, and delusions. Illusions represent misperceptions of actual external stimuli, such as mistaking a shadow for a person, and typically resolve upon increased attention, lighting adjustment, or sensory correction, whereas hallucinations occur without any real stimulus and persist independently. Delusions, in contrast, are fixed false beliefs without a sensory perceptual component, such as a conviction of being persecuted, lacking the vivid sensory quality of hallucinations; for instance, a patient may hold a delusional belief about external threats but not experience auditory voices unless true hallucinations are also present. These differences are crucial in psychiatric evaluation, as illusions often signal environmental or sensory deficits, while delusions and hallucinations indicate more profound disruptions in reality testing. Organic causes of hallucinations must be differentiated from functional psychiatric origins through clinical features and ancillary investigations. Organic hallucinations, often linked to , are typically acute, fluctuating in intensity, multimodal (involving multiple senses), and accompanied by altered , confusion, or identifiable medical triggers like infections, metabolic derangements, or medication side effects; for example, in hospitalized elderly patients may present with vivid, fragmented visual hallucinations that wax and wane over hours. In contrast, functional hallucinations in conditions like are chronic, organized, and modality-specific (e.g., complex auditory with commentary), occurring with preserved and often featuring impaired ; ruling out organic factors involves excluding via tools like the Confusion Assessment Method and conducting targeted labs or imaging. Pseudohallucinations, particularly in (BPD), differ from true hallucinations by their internal origin, reduced vividness, and preserved insight, where individuals recognize the experiences as unreal or self-generated. In BPD, auditory verbal hallucinations are often described as originating "inside the head," transient, stress-related, and less distressing or commanding than those in psychotic disorders, with prevalence estimates of approximately 25-27% in clinical samples; these may reflect dissociative processes rather than primary psychosis, and emphasizes evaluating for comorbid trauma or affective symptoms over trials. Cultural syndromes can mimic hallucinations through interpretive frameworks, requiring culturally sensitive assessment to avoid misdiagnosis. For instance, in Yoruba communities of , experiences labeled as "Ode-Ori" (head carrier or madness) may involve attributed to causes, presenting as auditory or visual perceptions interpreted as ancestral communications rather than pathological hallucinations; these differ from clinical hallucinations by their contextual acceptance within cultural rituals and lack of distress or functional impairment outside spiritual beliefs. Similarly, in practiced by Afro-Caribbean communities, beliefs in can complicate the diagnosis of mental illness, presenting as states that are not equated with psychotic hallucinations when into their cultural meaning is retained. Clinicians must collaborate with cultural experts to discern whether such experiences align with normative practices or indicate underlying . Key differentials include hallucinations in , which are often simple, recurrent visual phenomena (e.g., seeing deceased relatives or animals) without complex narratives, predominantly in (affecting up to 80% of cases) and associated with visuospatial deficits, contrasting with the novel, elaborate content of primary psychotic hallucinations. In (PTSD), flashbacks represent intrusive re-experiencing of past trauma—vivid but memory-based sensory reliving (e.g., reliving a scene)—rather than novel hallucinations, lacking the external attribution and persistence of true perceptual distortions; differentiation hinges on trauma history, episodic triggers, and absence of reality testing impairment beyond the event. Other mimics, such as Charles Bonnet syndrome in , feature preserved insight and non-threatening visuals, underscoring the need for modality-specific evaluation.

Treatment

Pharmacological Treatments

Pharmacological treatments for hallucinations primarily target the underlying neurochemical imbalances, such as excessive activity in conditions like . medications are the first-line intervention for hallucinations in schizophrenia spectrum disorders, rapidly reducing symptom severity through blockade of dopamine D2 receptors in the . Typical antipsychotics, exemplified by , exert potent antagonism, effectively suppressing positive symptoms including auditory and visual hallucinations. These agents are particularly useful in acute settings where rapid control is needed, though their use is tempered by a higher risk of extrapyramidal side effects (EPS), such as acute , , and , which require careful monitoring and potential adjunctive treatments like anticholinergics. Atypical antipsychotics, such as , , and , provide broader efficacy by modulating both and serotonin receptors, often leading to better tolerability and sustained symptom control. These medications have demonstrated efficacy in alleviating auditory verbal hallucinations, with showing particular benefit in treatment-resistant cases. Compared to typical agents, atypicals are associated with a lower incidence of EPS but may carry risks of metabolic disturbances like and , underscoring the need for individualized selection based on patient profile. For hallucinations arising in the context of mood disorders, such as , selective serotonin reuptake inhibitors (SSRIs) like address potential serotonergic dysregulation, though they are most effective when combined with antipsychotics for comprehensive symptom relief. In substance-induced cases, particularly alcohol withdrawal featuring tremors and perceptual disturbances, benzodiazepines such as or are standard to mitigate autonomic hyperactivity and prevent progression to severe hallucinations. Migraine-associated visual auras, considered a form of hallucination, respond acutely to like for aborting attacks, while prophylactic agents such as topiramate reduce aura frequency by stabilizing neuronal excitability. For neurological conditions, visual hallucinations in are often managed by reducing medications when possible, or using (approved specifically for psychosis), , or , which have lower risk of worsening motor symptoms. In , anticonvulsants such as or address seizure-related hallucinations by controlling underlying epileptiform activity. In September 2024, the FDA approved Cobenfy (xanomeline and ), a muscarinic receptor , for the treatment of in adults, demonstrating efficacy in reducing positive symptoms such as hallucinations through a novel non- mechanism. Across these treatments, ongoing clinical monitoring is essential to balance efficacy against adverse effects, including the EPS linked to blockade in antipsychotics.

Non-Pharmacological Interventions

Non-pharmacological interventions for hallucinations emphasize psychological therapies, supportive strategies, and lifestyle modifications to alleviate distress and enhance coping without relying on medications. These approaches target the emotional impact and functional impairment of hallucinations, fostering insight and resilience. Among the most established methods is (CBT), particularly tailored for voice-hearing experiences, which involves normalizing the phenomenon, challenging distressing content, and building adaptive responses. A of randomized and non-randomized trials indicates that CBT significantly reduces the frequency, severity, and distress of auditory hallucinations, with notable improvements in standardized measures like the Psychotic Symptom Rating Scales (PSYRATS). Reality testing, a core component of CBT protocols, equips individuals with techniques to evaluate the validity of hallucinatory experiences and differentiate them from . Common practices include journaling episodes to log sensory details, emotional triggers, and contradictory evidence, which promotes metacognitive awareness and diminishes the perceived power of hallucinations. This approach has been integrated into evidence-based manuals for treatment, showing benefits in reducing preoccupation and in delusional or hallucinatory beliefs. For visual hallucinations in syndrome, arising from due to vision loss, low-vision rehabilitation plays a key role by providing optical aids such as magnifiers and enhancing environmental stimulation to counteract deprivation. Clinical evaluations demonstrate that these interventions improve and functional outcomes, thereby decreasing the occurrence and intensity of hallucinations in affected patients. Peer support through organizations like the Hearing Voices Network offers practical coping strategies, including selective attention to positive voices, via engaging activities, and reframing to empower voice-hearers. These groups emphasize community validation and skill-sharing, which help mitigate isolation and emotional burden associated with hallucinations. Lifestyle adjustments further support management, such as implementing for hypnagogic hallucinations—characterized by vivid imagery at sleep onset—through consistent schedules, dark environments, and avoidance of stimulants to stabilize sleep-wake transitions and reduce episode frequency. Similarly, techniques cultivate non-reactive observation of experiences, leading to gradual declines in hallucination severity and related delusions, as evidenced in intervention studies tracking symptom trajectories over weeks.

Epidemiology

Prevalence and Incidence

In the general population, the lifetime prevalence of hallucinations is estimated to be between 5% and 15%, based on large-scale epidemiological studies. This figure encompasses various sensory modalities, with auditory verbal hallucinations being among the most commonly reported, occurring in approximately 5-28% of individuals over their lifetime. Rates are notably higher in specific non-pathological contexts, such as bereavement, where meta-analyses indicate that 50-60% of grieving individuals experience sensory perceptions of the deceased, often visual or auditory in . Among psychiatric populations, hallucinations are substantially more prevalent. In schizophrenia spectrum disorders, 60-80% of patients report hallucinations, predominantly auditory, though visual and multimodal experiences also occur frequently. In , lifetime prevalence of psychotic symptoms including hallucinations ranges from 50% to 70%, typically emerging during manic or mixed episodes and often involving auditory or visual content. These rates underscore the role of hallucinations as core symptoms in psychotic disorders, contributing to diagnostic criteria in frameworks like the DSM-5. Prevalence patterns vary by age and gender. Auditory hallucinations peak in young adults, with past-year rates reaching 7% among those aged 16-19 years and declining steadily across the lifespan to about 3% in individuals over 70. In contrast, visual hallucinations are more common in the elderly, affecting around 10% of older psychiatric outpatients and often linked to age-related sensory decline or neurological conditions. Overall, there is a slight female predominance in hallucination reporting across populations, potentially influenced by gender differences in symptom expression and help-seeking behaviors. Global variations in reported prevalence reflect cultural influences on disclosure and interpretation. Rates appear higher in some developing countries, such as West African nations like Ghana (90% auditory hallucinations in clinical samples) and Nigeria (85%), compared to European countries like Austria (66%), attributed to greater cultural acceptance of spiritual or supernatural explanations for such experiences. Recent surveys from the World Mental Health Initiative in the 2010s have highlighted increasing recognition of hallucinations in non-clinical samples worldwide, with median lifetime prevalence of psychotic experiences around 7%, emphasizing their continuum from normative to pathological.

Associated Risk Factors

Hallucinations exhibit a significant genetic component, particularly in the context of schizophrenia-spectrum disorders where they are a core symptom. Twin and family studies estimate the of at approximately 80%, with genetic factors accounting for 30-60% of the variance in psychotic experiences including hallucinations among affected and non-clinical individuals. Polygenic risk scores (PRS) derived from genome-wide association studies further support this, as elevated PRS for predict increased severity and frequency of auditory and visual hallucinations in both clinical and non-clinical populations. Childhood trauma represents a key environmental for developing hallucinations in adulthood, especially auditory forms. Experiences of physical, sexual, or emotional during childhood approximately double the of auditory hallucinations later in life, with odds ratios ranging from 1.2 to 2.5 across trauma categories. This association is thought to arise from altered stress response systems and neurodevelopmental disruptions, though the precise mechanisms remain under investigation. Substance use, particularly lifetime exposure to and other drugs, substantially elevates the risk of hallucinations. Individuals with heavy lifetime use (more than 50 occasions) face odds ratios of around 3.1 for psychotic experiences including hallucinations, while dependence on substances like can yield odds ratios as high as 3-5 times greater compared to non-users. These effects are dose-dependent and interact with genetic vulnerability, amplifying risk in those with high PRS. Sensory deprivation due to vision or hearing impairment also heightens hallucination susceptibility, often through deafferentation mechanisms where reduced input leads to spontaneous neural activity. increases the risk of visual hallucinations by approximately 20%, as seen in conditions like syndrome affecting 11-15% of those with significant vision loss. Similarly, hearing impairment raises the likelihood of auditory hallucinations by about 20%, with prevalence climbing to 16-24% in severe cases, correlating linearly with impairment degree. Comorbid conditions such as disorders and migraines serve as additional predictors of hallucinations. Chronic quadruples the odds of hallucinatory experiences compared to normal patterns, likely due to disrupted REM sleep and heightened perceptual sensitivity. Migraines, especially those with , are linked to transient hallucinations, with affected individuals showing elevated risk for visual and auditory perceptual distortions during attacks.

Other Contexts

Hallucinations in Artificial Intelligence

In , hallucinations refer to instances where models, particularly large language models (LLMs), generate plausible-sounding but false, fabricated, or unsubstantiated information presented as factual, often due to the probabilistic nature of their outputs. This phenomenon is particularly prevalent in large language models (LLMs) and generative systems, where the AI produces responses that appear plausible but lack grounding in or training data. Unlike perceptual distortions in , AI hallucinations are output errors arising from the model's inability to verify facts internally, leading to inventions such as non-existent references or events. The primary causes of AI hallucinations include limitations in training data, such as biases, inaccuracies, or incomplete coverage, which lead the model to overfit or extrapolate erroneously. In multimodal tasks like image description, models generate stereotypical outputs through pattern-matching and assumptions derived from training data, filling informational gaps with common tropes (e.g., contemplative figures in landscapes) rather than accurately representing unique or surreal elements in unexamined or ambiguously processed inputs. Additionally, the autoregressive generation process in LLMs—predicting sequentially based on statistical patterns without inherent —amplifies the issue, especially under ambiguous prompts or when handling out-of-distribution queries. For instance, when queried about impossible historical events, such as Albert Einstein meeting Isaac Newton, LLMs often generate plausible but false details, like specifying a particular year for the encounter, rather than admitting ignorance, because training and evaluation procedures incentivize confident responses over acknowledging uncertainty. Similar prompts that induce hallucinations include simple counting tasks, such as "How many 'r's are in 'strawberry'?", where models frequently report two instead of the correct three due to tokenization and pattern recognition limitations; queries about random past dates with no notable events, often resulting in fabricated news; predictions for future events beyond training data; and references to non-existent papers or events, where models invent plausible summaries or details. Model constraints, like limited context windows, further contribute by forcing reliance on compressed representations that may introduce distortions; even with expanded windows, LLMs hallucinate more in long contexts due to attention dilution (spread of focus across tokens reducing effectiveness), positional encoding drift (inaccurate position awareness in extended sequences), recency bias (over-reliance on recent information), and the "lost in the middle" effect (ignoring central details), which lead to overlooked middle information, error accumulation in autoregressive generation, and plausible but incorrect outputs in dense scenarios. In early models like GPT-3.5, benchmarks revealed hallucination rates as high as 39.6% in tasks involving factual recall or citation generation. Notable examples illustrate the impact: has been observed fabricating bibliographic citations, such as inventing scholarly articles or legal cases that do not exist, which can mislead users in research or professional contexts. In image generation, tools like or often produce anatomically impossible scenes, such as humans with extra limbs or objects defying physics, due to training on vast but noisy datasets that prioritize visual coherence over realism. These errors highlight the risks in applications like , healthcare—particularly in medical diagnosis, where erroneous outputs could lead to misdiagnoses or harmful decisions—or creative , where unverified outputs can propagate . Fine-tuned models aim to reduce these risks in such domains. As of 2025, hallucination rates in leading LLMs have declined by approximately 3% per year but remain significant in high-stakes domains like legal analysis, averaging 6.4–18.7%; however, in benchmarks like Vectara's hallucination evaluation, lighter or specialized models often achieve lower hallucination rates or outperform larger frontier models on factual grounding. To mitigate hallucinations, techniques such as retrieval-augmented generation (RAG) integrate external knowledge bases to ground responses in verifiable sources, reducing fabrication rates by up to 50% in controlled evaluations. Fine-tuning on domain-specific data and implementing confidence scoring—where models abstain from low-certainty outputs—further enhance reliability. By 2025, advancements in neurosymbolic AI, which combines neural pattern recognition with symbolic logic for verifiable reasoning, have shown promise in reducing hallucinations in targeted domains like legal analysis. However, complete elimination of hallucinations is impossible in general-purpose models due to their inherent probabilistic mechanisms. Detection and verification methods for AI hallucinations include real-time flagging or correction of issues. Internal approaches probe model activations, such as Cross-Layer Attention Probing for fine-grained detection, or employ prompt mutations, as in MetaQA, to identify inconsistencies without external data. External methods utilize fact-verification modules, chain-of-verification processes that break down claims into verifiable steps, and integration with tools for output checking. Human-in-the-loop oversight provides additional review for high-stakes applications. Unlike human hallucinations, which involve subjective perceptual experiences rooted in and often tied to of , AI versions lack any internal sensory or experiential component; they are purely computational confabulations driven by optimization for fluency over accuracy. This distinction underscores that AI hallucinations stem from design choices in probabilistic modeling, not cognitive phenomenology.

Cultural and Historical Perspectives

In ancient civilizations, hallucinations were frequently interpreted as divine communications rather than pathological phenomena. For instance, in 5th-century BCE , oracles at sites like are speculated to have employed hallucinogenic substances such as henbane or inhaled gas from geological fissures to induce visions believed to convey messages from gods like Apollo, influencing political and military decisions across the Hellenic world. This perspective framed altered sensory experiences as sacred intermediaries between humans and the divine, a view echoed in various prophetic traditions where visions were seen as infallible due to their inspirational origin. By the , Western shifted toward medicalizing hallucinations, classifying them as symptoms of . French psychiatrist Jean-Étienne-Dominique Esquirol played a pivotal role in this transition, providing the first modern definition of hallucinations as perceptions without external stimuli and integrating them into a semiology of mental illnesses, often linking them to or precursors. Esquirol's work, detailed in his 1838 Des Maladies Mentales, emphasized clinical observation and statistical analysis, marking hallucinations as indicators of "wandering mind" rather than events, which entrenched stigma in European medical discourse. Culturally, interpretations of hallucinations vary widely, with non-Western traditions often viewing them as spiritually significant. In Native American shamanic practices, such as peyote rituals within the , induced visions from the cactus Lophophora williamsii are regarded as pathways to healing, spiritual insight, and communal bonding, legally protected as religious sacraments since the 1978 . In contrast, Western medicine has historically stigmatized such experiences as deviant, while some African traditions, like those among the Xhosa in , interpret auditory hallucinations as ancestral communications or benevolent entities, fostering acceptance and integration rather than pathologization. This cultural shaping influences phenomenology, with voices in non-Western contexts often reported as more dialogic or supportive compared to the distressing, commanding tones prevalent in Western reports. Notable historical cases highlight these interpretive tensions. In the 15th century, Joan of Arc's visions of saints and divine missions, which guided her military campaigns during the , have been retrospectively debated as potential auditory hallucinations, possibly linked to or cultural religious fervor, though contemporary accounts framed them as holy inspirations leading to her in 1920. Similarly, in 1943, Swiss chemist Albert Hofmann's accidental ingestion of lysergic acid diethylamide (LSD-25) induced profound hallucinations, sparking 20th-century research into psychedelics as tools for exploring consciousness, though initially pursued for psychiatric applications before cultural backlash in the 1960s. The 21st century has seen efforts to destigmatize hallucinations through movements recognizing their non-pathological forms. The Hearing Voices Network, founded in 1987 and expanding globally, promotes peer support groups that reframe voice-hearing as a meaningful experience influenced by trauma or culture, challenging biomedical dominance and advocating for lived-experience perspectives in mental health policy. This shift aligns with broader philosophical debates on , as in ' 1641 Meditations on First Philosophy, where the "evil demon" hypothesis posits a deceptive entity capable of fabricating illusory perceptions indistinguishable from , underscoring about sensory reliability and paralleling modern discussions of hallucinatory .

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