A hallucination is a sensory experience that feels completely real but has no external source. You might see a person who isn’t there, hear a voice in an empty room, or smell something no one else can detect. Unlike imagination or daydreaming, hallucinations aren’t under your control, and in the moment, they’re often indistinguishable from reality. They’re also far more common than most people realize: research estimates that up to 38.7% of the general population experiences some form of hallucination in their lifetime, and many of those people have no psychiatric condition at all.
How Hallucinations Differ From Delusions and Illusions
These three terms get mixed up constantly, but they describe very different things. A hallucination is a sensory experience: you perceive something through your senses that isn’t actually there. A delusion is a false belief, like being convinced you’re being followed by a government agency when you’re not. An illusion is a misinterpretation of something real, like mistaking a coat rack for a person in a dark room. The key distinction is that hallucinations create perception out of nothing, illusions distort something that exists, and delusions aren’t sensory at all.
The Five Types of Hallucinations
Auditory hallucinations are the most common type across nearly all conditions. They range from hearing footsteps, knocking, or music to hearing distinct voices speaking words or full sentences. The voices can be neutral, encouraging, or hostile. Among hospitalized patients with schizophrenia, bipolar disorder, or depression, auditory hallucinations consistently rank as the most frequent form.
Visual hallucinations involve seeing objects, shapes, people, animals, or lights that aren’t present. These are especially common in neurological conditions like Parkinson’s disease, where visual hallucinations are the most frequent psychotic symptom. They also occur in people with significant vision loss from conditions like macular degeneration or glaucoma, a phenomenon called Charles Bonnet syndrome.
Tactile hallucinations create physical sensations on or inside the body. People describe feeling insects crawling on their skin, tingling, pressure, or the sensation of internal organs shifting. These are particularly associated with substance use and withdrawal.
Olfactory hallucinations involve smelling things that don’t exist or that nobody else can detect. Across psychiatric diagnoses, olfactory hallucinations have a notably strong link to delusions, more so than any other type.
Gustatory hallucinations produce tastes, often metallic or unpleasant, with no source. These are relatively common in people with epilepsy, where they can occur as part of a seizure.
What Happens in the Brain
Hallucinations generally involve abnormal activity in the brain areas that normally process the relevant sense. In people who hear voices, brain imaging studies consistently show increased activity in the auditory processing areas of the brain, the same regions that light up when you listen to someone actually speaking. There’s also altered connectivity between these hearing centers and the language-processing areas that help you distinguish your own inner speech from external sound.
One leading theory focuses on the communication loop between the brain’s sensory relay station (the thalamus) and the outer cortex. Normally, incoming sensory information from your eyes, ears, and skin drives this loop. But when that incoming signal is weakened or disrupted, whether by disease, drugs, sensory loss, or stress, the brain’s internal attention mechanisms can take over and generate perceptions on their own. Excess dopamine activity, which is characteristic of schizophrenia, appears to be one factor that tips this balance toward internally generated experiences.
Psychiatric Causes
Schizophrenia is the condition most strongly associated with hallucinations. In a large study of nearly 5,000 hospitalized patients, 61.1% of those with schizophrenia were experiencing hallucinations at the time of admission. The hallucinations in schizophrenia are predominantly auditory and tend to be more severe and persistent than in other conditions.
Bipolar disorder also produces hallucinations, though less frequently and with different characteristics. About 22.9% of patients in a mixed bipolar episode reported hallucinations, compared to 11.2% during mania and 10.5% during bipolar depression. Hallucinations in bipolar disorder tend to be less severe than in schizophrenia, more often visual, and frequently accompanied by paranoid beliefs. Even unipolar depression can produce hallucinations, though only about 5.9% of hospitalized patients with depression experience them.
Non-Psychiatric Causes
Many hallucinations have nothing to do with mental illness. Parkinson’s disease commonly causes visual hallucinations as the condition progresses. Charles Bonnet syndrome produces vivid visual hallucinations in people who have lost significant vision, whether from macular degeneration, glaucoma, retinitis pigmentosa, or even something as simple as an eye patch blocking visual input. The brain, deprived of its normal visual data, essentially fills in the gaps with images of its own creation. People with Charles Bonnet syndrome typically recognize that what they’re seeing isn’t real, which distinguishes it from psychotic hallucinations.
High fevers, dehydration, severe infections, and sleep deprivation can all trigger hallucinations. So can epilepsy, particularly the gustatory and olfactory types that accompany certain seizures. Dementia, including Alzheimer’s and vascular dementia, frequently involves hallucinations as the disease advances.
Hallucinations During Sleep Transitions
Some of the most common hallucinations happen right at the edges of sleep, and they’re considered normal. Hypnagogic hallucinations occur as you’re falling asleep, while hypnopompic hallucinations happen as you’re waking up. About 86% of these are visual, typically involving geometric patterns, shifting shapes, light flashes, or kaleidoscope-like images. Between 25% and 44% involve physical sensations like floating, falling, or feeling a presence in the room. Auditory versions, occurring in 8% to 34% of cases, might include hearing your name called or fragments of conversation.
These differ from dreams in that they’re brief, fragmented experiences rather than narrative storylines. They’re usually fleeting and harmless, though they can be startling if you don’t know what they are. They’re especially common in people with disrupted sleep patterns or narcolepsy.
Substance-Induced Hallucinations
A wide range of drugs can trigger hallucinations, each through different brain mechanisms. Classic psychedelics like LSD and psilocybin work primarily by flooding serotonin receptors, producing intense visual and sometimes auditory distortions. Stimulants like methamphetamine and cocaine boost dopamine levels dramatically, which can produce paranoid auditory hallucinations, especially after prolonged use or binge patterns. Cannabis activates specific receptors in the brain’s cannabinoid system that, at high enough doses, can produce perceptual distortions and occasionally full hallucinations. Ketamine and PCP block a type of receptor involved in learning and sensory processing, creating dissociative hallucinations where the boundary between self and environment breaks down.
Alcohol withdrawal is another well-known trigger. Severe withdrawal can cause delirium tremens, which involves vivid, often terrifying visual and tactile hallucinations.
How Hallucinations Are Treated
Treatment depends entirely on the cause. When hallucinations stem from a medication side effect, a fever, or substance use, addressing that underlying trigger often resolves them. For Charles Bonnet syndrome, reassurance and understanding that the hallucinations are a product of vision loss, not mental illness, is often the most important intervention.
For hallucinations linked to psychiatric conditions like schizophrenia, antipsychotic medications are the first-line treatment. However, even with medication, 25% to 50% of people with psychotic disorders continue to experience persistent hallucinations. For these cases, a specialized form of talk therapy called cognitive behavioral therapy for psychosis has become a widely recommended addition to treatment. The goal isn’t necessarily to eliminate hallucinations entirely but to help people understand their experiences, develop coping strategies, and reduce the distress and disruption the hallucinations cause. Nearly all current treatment guidelines for schizophrenia now include this approach alongside medication.
For hallucinations in Parkinson’s disease and dementia, treatment involves carefully balancing medications, since some drugs used for these conditions can actually worsen hallucinations while others can reduce them.