A hallucination is a sensory experience that feels entirely real but occurs without any corresponding external stimulus. The DSM-5 describes it as “a perception-like experience with the clarity and impact of a true perception but without the external stimulation of the relevant sensory organ.” That compact definition does a lot of work: it captures the fact that a hallucination is not a vague impression or a stray thought but something the person genuinely sees, hears, smells, tastes, or feels, even though nothing in the outside world is producing it. The concept is richer and more contested than most people assume, touching everything from grief to extreme altitude to the outputs of artificial intelligence.
What Makes a Hallucination Different from an Illusion or Delusion
People frequently use “hallucination,” “illusion,” and “delusion” interchangeably, but they describe fundamentally different mental events. An illusion is a distortion of something real: you see a coat hanging on a door and briefly mistake it for a person. The coat is actually there; your brain just processes it incorrectly. A hallucination, by contrast, has no external source at all. You hear a voice in an empty room, or you see a figure where nothing exists. The perceptual experience is generated entirely from within.
A delusion is different from both. It is not a sensory event but a fixed, false belief. Believing that a government agency is monitoring your phone calls is a delusion, not a hallucination. A person can have both simultaneously, of course, but the categories refer to separate processes: one is about perception, the other about conviction.
There is also a less well-known distinction between hallucinations and pseudohallucinations. In clinical terminology, a true hallucination is experienced as coming from the external world and can be located in three-dimensional space. A pseudohallucination, on the other hand, is experienced as coming from inside the person’s own mind. Someone might hear a voice but recognize that it is “in their head” rather than originating from outside. The key distinguishing features of pseudohallucinations are a lack of the sense of objective external reality and the preservation of insight, meaning the person typically knows the experience is not real.1PubMed Central. Hallucinations and related concepts-their conceptual background This distinction matters clinically because pseudohallucinations often suggest a different diagnostic picture than true hallucinations.2PubMed. Hallucinations, psuedohallucinations, and parahallucinations
The Senses Involved
When most people think of hallucinations, they think of hearing voices or seeing things that are not there. Those are the two most common forms, but hallucinations can involve any sense. A study of people with Parkinson’s disease who experienced psychotic symptoms found that visual hallucinations were the most common (about a quarter of participants), followed by olfactory hallucinations involving phantom smells, then tactile hallucinations (feeling things on the skin), auditory hallucinations, and gustatory hallucinations involving phantom tastes.3PubMed. Comprehensive identification of delusions and olfactory, tactile, gustatory, and minor hallucinations in Parkinson’s disease psychosis The ranking differs by condition. In schizophrenia, auditory hallucinations dominate, with voice-hearing being the most recognized feature of the illness. In Parkinson’s and Lewy body dementia, visual hallucinations are far more typical.
Tactile, olfactory, and gustatory hallucinations (sometimes grouped as “TOGH” experiences) are less discussed but not rare. In one study across schizophrenia, schizoaffective disorder, and bipolar disorder, about one in five people reported hallucinations involving touch, smell, or taste, and these tended to cluster together. If someone experienced one of these less common types, they were more likely to experience the others as well.4Annals, Academy of Medicine, Singapore. Hallucinations in Schizophrenia, Schizoaffective Disorder and Bipolar Disorder
Hallucinations Without Mental Illness
One of the most common misconceptions about hallucinations is that they always signal psychiatric disease. They do not. Healthy people experience them under a surprising range of circumstances.
The transition between waking and sleep is one of the most familiar settings. Hypnagogic hallucinations happen as you fall asleep, and hypnopompic hallucinations happen as you wake up. These can involve vivid images, sounds, or the sensation of a presence in the room. They are common enough to be considered a normal feature of sleep, particularly during periods of poor rest or unusual sleep schedules.
Bereavement is another. Between roughly 30% and 60% of widowed people report sensing the presence of their deceased partner in some way: hearing their voice, briefly seeing them, or feeling their touch.5PubMed. Post-bereavement hallucinatory experiences: A critical overview of population and clinical studies These experiences tend to differ from psychotic hallucinations in an important way. The voices heard during grief are almost never anonymous. They are clearly identified as belonging to the deceased person and are often linked to specific biographical memories, like hearing a grandmother’s voice saying something she used to say at bedtime.6Schizophrenia Bulletin. Sensory and Quasi-Sensory Experiences of the Deceased in Bereavement: An Interdisciplinary and Integrative Review Many people find these experiences comforting rather than distressing, and they generally do not indicate a need for psychiatric treatment.
Physical extremes can also trigger hallucinations in otherwise healthy people. At very high altitude, isolated psychotic episodes are a recognized medical phenomenon. A systematic investigation of altitude-related psychosis found hallucinations in about 42% of documented episodes, with perceptual disturbances of some kind in close to 60%. Most people who experienced them described the hallucinations as neutral or even comforting. Risk factors included starvation, frostbite, and the use of supplemental oxygen, suggesting that metabolic stress and hypoxia play a role.7Psychological Medicine. Isolated psychosis during exposure to very high and extreme altitude – characterisation of a new medical entity
Voice-hearing in the general population is more widespread than the clinical literature might suggest. A review of population studies found prevalence rates that varied enormously depending on how the question was asked and who was surveyed, but the median was around 13%.8PubMed. The prevalence of voice-hearers in the general population: a literature review Most of these voice-hearers had no psychiatric diagnosis. The findings support what researchers call the “continuum model,” in which hallucination-like experiences shade from fleeting, everyday events (like hearing your name called in a crowd when nobody said it) into the persistent, distressing forms that characterize illness.
What the Brain Is Doing During a Hallucination
Brain imaging has shown that hallucinations are not vague disruptions. They activate the same sensory regions that would fire if the person were actually perceiving something from the outside world. During auditory hallucinations in people with schizophrenia, functional MRI studies have found activation in areas including the superior and middle temporal gyri (regions involved in processing speech and language), the inferior frontal gyrus, the anterior cingulate, the thalamus, and even structures as deep as the inferior colliculus, a midbrain hub for auditory processing.9JAMA Psychiatry. Mapping Auditory Hallucinations in Schizophrenia Using Functional Magnetic Resonance Imaging In other words, the brain’s hearing system fires as though real sound is arriving, even when none is.
Magnetoencephalography studies have confirmed that auditory cortex activity occurs during hallucinations, including activation in Heschl’s gyrus, a structure at the core of early sound processing. The timing and shape of this brain activity varied considerably from person to person, but the average pattern was clearly tied to the hallucinatory events themselves.10NeuroImage. The spatiotemporal pattern of auditory cortical responses during verbal hallucinations
Visual hallucinations in Lewy body disease involve their own distinct network disruptions. Research has linked the severity of complex visual hallucinations to altered connectivity between early visual areas and higher-order visual and attentional networks. The emerging picture is that different features of a hallucination, how long it lasts versus how often it happens, may arise from different types of connectivity changes within these networks.11PubMed Central. Visual hallucinations in Lewy body disease: pathophysiological insights from phenomenology In Parkinson’s disease with dementia, pathology in a region called the nucleus basalis of Meynert, a major source of the brain’s cholinergic signaling, has been linked to both visual hallucinations and delusions.12PubMed. Delusions and visual hallucinations in a patient with Parkinson’s disease with dementia showing pronounced Lewy body pathology in the nucleus basalis of Meynert
Chemical Triggers and Pharmacological Pathways
Hallucinations can be produced through at least three distinct chemical routes in the brain. Stimulants like amphetamines trigger hallucinations primarily by ramping up activity at dopamine receptors. Classic psychedelics like LSD and psilocybin produce their effects by activating serotonin receptors. And dissociative drugs like ketamine and PCP cause hallucinations by blocking glutamate receptors.13PubMed Central. Pharmacology of hallucinations: several mechanisms for one single symptom? The fact that these three routes are pharmacologically independent of one another, involving entirely different neurotransmitter systems, is one reason hallucinations have proven so difficult to treat with a single drug. The hallucinations someone experiences from methamphetamine are mechanistically different from the visual distortions produced by psilocybin, even though both qualify as hallucinations under the same clinical definition.
This pharmacological diversity also helps explain why antipsychotic medications, which primarily work by blocking dopamine receptors, are effective against the hallucinations of schizophrenia but often less effective against visual hallucinations in Lewy body disease, where cholinergic deficiency plays a bigger role. The underlying chemistry shapes which drug approach works.
A Prediction Machine That Gets It Wrong
One influential framework for understanding hallucinations is the predictive processing model. The idea is that your brain does not passively receive sensory input and then react to it. Instead, the brain is constantly generating predictions about what it expects to see, hear, and feel, and then checking those predictions against what actually arrives through the senses. When the predictions are confident enough, they can override or fill in gaps in the incoming data. Normally this works well: it is why you can recognize a friend’s face in a dark room or finish someone’s sentence before they say the last word. But when the balance tips too far toward prediction and away from actual sensory input, the brain can generate experiences with no external source. That, in this framework, is a hallucination.14Neuroscience & Biobehavioral Reviews. Predictive coding in neuropsychiatric disorders: A systematic transdiagnostic review
This view has gained traction in part because it elegantly accounts for the continuum of hallucination-like experiences. In ambiguous or deprived sensory environments, even healthy brains lean more heavily on prediction, which is why sensory deprivation, darkness, extreme fatigue, and grief (all states with heightened uncertainty) are all associated with hallucinatory experiences. In clinical disorders, the balance may be chronically disrupted, leading to persistent hallucinations even in clear sensory conditions.
How Culture Shapes What People Hallucinate
The content and emotional tone of hallucinations are not universal. A cross-cultural study comparing voice-hearing experiences in the United States, South India, and West Africa found striking differences in how people related to their voices. Americans with schizophrenia were more likely to describe their voices as intrusive, unreal thoughts and to find them distressing. South Indian participants were more likely to describe the voices as providing useful guidance, often attributing them to relatives. West African participants were more likely to characterize their voices as morally good and causally powerful.15PubMed. Hearing Voices in Different Cultures: A Social Kindling Hypothesis
These differences suggest that cultural expectations and social learning shape how hallucinations are interpreted, experienced, and reported. In societies where spirit communication or ancestor contact is a recognized experience, voice-hearing may be incorporated into a framework that makes it less frightening and potentially meaningful. In societies that pathologize any departure from consensus reality, the same experience may become a source of shame and distress. The underlying perceptual event may be broadly similar across populations, but the subjective experience built around it varies enormously.
Where the Word Came From
The word “hallucination” traces to the Latin “alucinari,” meaning to wander in the mind. Jean-Étienne Esquirol, a French psychiatrist, is credited with bringing the term into clinical use in the early 1800s after observing hallucinations in his patients and apparently experiencing them himself.16PubMed Central. The evolution of the perspectives on hallucinations from the 1600s until present day Before Esquirol’s time, in the 17th and 18th centuries, hallucinatory experiences were often interpreted as spiritually meaningful rather than pathological. The medicalization of hallucinations as a symptom of illness is relatively recent in the long arc of human experience, and it coexists today with religious and cultural frameworks in which certain forms of hallucination are welcomed rather than treated.
When AI “Hallucinates”
The term has taken on a second, rapidly growing meaning in the world of artificial intelligence. When a large language model produces a response that sounds confident and coherent but is factually wrong or entirely fabricated, researchers and engineers call it a hallucination. The analogy to human hallucinations is loose but deliberate: in both cases, a system generates output that does not correspond to external reality but has the internal feel of something real and reliable.
AI researchers distinguish between intrinsic hallucinations, where the model contradicts something it was directly given in the input, and extrinsic hallucinations, where the model produces claims that are inconsistent with its training data or with verifiable facts about the world.17arXiv. A comprehensive taxonomy of hallucinations in Large Language Models Some theoretical work has argued that hallucination is not merely a bug to be fixed but a structural feature of systems that predict the next piece of output based on patterns rather than grounded understanding, suggesting it may be unavoidable in any sufficiently flexible predictive system.
The comparison between human and machine hallucinations has been explored more formally. Both systems rely on predictive architectures: the human brain sends top-down predictions about what is happening in the world and then updates based on mismatches with sensory input, while a language model predicts the next token in a sequence based on learned statistical patterns. Both can go wrong when prediction outstrips grounding. A theoretical review has argued that the tendency to generate incorrect or confabulated responses may be an inherent feature of any advanced system that processes incomplete information, whether biological or computational. The capacity for flexible prediction that enables creativity and rapid adaptation is the same capacity that introduces the risk of hallucination.18arXiv. I Think, Therefore I Hallucinate: Minds, Machines, and the Art of Being Wrong
The borrowed terminology has drawn some criticism from clinicians and philosophers who argue that using “hallucination” for machine outputs obscures the experiential, subjective quality that is central to the human phenomenon. A language model that fabricates a citation does not experience anything; it has no sensory qualia, no sense of reality, no distress. Whether the shared label is a useful metaphor or a misleading conflation remains an open question, but at this point the AI usage is firmly entrenched in both technical and popular discourse.