A hallucination is a perception that feels completely real but occurs without any external stimulus to trigger it. Unlike imagining something or daydreaming, a hallucination carries a compelling sense of reality. You don’t choose it, and in the moment, it can be indistinguishable from genuine sensory experience. Hallucinations are far more common than most people realize, affecting an estimated 6 to 15% of the general population at some point in their lives, and many cases have nothing to do with mental illness.
How Hallucinations Differ From Illusions and Delusions
These three terms get mixed up constantly, but they describe very different things. A hallucination is perceiving something that isn’t there at all, like hearing a voice in an empty room. An illusion is a misinterpretation of something that actually exists, like hearing wind blow and mistaking it for a bird chirping. The real stimulus is there; your brain just reads it wrong.
A delusion, on the other hand, isn’t a sensory experience at all. It’s a firmly held false belief, like being convinced that a neighbor is secretly monitoring your phone calls. Delusions involve thinking, not perceiving. A person can experience hallucinations and delusions at the same time, but they’re separate phenomena.
Types of Hallucinations by Sense
Hallucinations can involve any of the five senses, and sometimes more than one at once.
- Auditory: The most common type overall. These range from hearing simple sounds like buzzing or knocking to hearing full conversations or distinct voices. In conditions like schizophrenia, auditory hallucinations often take the form of spoken words.
- Visual: The second most common type. These can be as simple as flashing lights or geometric shapes, or as complex as seeing fully formed people, animals, or objects that aren’t there.
- Tactile: Feeling sensations on or under the skin without a physical cause, such as crawling, tingling, or pressure. These are sometimes associated with stimulant use or alcohol withdrawal.
- Olfactory: Smelling odors that have no source. People with epilepsy affecting the temporal lobe are particularly prone to this type.
- Gustatory: Tasting something without eating or drinking. This is the rarest type and sometimes occurs alongside olfactory hallucinations.
What Happens in the Brain
Your brain is constantly building your perception of reality by combining sensory input from the outside world with internal expectations and predictions. Hallucinations appear to arise when that system misfires, and internal brain activity generates a percept without any matching input from the senses.
One prominent theory focuses on communication between the thalamus (a relay hub deep in the brain) and the cortex (the outer layer responsible for processing sensory information). Normally, incoming sensory data keeps this communication grounded. When that external anchoring weakens, the brain’s own attention and expectation systems can take over, essentially generating perceptions from the inside out. In people with schizophrenia, there’s evidence that this thalamus-to-cortex signaling is poorly regulated by external input, allowing internally generated activity to dominate.
For auditory hallucinations specifically, the brain regions involved in producing and understanding language activate as though a real conversation is happening. The areas that process speech sounds, assign meaning to words, and even drive the impulse to speak all light up. This is part of why voice-hearing feels so vivid: the brain is running the same machinery it would use for an actual voice.
Psychiatric Conditions
Schizophrenia is the condition most strongly associated with hallucinations. Hallucinations are one of five core symptom domains used to diagnose schizophrenia spectrum disorders, alongside delusions, disorganized speech, abnormal motor behavior, and what clinicians call “negative symptoms” (like reduced emotional expression). Auditory hallucinations, particularly hearing voices, are the hallmark. But hallucinations also occur in schizoaffective disorder, bipolar disorder during severe manic or depressive episodes, and major depression with psychotic features.
Neurological and Medical Causes
Many hallucinations have nothing to do with psychiatric illness. Several neurological conditions produce them through entirely different mechanisms.
Parkinson’s Disease and Lewy Body Dementia
Up to half of people with Parkinson’s disease experience visual hallucinations at some point during their illness. These often involve seeing people, animals, or objects that aren’t present. Lewy body dementia, the second most common form of dementia, produces similar hallucinations. In fact, visual hallucinations are so characteristic of Lewy body dementia that they help distinguish it from Alzheimer’s with an 83% positive predictive value. The hallucinations in both conditions are linked to abnormal protein clumps called Lewy bodies that accumulate in brain regions involved in visual processing and memory.
Charles Bonnet Syndrome
People who are losing their vision, whether from macular degeneration, glaucoma, cataracts, or other causes, can develop vivid visual hallucinations of faces, people, animals, or detailed objects. This is known as Charles Bonnet syndrome and is thought to be a “release” phenomenon: when the visual cortex stops receiving normal input from the eyes, it starts generating its own images. The key feature is that the person typically knows these visions aren’t real. Risk factors include vision loss in both eyes, social isolation, and cognitive decline.
Epilepsy
Seizures involving the brain’s visual processing areas produce hallucinations that tend to be brief and simple, often appearing as brightly colored spots, shapes, or flashes. About 4.6% of people with epilepsy experience occipital seizures, and nearly all of these involve some visual disturbance. Seizures in the temporal lobe more commonly produce olfactory hallucinations, like suddenly smelling something that isn’t there.
Substance-Induced Hallucinations
Three major classes of drugs reliably produce hallucinations, each through a different brain mechanism.
Psychedelics like LSD and psilocybin work by stimulating serotonin receptors in the brain, producing complex visual distortions, geometric patterns, and sometimes fully formed imagery. Stimulants like cocaine and amphetamines increase dopamine signaling and can trigger hallucinations, particularly with heavy or prolonged use. These tend to be paranoid in nature, sometimes involving seeing shadows or hearing threatening voices. Dissociative drugs like ketamine and PCP block a type of receptor involved in learning and perception, producing a broader set of symptoms that can resemble schizophrenia, including hallucinations, confusion, and detachment from reality.
Alcohol withdrawal is another well-known trigger. Severe withdrawal (delirium tremens) can produce intense visual and tactile hallucinations, typically within 48 to 72 hours after the last drink.
Hallucinations During Sleep
Some of the most common hallucinations happen to perfectly healthy people during the transition between sleep and wakefulness. Up to 70% of people experience these at least once. Hallucinations that occur while falling asleep are called hypnagogic hallucinations; those that happen while waking up are called hypnopompic hallucinations.
About 86% of hypnagogic hallucinations are visual, typically involving changing geometric patterns, shapes, or flashes of light. Between 8% and 34% are auditory, like hearing someone call your name or hearing a loud bang. And 25% to 44% involve a physical sensation, like feeling yourself falling or sensing pressure on your body. These experiences are generally harmless and happen because your brain enters a dream-like state while you’re still partially conscious.
How Hallucinations Are Managed
Treatment depends entirely on the underlying cause. For hallucinations tied to schizophrenia or other psychotic disorders, antipsychotic medications are the primary treatment. These work by reducing overactive dopamine signaling in the brain. Cognitive behavioral therapy adapted for psychosis can also help people develop strategies for managing distressing voices or visions, learning to reframe them and reduce their emotional impact.
For hallucinations caused by neurological conditions like Parkinson’s or Lewy body dementia, treatment is more delicate because some standard antipsychotic medications can worsen movement symptoms. Adjusting existing medications is often the first step, since some Parkinson’s drugs can themselves contribute to hallucinations.
Hallucinations from Charles Bonnet syndrome often improve if vision can be corrected, or as the brain adjusts over time. Simply understanding that the hallucinations have a known, non-psychiatric cause provides significant relief for many people. Sleep-related hallucinations rarely need any treatment at all, though improving sleep hygiene and maintaining a consistent sleep schedule can reduce their frequency.