When people hear the word hallucination, they often immediately associate it with severe mental illness. That connection is understandable, but the reality is much broader and far more nuanced.

A hallucination is a perception-like experience that occurs without a corresponding external stimulus. A person may hear a voice when no one is speaking, see something that is not physically present, or experience sensations of touch, smell, taste, or movement without any identifiable source.

Hallucinations certainly can occur in psychiatric disorders, but they are not limited to them. They may also appear in neurological conditions, during substance intoxication or withdrawal, around the natural transition between sleep and wakefulness, and sometimes even in people who do not have any diagnosed psychiatric or medical disorder at all.

Not Every Distorted Perception Is a Hallucination

This distinction matters deeply because many unusual sensory experiences are casually described as hallucinations even when they are clinically something quite different.

An illusion, for example, involves a real external stimulus that is interpreted incorrectly. Something is actually physically present in the environment, but the brain misidentifies or misinterprets it.

Other perceptual disturbances can change the subjective qualities of something that really exists. An object may appear unusually large or small, closer or farther away than it actually is, or somehow distorted in shape, color, or intensity. These experiences are functionally different from hallucinations because an external object is still present in the environment.

There are also sensory disturbances involving the intensity of a sensation, unusual bodily sensations, and agnosia, a condition in which a person has difficulty recognizing or identifying something despite successfully receiving sensory information about it.

Hallucination is therefore not a catch-all general term for every unusual sensory experience. It describes a very particular type of perceptual phenomenon.

Hallucinations Can Involve More Than Hearing Voices

Hallucinations are often classified according to the specific sensory system involved.

Auditory hallucinations may involve voices, music, sounds, knocking, footsteps, or other noises without an external source. Hearing voices is especially well known in popular culture because auditory verbal hallucinations are common in psychotic disorders, including schizophrenia. However, hearing a voice by itself does not automatically mean that a person has schizophrenia.

Visual hallucinations involve seeing people, objects, shapes, lights, animals, scenes, or other images that are not actually present.

There can also be olfactory hallucinations, involving smells; gustatory hallucinations, involving taste; and tactile hallucinations, involving sensations of touch on the skin.

Some people experience unusual sensations coming from inside the body, sometimes described as somatic or bodily hallucinations. Experiences involving a distorted sense of movement, position, or balance may also occur.

More than one sensory modality can be involved at the exact same time. Modern psychological research increasingly recognizes these multimodal hallucinations rather than treating every sensory experience as completely separate and isolated from the others.

What Happens in the Brain?

It is tempting to explain hallucinations by saying that one specific brain chemical simply becomes “too high.” However, the neurobiology is much more complicated.

Hallucinations appear to arise from complex disruptions in the brain systems responsible for processing sensory information, expectations, attention, memory, and the vital distinction between internally generated experiences and information coming from the outside world.

In schizophrenia, abnormalities involving the neurotransmitter dopamine are undoubtedly important, but dopamine does not provide a complete explanation. Research also points heavily to glutamate and the complex interactions among multiple neural systems. In other words, there is no single neurotransmitter imbalance that explains every hallucination in every clinical condition.

The underlying mechanisms may also differ significantly depending on the root cause. Hallucinations related to a primary psychotic disorder are not necessarily produced in the exact same biological way as hallucinations associated with sleep, neurological illness, medication side effects, substance use, or withdrawal.

Alcohol withdrawal is one prominent example. Significant changes in inhibitory and excitatory brain signaling occur after prolonged, heavy alcohol exposure is suddenly reduced or stopped, and hallucinations can quickly appear as part of the withdrawal syndrome. Severe withdrawal can also dangerously progress to seizures or delirium.

“True Hallucinations” and “Pseudohallucinations”

Older psychiatric traditions often divided these sensory experiences into true hallucinations and pseudohallucinations.

A so-called true hallucination was generally described as being experienced in external space and having the unmistakable quality of ordinary perception. For example, a person might experience a voice as coming from somewhere in the room and react as if an actual, physical speaker were present.

The term pseudohallucination was historically used for experiences that seemed to occur within a person’s internal or subjective space. Someone might describe hearing something “inside the mind” rather than through the ears, while sometimes maintaining greater awareness or insight that the experience is unusual.

The problem is that the term pseudohallucination has been defined in contradictory ways throughout psychiatric history. Furthermore, the boundary between “internal” and “external” voices is simply not as clinically clear-cut as the old terminology suggests.

For this reason, contemporary clinical assessment—particularly in modern U.S. practice—is often much more interested in exploring how the experience actually occurs rather than forcing it into this older, rigid two-part classification.

A modern clinician may ask whether the experience feels internal or external, whether the person recognizes that others cannot perceive it, how convincing it feels, whether it causes distress, whether the person feels able to resist or ignore it, and whether it occurs together with delusions, confusion, mood symptoms, substance use, or neurological changes.

Those detailed nuances often tell us much more about the patient's reality than a single diagnostic label alone.

Hallucinations Do Not Automatically Mean Schizophrenia

Perhaps the single most important point to understand is that a hallucination is a symptom, not a diagnosis.

Hallucinations can indeed occur in schizophrenia and other psychotic disorders, but they may also occur in mood disorders with psychotic features, neurological disorders, substance-related conditions, sleep-related experiences, and various other medical situations. Research has also thoroughly documented hallucinatory experiences among many people without any psychotic disorders.

Historically, psychiatrists tried to group specific symptoms together to form distinct clinical pictures. They described combinations of hallucinations, beliefs that outside forces were controlling thoughts or actions, and experiences of external influence under terms such as Kandinsky–Clérambault syndrome and related concepts of mental automatism.

While these descriptions remain a fascinating part of the history of psychopathology, they should not be presented as a stand-alone diagnostic criterion for schizophrenia in contemporary U.S. clinical practice. Today, clinicians evaluate the complete pattern of symptoms, their duration, their profound effect on daily functioning, and all possible psychiatric, neurological, medical, or substance-related explanations.

That careful distinction can change the entire meaning and trajectory of the symptom.

The most useful question a professional can ask is therefore not simply, “Is this a hallucination?” It is: What is the person experiencing, how is it being experienced, when does it happen, and what else is happening at the same time?

Understanding those deeply personal details is what turns an unusual perception from a frightening label into actionable information that can actually be clinically understood and treated.

References

• Montagnese, M., Leptourgos, P., Fernyhough, C., Waters, F., Larøi, F., Jardri, R., et al. (2021). A Review of Multimodal Hallucinations: Categorization, Assessment, Theoretical Perspectives, and Clinical Recommendations. Schizophrenia Bulletin, 47(1), 237–248. doi:10.1093/schbul/sbaa101.
  Relevance: Provides a modern overview of hallucinations across different sensory modalities and discusses their classification, assessment, and possible underlying mechanisms. Relevant pages: 237–248.
• McCutcheon, R. A., Krystal, J. H., & Howes, O. D. (2020). Dopamine and glutamate in schizophrenia: biology, symptoms and treatment. World Psychiatry, 19(1), 15–33. doi:10.1002/wps.20693.
  Relevance: Reviews evidence concerning dopamine and glutamate abnormalities in schizophrenia and demonstrates why psychotic symptoms cannot accurately be reduced to a single neurotransmitter explanation. Relevant pages: 15–33.
• de Leede-Smith, S., & Barkus, E. (2013). A comprehensive review of auditory verbal hallucinations: lifetime prevalence, correlates and mechanisms in healthy and clinical individuals. Frontiers in Human Neuroscience, 7, Article 367. doi:10.3389/fnhum.2013.00367.
  Relevance: Reviews auditory verbal hallucinations in both clinical and nonclinical populations and discusses psychological and neurobiological explanations. The journal uses an article number rather than a conventional page range.
• Telles-Correia, D., Moreira, A. L., & Gonçalves, J. S. (2015). Hallucinations and related concepts—their conceptual background. Frontiers in Psychology, 6, Article 991. doi:10.3389/fpsyg.2015.00991.
  Relevance: Particularly useful for distinguishing hallucinations from illusions and for understanding the historical and controversial concept of pseudohallucinations. The journal uses Article 991 rather than conventional page numbering.
• Bless, J. J., Hugdahl, K., Kråkvik, B., Vedul-Kjelsås, E., Kalhovde, A. M., Grønli, J., & Larøi, F. (2021). In the twilight zone: An epidemiological study of sleep-related hallucinations. Comprehensive Psychiatry, 108, 152247. doi:10.1016/j.comppsych.2021.152247.
  Relevance: Examines hallucinations associated with sleep and supports the point that hallucinatory experiences are not limited to psychotic disorders. The publication uses article number 152247 rather than a page range.
• Jesse, S., Bråthen, G., Ferrara, M., Keindl, M., Ben-Menachem, E., Tanasescu, R., et al. (2017). Alcohol withdrawal syndrome: mechanisms, manifestations, and management. Acta Neurologica Scandinavica, 135(1), 4–16. doi:10.1111/ane.12671.
  Relevance: Describes the neurobiology and clinical manifestations of alcohol withdrawal, including auditory, visual, and tactile hallucinations, seizures, and delirium. Relevant pages: 4–16.