Hallucinations are real experiences. The sights, sounds, smells, or sensations a person perceives during a hallucination are genuinely felt, even though they don’t come from anything in the outside world. Brain imaging studies show that the same regions responsible for processing actual sensory input fire during hallucinations, meaning the brain treats these experiences much like it treats real ones. The distinction isn’t between “real” and “fake” but between externally caused and internally generated.
What Happens in the Brain
When someone hears a voice that isn’t there, their primary auditory cortex lights up on brain scans, the same area that processes actual speech and sound. This is why hallucinations feel so convincing. The brain’s sensory machinery is genuinely activated, not in response to something in the environment, but from signals generated internally.
One leading explanation centers on the communication loop between the thalamus (a relay station that filters incoming sensory data) and the cortex (where that data gets interpreted). Normally, external stimuli drive this loop. But in people experiencing hallucinations, that loop can fire on its own. There’s evidence that in conditions like schizophrenia, the thalamus struggles to properly gate sensory signals, allowing internal mental activity to be processed as though it came from outside.
This connects to a concept researchers call “source monitoring,” the brain’s ability to distinguish between thoughts it generated and perceptions it received. When source monitoring breaks down, internally generated experiences get misattributed to the outside world. Your inner voice gets tagged as someone else’s voice. An internally generated image gets tagged as something you’re actually seeing. The experience is real. The source is misidentified.
How Common Hallucinations Actually Are
Hallucinations are far more common than most people assume. Epidemiological studies estimate that 6 to 15 percent of the general population experiences hallucinations, many of whom have no psychiatric diagnosis at all. Some hallucinations are so ordinary that most people don’t even recognize them as such.
The most familiar example is the hypnagogic hallucination, which happens as you’re falling asleep. You might hear someone call your name, see a flash of a face, or feel a sudden jolt. These experiences range from vague impressions to vivid, complex scenes. A related phenomenon, hypnopompic hallucinations, occurs when waking up and can include feelings of paralysis combined with visual or auditory perceptions. About 8 percent of the general population experiences sleep paralysis at some point, often accompanied by these kinds of perceptions.
Grief is another surprisingly common trigger. A meta-analysis of 21 studies found that roughly 56.6 percent of bereaved people experience some form of hallucination involving their deceased loved one. These range from hearing the person’s voice to sensing their presence to briefly seeing them. These experiences span all five senses, with prevalence in individual sensory types ranging from about 7 to 40 percent.
The Five Sensory Types
Hallucinations can occur in any sense:
- Auditory hallucinations are the most common type overall. They range from hearing music, footsteps, or banging doors to hearing full voices. The voices can be neutral, positive, or negative.
- Visual hallucinations involve seeing objects, shapes, people, animals, or lights that aren’t present.
- Tactile hallucinations create the sensation of touch, like bugs crawling on your skin or movement inside your body.
- Olfactory hallucinations involve smelling something that no one else can detect.
- Gustatory hallucinations produce tastes, often strange or metallic. These are particularly associated with epilepsy.
Why Vision Loss Can Cause Hallucinations
One of the clearest demonstrations that hallucinations are real brain events comes from Charles Bonnet Syndrome. People who lose significant vision from conditions like macular degeneration, glaucoma, or diabetic retinopathy sometimes begin seeing vivid, detailed images: figures, animals, patterns, faces. They aren’t experiencing a psychiatric illness. Their visual cortex, starved of input from damaged eyes, becomes hyperexcitable and starts generating its own images.
Brain scans of people with this condition show heightened activity in the visual association areas of the cortex, despite no corresponding input from the retina. It’s the visual equivalent of phantom limb pain: the brain expects input, doesn’t get it, and fills in the gap with internally generated perceptions. Diagnosis requires documented visual impairment and the confirmed absence of any psychiatric or cognitive condition.
Psychiatric and Medical Causes
Hallucinations are most strongly associated with psychosis-related disorders, particularly schizophrenia, where auditory hallucinations (often commanding or critical voices) are a hallmark symptom. They also occur in bipolar disorder and severe depression. But the list of medical causes is long: high fevers, epilepsy, Parkinson’s disease, delirium, and various neurological conditions can all produce hallucinations. Chemical imbalances and abnormalities in the brain underlie all of these cases, though the specific mechanism differs.
How Substances Trigger Hallucinations
Different drugs produce hallucinations through entirely different brain pathways. Psychedelics like LSD and psilocybin work by overstimulating serotonin receptors on cortical neurons. This hyperactivation disrupts the thalamus’s normal filtering of sensory information, essentially flooding the cortex with unregulated signals and producing the characteristic visual distortions and perceptual shifts.
Dissociative drugs like ketamine and PCP take a different route. They block glutamate receptors involved in normal brain signaling, producing a more complex set of symptoms that can resemble schizophrenia, including hallucinations, detachment from reality, and disordered thinking. Stimulants like cocaine and amphetamines represent yet a third pathway, particularly when used heavily or over long periods. Three different drug classes, three different mechanisms, all producing the same category of symptom.
AI “Hallucinations” Are Something Else Entirely
If you came to this question because of AI, it’s worth knowing that the term “hallucination” means something fundamentally different when applied to language models like ChatGPT. An AI hallucination is when a system generates a confident, plausible-sounding response that is entirely fabricated. It’s not perceiving something that isn’t there. It has no perception at all.
Human hallucinations involve genuine sensory processing in a conscious brain. AI systems lack consciousness and subjective experience. They don’t “see” or “hear” anything. They produce text based on statistical patterns in training data. Some researchers have argued the term “hallucination” is misleading when applied to AI, since the machine isn’t misperceiving reality. It’s fabricating information, a fundamentally different process despite the shared label.