Free to follow every thread. No paywall, no dead ends.
Hallucination: the story on HearLore | HearLore
Hallucination
In 1646, the physician Sir Thomas Browne introduced the word hallucination into the English language, deriving it from the Latin alucinari, which means to wander in the mind. Browne defined this phenomenon as a sort of vision that is depraved and receives its objects erroneously, establishing a framework that would persist for centuries. Today, we understand hallucinations as perceptions that possess the compelling sense of reality despite the absence of an external stimulus. This definition distinguishes them from dreams, which occur during sleep, and illusions, which involve distorted but real perceptions. The experience is not merely a visual glitch but a complex neurological event that can occur in any sensory modality, from the sight of a giraffe to the smell of burning rubber. The history of the term reveals a shift from a moral judgment of a depraved mind to a clinical understanding of brain function, yet the core mystery remains: how does the brain generate a world that feels entirely real when no such world exists?
Voices Behind The Back
Auditory hallucinations, also known as paracusia, are the most common form of hallucination, with an estimated lifetime prevalence of 9.6% in the general population. These are not merely abstract sounds but often take the form of voices that speak to the subject, comment on their actions, or curse them. In schizophrenia, these voices are typically perceived as coming from outside the person, often from behind their back, creating a feeling of being watched or targeted. Research indicates that 55% of auditory hallucinations are malicious in content, involving people talking about the subject rather than speaking directly to them. This phenomenon is so prevalent that it is a key diagnostic criterion for psychotic disorders, yet it is not exclusive to them. Individuals with lateral temporal lobe epilepsy, dissociative disorders, or even those suffering from severe sleep deprivation can hear voices. The distinction between schizophrenia and other conditions is often blurred, as the same auditory experience can arise from a wide array of neurological and psychological causes, including high caffeine consumption, where five cups of coffee a day can trigger the phenomenon.
The Geometry Of Sight
Visual hallucinations are classified into simple and complex forms, ranging from the geometric to the lifelike. Simple visual hallucinations, or phosphenes, are amorphous lights and colors without structure, while photopsias involve geometric shapes that may move across the visual field. These elementary hallucinations are often the first sign of a seizure in the occipital lobe, where brightly colored geometric patterns may multiply or form concentric rings. In contrast, complex visual hallucinations are clear, lifelike images of people, animals, or scenes, such as a giraffe standing in a room. These formed hallucinations are frequently associated with Charles Bonnet syndrome, a condition affecting partially or severely sight-impaired individuals who may initially fear they are losing their minds. The hallucinations in Charles Bonnet syndrome can be so vivid that the subject must use logic to distinguish them from reality, reasoning that fire without smoke or rats with pink ribbons cannot be real. Despite the distress, insight is often preserved, and the hallucinations may persist for months or years depending on the speed of eye deterioration.
Common questions
Who introduced the word hallucination into the English language and when?
Sir Thomas Browne introduced the word hallucination into the English language in 1646. He derived the term from the Latin word alucinari, which means to wander in the mind. Browne defined the phenomenon as a depraved vision that receives its objects erroneously.
What percentage of the general population experiences auditory hallucinations in their lifetime?
An estimated 9.6% of the general population experiences auditory hallucinations in their lifetime. These hallucinations often take the form of voices that speak to the subject or comment on their actions. Research indicates that 55% of these auditory hallucinations are malicious in content.
What is Charles Bonnet syndrome and who does it affect?
Charles Bonnet syndrome is a condition affecting partially or severely sight-impaired individuals who experience complex visual hallucinations. These formed hallucinations are clear and lifelike, often causing the subject to fear they are losing their minds. The hallucinations may persist for months or years depending on the speed of eye deterioration.
What causes tactile hallucinations known as formication?
Tactile hallucinations known as formication create the sensation of insects crawling underneath the skin and are frequently associated with prolonged cocaine use. This sensation also occurs during normal hormonal changes like menopause. The neurological basis for these sensations lies in the postcentral gyrus and the posterior frontal lobe.
What is the difference between hypnagogic and hypnopompic hallucinations?
Hypnagogic hallucinations occur as one is falling asleep while hypnopompic hallucinations happen when one is waking up. Surveys suggest that 37% of respondents experience these hallucinations twice a week while remaining aware of the true nature of the images. These events are sometimes associated with narcolepsy or brainstem abnormalities yet can occur in healthy individuals.
What percentage of hallucinations report religious content according to a 2015 review?
A 2015 review of 55 publications from 1962 to 2014 found that 16 to 28.6% of those experiencing hallucinations report at least some religious content. This suggests that hallucinations are not merely random noise but can be deeply tied to the subject's cultural and spiritual framework. In 90% of cases of psychosis, a visual hallucination occurs in combination with another sensory modality.
Tactile hallucinations, known as formication, create the sensation of insects crawling underneath the skin, a symptom frequently associated with prolonged cocaine use but also occurring during normal hormonal changes like menopause. This sensation is not limited to the skin; somatic hallucinations can involve the visceral organs, causing feelings of pressure, burning, or the invasion of animals like snakes in the stomach or frogs in the rectum. These experiences are categorized as cenesthopathic hallucinations, a pathological alteration in the sense of bodily existence that is unique to the sufferer and often hard to describe. In schizophrenia, this subtype is recognized as a distinct marker, where the patient feels their flesh is decomposing or their body is being mutilated. The neurological basis for these sensations lies in the postcentral gyrus and the posterior frontal lobe, areas responsible for arousal and ejaculation, suggesting that the brain's map of the body can be rewritten to produce sensations of pain, movement, or invasion without any external cause.
The Sleep And The Waking
Hypnagogic hallucinations occur as one is falling asleep, while hypnopompic hallucinations happen when one is waking up, and both are considered normal phenomena affecting a high proportion of the population. Surveys suggest that 37% of respondents experience these hallucinations twice a week, lasting from seconds to minutes while the subject remains aware of the true nature of the images. These events are sometimes associated with narcolepsy or brainstem abnormalities, yet they can occur in healthy individuals without any transient trigger factor. The evidence for benign hallucinatory experiences goes back to 1886, with the Society for Psychical Research suggesting that approximately 10% of the population has experienced at least one hallucinatory episode in the course of their life. This normality contrasts sharply with the pathological hallucinations of delirium tremens, which occur during alcohol withdrawal and are associated with agitation, confusion, and a gradual reduction in insight. The transition from the benign hypnagogic state to the terrifying delirium tremens highlights the fragile boundary between normal brain function and neurological crisis.
The Chemical And The Clinical
Drug-induced hallucinations are caused by hallucinogens, dissociatives, and deliriants, including many drugs with anticholinergic actions and certain stimulants. Some psychedelics such as lysergic acid diethylamide and psilocybin can cause hallucinations that range in the spectrum of mild to intense, while opioids like fentanyl and pethidine are more associated with this side effect than natural opioids like morphine. The mechanism of these hallucinations involves the activation of specific receptors, such as the kappa opioid receptor, sigma receptors, and the NMDA receptors. In the 1930s, a combination drug called Skophedal, invented in Germany, was hailed as the wonder drug of its time, yet its psychotomimetic side effects were a limiting factor in its use. The pathophysiology of hallucinations is further complicated by the role of dopamine and serotonin, where dopaminergic hallucinations are often associated with a lack of awareness that one is hallucinating, unlike serotonergic hallucinations where the person maintains an awareness of the unreality of the experience.
The Brain And The Mind
Hallucinations are associated with structural and functional abnormalities in primary and secondary sensory cortices, with reduced grey matter in regions of the superior temporal gyrus and middle temporal gyrus linked to auditory hallucinations. One proposed model 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. This dysfunction in thalamocortical circuitry may underlie the observed top-down and bottom-up dysfunction, where expectations distort sensory input. The brain's ability to gate sensory stimuli is compromised, leading to the generation of hallucinations that feel more real than reality itself. In diseases such as Alzheimer's, grey and white matter abnormalities in visual regions support the notion of dysfunction in sensory regions underlying hallucinations, while in schizophrenia, the failure to reduce P50 amplitude in response to a second stimulus represents a failure to gate sensory stimuli.
The Voices Of Faith
A 2015 review of 55 publications from 1962 to 2014 found that 16 to 28.6% of those experiencing hallucinations report at least some religious content in them, along with 20 to 60% reporting some religious content in delusions. This suggests that hallucinations are not merely random noise but can be deeply tied to the subject's cultural and spiritual framework. In 90% of cases of psychosis, a visual hallucination occurs in combination with another sensory modality, most often being auditory or somatic, creating a multimodal experience that is correlated to poorer mental health outcomes. The prevalence of hallucinations varies depending on underlying medical conditions, age, and culture, with children and adolescents experiencing similar rates to adults, though these are not necessarily indicative of later psychopathology. The Hearing Voices Movement, starting in Europe, aims to utilize the knowledge and experience of voice hearers combined with experts in disorders such as schizophrenia, advocating for individuals who hear voices without necessarily having a diagnosable mental illness.