What was the Rosenham Experiment?


The Rosenhan experiment or Thud experiment was an experiment conducted to determine the validity of psychiatric diagnosis. The participants feigned hallucinations to enter psychiatric hospitals but acted normally afterwards. They were diagnosed with psychiatric disorders and were given antipsychotic medication. The study was conducted by psychologist David Rosenhan, a Stanford University professor, and published by the journal Science in 1973 under the title “On Being Sane in Insane Places”. It is considered an important and influential criticism of psychiatric diagnosis, and broached the topic of wrongful involuntary commitment.

Rosenhan’s study was done in two parts. The first part involved the use of healthy associates or “pseudopatients” (three women and five men, including Rosenhan himself) who briefly feigned auditory hallucinations in an attempt to gain admission to 12 psychiatric hospitals in five states in the United States. All were admitted and diagnosed with psychiatric disorders. After admission, the pseudopatients acted normally and told staff that they no longer experienced any additional hallucinations. As a condition of their release, all the patients were forced to admit to having a mental illness and had to agree to take antipsychotic medication. The average time that the patients spent in the hospital was 19 days. All but one were diagnosed with schizophrenia “in remission” before their release.

The second part of his study involved a hospital administration challenging Rosenhan to send pseudopatients to its facility, whose staff asserted that they would be able to detect the pseudopatients. Rosenhan agreed, and in the following weeks 41 out of 193 new patients were identified as potential pseudopatients, with 19 of these receiving suspicion from at least one psychiatrist and one other staff member. Rosenhan sent no pseudopatients to the hospital.

While listening to a lecture by R.D. Laing, who was associated with the anti-psychiatry movement, Rosenhan conceived of the experiment as a way to test the reliability of psychiatric diagnoses. The study concluded “it is clear that we cannot distinguish the sane from the insane in psychiatric hospitals” and also illustrated the dangers of dehumanisation and labelling in psychiatric institutions. It suggested that the use of community mental health facilities which concentrated on specific problems and behaviours rather than psychiatric labels might be a solution, and recommended education to make psychiatric workers more aware of the social psychology of their facilities.

Pseudopatient Experiment

Rosenhan himself and seven mentally healthy associates, called “pseudopatients”, attempted to gain admission to psychiatric hospitals by calling for an appointment and feigning auditory hallucinations. The hospital staff were not informed of the experiment. The pseudopatients included a psychology graduate student in his twenties, three psychologists, a pediatrician, a psychiatrist, a painter, and a housewife. None had a history of mental illness. Pseudopatients used pseudonyms, and those who worked in the mental health field were given false jobs in a different sector to avoid invoking any special treatment or scrutiny. Apart from giving false names and employment details, further biographical details were truthfully reported.

During their initial psychiatric assessment, the pseudopatients claimed to be hearing voices of the same sex as the patient which were often unclear, but which seemed to pronounce the words “empty”, “hollow”, or “thud”, and nothing else. These words were chosen as they vaguely suggest some sort of existential crisis and for the lack of any published literature referencing them as psychotic symptoms. No other psychiatric symptoms were claimed. If admitted, the pseudopatients were instructed to “act normally”, reporting that they felt fine and no longer heard voices. Hospital records obtained after the experiment indicate that all pseudopatients were characterized as friendly and cooperative by staff.

All were admitted, to 12 psychiatric hospitals across the United States, including rundown and underfunded public hospitals in rural areas, urban university-run hospitals with excellent reputations, and one expensive private hospital. Though presented with identical symptoms, seven were diagnosed with schizophrenia at public hospitals, and one with manic-depressive psychosis, a more optimistic diagnosis with better clinical outcomes, at the private hospital. Their stays ranged from 7 to 52 days, and the average was 19 days. All but one were discharged with a diagnosis of schizophrenia “in remission”, which Rosenhan considered as evidence that mental illness is perceived as an irreversible condition creating a lifelong stigma rather than a curable illness.

Despite constantly and openly taking extensive notes on the behaviour of the staff and other patients, none of the pseudopatients were identified as impostors by the hospital staff, although many of the other psychiatric patients seemed to be able to correctly identify them as impostors. In the first three hospitalisations, 35 of the total of 118 patients expressed a suspicion that the pseudopatients were sane, with some suggesting that the patients were researchers or journalists investigating the hospital. Hospital notes indicated that staff interpreted much of the pseudopatients’ behaviour in terms of mental illness. For example, one nurse labelled the note-taking of one pseudopatient as “writing behaviour” and considered it pathological. The patients’ normal biographies were recast in hospital records along the lines of what was expected of schizophrenics by the then-dominant theories of its cause.

The experiment required the pseudopatients to get out of the hospital on their own by getting the hospital to release them, though a lawyer was retained to be on call for emergencies when it became clear that the pseudopatients would not ever be voluntarily released on short notice. Once admitted and diagnosed, the pseudopatients were not able to obtain their release until they agreed with the psychiatrists that they were mentally ill and began taking antipsychotic medications, which they flushed down the toilet. No staff member reported that the pseudopatients were flushing their medication down the toilets.

Rosenhan and the other pseudopatients reported an overwhelming sense of dehumanisation, severe invasion of privacy, and boredom while hospitalised. Their possessions were searched randomly, and they were sometimes observed while using the toilet. They reported that though the staff seemed to be well-meaning, they generally objectified and dehumanised the patients, often discussing patients at length in their presence as though they were not there, and avoiding direct interaction with patients except as strictly necessary to perform official duties. Some attendants were prone to verbal and physical abuse of patients when other staff were not present. A group of patients waiting outside the cafeteria half an hour before lunchtime were said by a doctor to his students to be experiencing “oral-acquisitive” psychiatric symptoms. Contact with doctors averaged 6.8 minutes per day.

Non-Existent Impostor Experiment

For this experiment, Rosenhan used a well-known research and teaching hospital, whose staff had heard of the results of the initial study but claimed that similar errors could not be made at their institution. Rosenhan arranged with them that during a three-month period, one or more pseudopatients would attempt to gain admission and the staff would rate every incoming patient as to the likelihood they were an impostor. Out of 193 patients, 41 were considered to be impostors and a further 42 were considered suspect. In reality, Rosenhan had sent no pseudopatients; all patients suspected as impostors by the hospital staff were ordinary patients. This led to a conclusion that “any diagnostic process that lends itself too readily to massive errors of this sort cannot be a very reliable one.”


Rosenhan published his findings in Science, in which he criticised the reliability of psychiatric diagnosis and the disempowering and demeaning nature of patient care experienced by the associates in the study. In addition, he described his work in a variety of news appearances, including to the BBC:

I told friends, I told my family: “I can get out when I can get out. That’s all. I’ll be there for a couple of days and I’ll get out.” Nobody knew I’d be there for two months … The only way out was to point out that they’re [the psychiatrists are] correct. They had said I was insane, “I am insane; but I am getting better.” That was an affirmation of their view of me.

The experiment is argued to have “accelerated the movement to reform mental institutions and to deinstitutionalize as many mental patients as possible”.

Many respondents to the publication defended psychiatry, arguing that as psychiatric diagnosis relies largely on the patient’s report of their experiences, faking their presence no more demonstrates problems with psychiatric diagnosis than lying about other medical symptoms. In this vein, psychiatrist Robert Spitzer quoted Seymour S. Kety in a 1975 criticism of Rosenhan’s study:

If I were to drink a quart of blood and, concealing what I had done, come to the emergency room of any hospital vomiting blood, the behavior of the staff would be quite predictable. If they labeled and treated me as having a bleeding peptic ulcer, I doubt that I could argue convincingly that medical science does not know how to diagnose that condition.

Kety also argued that psychiatrists should not necessarily be expected to assume that a patient is pretending to have mental illness, thus the study lacked realism. Rosenhan called this the “experimenter effect” or “expectation bias”, something indicative of the problems he uncovered rather than a problem in his methodology.

In The Great Pretender, a 2019 book on Rosenhan, author Susannah Cahalan questions the veracity and validity of the Rosenhan experiment. Examining documents left behind by Rosenhan after his death, Cahalan finds apparent distortion in the Science article: inconsistent data, misleading descriptions, and inaccurate or fabricated quotations from psychiatric records. Moreover, despite an extensive search, she is only able to identify two of the eight pseudopatients: Rosenhan himself, and a graduate student whose testimony is allegedly inconsistent with Rosenhan’s description in the article. In light of Rosenhan’s seeming willingness to bend the truth in other ways regarding the experiment, Cahalan questions whether some or all of the six other pseudopatients might have been simply invented by Rosenhan.

Related Experiments

In 1887 American investigative journalist Nellie Bly feigned symptoms of mental illness to gain admission to a lunatic asylum and report on the terrible conditions therein. The results were published as Ten Days in a Mad-House.]

In 1968 Maurice K. Temerlin split 25 psychiatrists into two groups and had them listen to an actor portraying a character of normal mental health. One group was told that the actor “was a very interesting man because he looked neurotic, but actually was quite psychotic” while the other was told nothing. Sixty percent of the former group diagnosed psychoses, most often schizophrenia, while none of the control group did so.

In 1988, Loring and Powell gave 290 psychiatrists a transcript of a patient interview and told half of them that the patient was black and the other half white; they concluded of the results that “clinicians appear to ascribe violence, suspiciousness, and dangerousness to black clients even though the case studies are the same as the case studies for the white clients.”

In 2004, psychologist Lauren Slater claimed to have conducted an experiment very similar to Rosenhan’s for her book Opening Skinner’s Box. Slater wrote that she had presented herself at 9 psychiatric emergency rooms with auditory hallucinations, resulting in being diagnosed “almost every time” with psychotic depression. However, when challenged to provide evidence of actually conducting her experiment, she could not. The serious methodologic and other concerns regarding Slater’s work appeared as a series of responses to a journal report, in the same journal.

In 2008, the BBC’s Horizon science programme performed a similar experiment over two episodes entitled “How Mad Are You?”. The experiment involved ten subjects, five with previously diagnosed mental health conditions, and five with no such diagnosis. They were observed by three experts in mental health diagnoses and their challenge was to identify the five with mental health problems solely from their behaviour, without speaking to the subjects or learning anything of their histories. The experts correctly diagnosed two of the ten patients, misdiagnosed one patient, and incorrectly identified two healthy patients as having mental health problems. Unlike the other experiments listed here, however, the aim of this journalistic exercise was not to criticise the diagnostic process, but to minimise the stigmatisation of the mentally ill. It aimed to illustrate that people with a previous diagnosis of a mental illness could live normal lives with their health problems not obvious to observers from their behaviour.

What is Hallucination?


A hallucination is a perception in the absence of external stimulus that has qualities of real perceptions.

Hallucinations are vivid, substantial, and are perceived to be located in external objective space. They are distinguishable from several related phenomena, such as dreaming, 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 imagery (imagination), which does not mimic real perception, and is under voluntary control. Hallucinations also differ from “delusional perceptions”, in which a correctly sensed and interpreted stimulus (i.e. a real perception) is given some additional (and typically absurd) significance.

Hallucinations can occur in any sensory modality – visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.

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, etc. 55% of auditory hallucinations are malicious in content, for example, people talking about the subject behind their back, etc. Like auditory hallucinations, the source of the visual counterpart can also be behind the subject’s back. This can produce a feeling of being looked or stared at, usually with malicious intent. Frequently, auditory hallucinations and their visual counterpart are experienced by the subject together.

Hypnagogic 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, neurological disorders, and delirium tremens.

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


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


A visual hallucination is “the perception of an external visual stimulus where none exists”. A separate but related phenomenon is a visual illusion, which is a distortion of a real external stimulus. Visual hallucinations are classified as simple or complex:

  • Simple visual hallucinations (SVH) are also referred to as non-formed visual hallucinations and elementary visual hallucinations.
    • These terms refer to lights, colours, 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 colour to a giraffe (looks like a giraffe), while a complex visual hallucination is a discrete, lifelike image that is, unmistakably, a giraffe.


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

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

Another typical disorder where auditory hallucinations are very common is dissociative identity disorder. 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. However, many people not suffering from diagnosable mental illness may sometimes hear voices as well. One important example to consider when forming a differential diagnosis for a patient with paracusia is lateral temporal lobe epilepsy. 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, he/she does not necessarily suffer from 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.

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, arteriovenous malformation, stroke, lesion, abscess, or tumour.

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.

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


Command hallucinations are hallucinations in the form of commands; they can be auditory or inside of the person’s mind or consciousness. The contents of the hallucinations can range from the innocuous to commands to cause harm to the self or others. Command hallucinations are often associated with schizophrenia. People experiencing command hallucinations may or may not comply with the hallucinated commands, depending on the circumstances. Compliance is more common for non-violent commands.

Command hallucinations are sometimes used to defend a crime that has been committed, often homicides. In essence, it is a voice that one hears and it tells the listener what to do. Sometimes the commands are quite benign directives such as “Stand up” or “Shut the door.” Whether it is a command for something simple or something that is a threat, it is still considered a “command hallucination.” Some helpful questions that can assist one in figuring out if they may be suffering from 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?”


Phantosmia (olfactory hallucinations), smelling an odour that is not actually there, and parosmia (olfactory illusions), inhaling a real odour but perceiving it as different scent than remembered, 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. It can result from a range of conditions such as nasal infections, nasal polyps, dental problems, migraines, head injuries, seizures, strokes, or brain tumours. 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. It can also be a symptom of certain mental disorders such as depression, bipolar disorder, intoxication or withdrawal from drugs and alcohol, or psychotic disorders (e.g. schizophrenia). The perceived odours are usually unpleasant and commonly described as smelling burned, foul spoiled, or rotten.


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. 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.


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.

General Somatic Sensations

General somatic sensations of a hallucinatory nature are experienced when an individual feels that their body is being mutilated, i.e. twisted, torn, or disembowelled. 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 hallucination.


Hallucinations can be caused by a number of factors.

Hypnagogic Hallucination

These hallucinations occur just before falling asleep and affect a high proportion of the population: in one survey 37% of the respondents experienced them twice a week. The hallucinations can last from seconds to minutes; all the while, the subject usually remains aware of the true nature of the images. These may be associated with narcolepsy. Hypnagogic hallucinations are sometimes associated with brainstem abnormalities, but this is rare.

Peduncular Hallucinosis

Peduncular means pertaining to the peduncle, which is a neural tract running to and from the pons 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.

Delirium Tremens

One of the more enigmatic forms of visual hallucination is the highly variable, possibly polymodal delirium tremens. Individuals suffering from delirium tremens may be agitated and confused, especially in the later stages of this disease. Insight is gradually reduced with the progression of this disorder. Sleep is disturbed and occurs for a shorter period of time, with rapid eye movement sleep.

Parkinson’s Disease and Lewy Body Dementia

Parkinson’s disease is linked with Lewy body dementia for their similar hallucinatory symptoms. The symptoms strike during the evening in any part of the visual field, and are rarely polymodal. The segue into hallucination may begin with illusions where sensory perception is greatly distorted, but no novel sensory information is present. These typically last for several minutes, during which time the subject may be either conscious and normal or drowsy/inaccessible. Insight into these hallucinations is usually preserved and REM sleep is usually reduced. Parkinson’s disease is usually associated with a degraded substantia nigra pars compacta, but recent evidence suggests that PD affects a number of sites in the brain. Some places of noted degradation include the median raphe nuclei, the noradrenergic parts of the locus coeruleus, and the cholinergic neurons in the parabrachial area and pedunculopontine nuclei of the tegmentum.

Migraine Coma

This type of hallucination is usually experienced during the recovery from a comatose state. The migraine coma can last for up to two days, and a state of depression is sometimes comorbid. The hallucinations occur during states of full consciousness, and insight into the hallucinatory nature of the images is preserved. It has been noted that ataxic lesions accompany the migraine coma.

Charles Bonnet Syndrome

Charles Bonnet syndrome 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 manifestation of the hallucinations may change, becoming more or less frequent with changes in ability to see. The length of time that the sight impaired person can suffer from these hallucinations varies according to the underlying speed of eye deterioration. A differential diagnosis are ophthalmopathic hallucinations.

Focal Epilepsy

Visual hallucinations due to focal seizures differ depending on the region of the brain where the seizure occurs. For example, visual hallucinations during occipital lobe seizures are typically visions of brightly coloured, 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 localised to one part of the visual field on the contralateral side of the seizure focus, typically the temporal field. However, unilateral visions moving horizontally across the visual field begin on the contralateral side and move toward the ipsilateral side.

Temporal lobe seizures, on the other hand, can produce complex visual hallucinations of people, scenes, animals, and more as well as distortions of visual perception. 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.

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. Even when consciousness is impaired, insight into the hallucination or illusion is typically preserved.

Drug-Induced Hallucination

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.

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. 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 psychotomometic 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.).

Sensory Deprivation Hallucination

Hallucinations can be caused by sensory deprivation when it occurs for prolonged periods of time, and almost always occurs in the modality being deprived (visual for blindfolded/darkness, auditory for muffled conditions, etc).

Experimentally-Induced Hallucinations

Anomalous experiences, such as so-called benign hallucinations, may occur in a person in a state of good mental and physical health, even in the apparent absence of a transient trigger factor such as fatigue, intoxication or sensory deprivation.

The evidence for this statement has been accumulating for more than a century. Studies of benign hallucinatory experiences go back to 1886 and the early work of the Society for Psychical Research, which suggested approximately 10% of the population had experienced at least one hallucinatory episode in the course of their life. More recent studies have validated these findings; the precise incidence found varies with the nature of the episode and the criteria of “hallucination” adopted, but the basic finding is now well-supported.

Non-Celiac Gluten Sensitivity

There is tentative evidence of a relationship with non-celiac gluten sensitivity, the so-called “gluten psychosis”.


Dopaminergic and Serotoninergic Hallucinations

It has been reported that in serotoninergic hallucinations, the person maintains an awareness that they is hallucinating, unlike dopaminergic hallucinations.


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. Grey and white matter abnormalities in visual regions are associated with visual hallucinations in diseases such as Alzheimer’s disease, further supporting the notion of dysfunction in sensory regions underlying hallucinations.

One proposed model of hallucinations posits that over-activity in sensory regions, which is normally attributed to internal sources via feedforward networks to the inferior frontal gyrus, is interpreted as originating externally due to abnormal connectivity or functionality of the feedforward network. This is supported by cognitive studies those with hallucinations, who have demonstrated abnormal attribution of self generated stimuli.

Disruptions in thalamocortical circuitry may underlie the observed top down and bottom up dysfunction. Thalamocortical circuits, composed of projections between thalamic and cortical neurons and adjacent interneurons, underlie certain electrophysical characteristics (gamma oscillations) that are underlie sensory processing. Cortical inputs to thalamic neurons enable attentional modulation of sensory neurons. Dysfunction in sensory afferents, and abnormal cortical input may result in pre-existing expectations modulating sensory experience, potentially resulting in the generation of hallucinations. Hallucinations are associated with less accurate sensory processing, and more intense stimuli with less interference are necessary for accurate processing and the appearance of gamma oscillations (called “gamma synchrony”). Hallucinations are also associated with the absence of reduction in P50 amplitude in response to the presentation of a second stimuli after an initial stimulus; this is thought to represent failure to gate sensory stimuli, and can be exacerbated by dopamine release agents.

Abnormal assignment of salience to stimuli may be one mechanism of hallucinations. Dysfunctional dopamine signalling may lead to abnormal top down regulation of sensory processing, allowing expectations to distort sensory input.


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 utilised to treat the illness if the symptoms are severe and cause significant distress. For other causes of hallucinations there is no factual evidence to support any one treatment is scientifically tested and proven. However, abstaining from 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.


Several recent studies on the prevalence of hallucinations in the general population have appeared. A 2020 US study indicated a lifetime prevalence of 10-15% for vivid sensory hallucinations. Compared with the English Sidgewick Study of 1894, relative frequencies of sensory modalities differed in the US with fewer visual hallucinations.