A hallucination (huh-loo-suh-NAY-shun) is something that a person, while awake, perceives as real but it is not; it is usually caused by an outside stimulus * .

Is It All in Their Heads?

A good magician can make audience members think that they are seeing something they really are not, such as an animal disappearing into thin air or a bouquet appearing from under a handkerchief. These tricks are often referred to as illusions. The magician knows how to perform the illusion so that the viewer's eyes and brain are likely to misinterpret what is really happening.

Hallucinations are different from illusions. Illusions are mistaken perceptions of real events. During a hallucination, the person is not reacting to something real in the outside world. The brain creates its own stimulation instead of relying on input from the five senses. In other words, the entire experience, its cause and its response, takes place inside the brain.

Hallucinations occur when a person is conscious or awake, so although dreams may cause people to think that what they are dreaming is real, these are not true hallucinations. However, a large percentage of the population reports having brief hallucinations when they are just at the edge of consciousness while falling asleep or waking up. These are true hallucinations, and they are considered normal and harmless.

Hallucinations can involve any of the five senses: sight, hearing, taste, smell, and touch. They can appear in the form of seeing visions, hearing voices or music, smelling odors that are not really present in the environment, distorting the taste of food, or feeling as if something is crawling over or touching either the skin or the interior of the body. These sensations can be brief or extended. Often they are quite vivid and seem absolutely real to the person who experiences them. In people with mental illness, hallucinations are often accompanied by delusions. A delusion is a strong belief about something that is clearly false. For example, a delusional person might believe that the television is sending out signals that control his brain and cause him to hear voices.

What Triggers Hallucinations?

Hallucinations can occur when a person is mentally ill, uses certain drugs, is under prolonged stress, is exhausted or sleep deprived for a long time, experiences extremely high fever, or has certain nervous system diseases. People who are blind or deaf may also have hallucinations due to lack of sensory input from the eyes or ears. Scientists are not exactly sure why some people hallucinate, but they do know that nerves in specific areas of the brain responsible for interpreting sensory information are activated on their own without outside stimuli when a person hallucinates.

The most common hallucinations are those that accompany psychotic (sy-KOT-ik) disorders * , such as schizophrenia *


People who have undergone amputation of a limb or other appendage help researchers understand what may happen in the brain when it hallucinates. Many of these patients report that they feel like the missing body part is still there, even though they know the arm, leg, hand, or other body part is gone. For example, it is common for people who lose a leg to try to stand up and walk after their surgery.

Feeling like an amputated limb is still present is called phantom limb syndrome, and there are two main theories about why it happens. It may be that the nerve cells in the brain area that used to receive signals from that limb go into overdrive and stimulate themselves because that input has disappeared. The second theory is that the brain is programmed for a body that is whole and intact and in the right place so that when certain signals are missing, spontaneous nerve cell activity takes over. In either case, the brain is compensating for the lack of sensory input.

Anxiety * disorders and intense emotions or a highly traumatic experience also can generate hallucinations. For example, people who experience the death of a loved one often report hallucinations in which they see that person or hear the person's voice. Similarly, people who undergo extreme abuse sometimes report later visions of their abuser. Soldiers or others with post-traumatic stress disorder * may also experience hallucinations as flashbacks * to especially traumatic events.

Hallucinogenic drugs * such as the street drugs Ecstasy, LSD, mescaline, peyote, and psilocybin (magic mushrooms) are artificial sources of brain overstimulation. They excite the central nervous system * so much that certain areas of the brain produce visions, sounds, and feelings that are not based in reality. Some hallucinogenic drug users continue to experience bizarre visions and sounds even long after they stop using the drug. Some prescription drugs also can produce hallucinations as an unwanted side effect.

That the brain may compensate for a lack of sensory input helps to explain why people who are deaf or blind or who are placed in extended solitary confinement often experience hallucinations. Under such circumstances, the different areas of the brain that were used to receiving signals through the senses start to stimulate themselves. Reduced sensory input also may explain why people tend to experience seeing ghosts at night instead of during the day; the brain is more likely to create the vision of ghosts when other visual stimuli are absent.

Some nervous system diseases such as Alzheimer's disease * may also cause hallucinations. Brain damage, brain tumors, or lesions * in the brain may also stimulate the production of hallucinations.

See also Anxiety and Anxiety Disorders: Overview • Delusions, Delusional Disorders, and Paranoia • Psychosis • Schizophrenia


Books and Articles

Dovey, Dana. “What Hearing Voices Is Like, as Told by Those with Auditory Hallucinations.” Medical Daily, March 13, 2015. http://www.medicaldaily.com/what-hearing-voices-told-those-auditory-hallucinations-325552 (accessed September 21, 2015).

Sacks, Oliver. Hallucinations. New York: Vintage Press, 2013.


Alzheimer's Association. “Hallucinations and Alzheimer's.” http://www.alz.org/care/alzheimers-dementia-hallucinations.asp (accessed October 18, 2015).

International Consortium on Hallucination Research and Related Symptoms. “Multisensory Hallucination Scale for Children.” http://hallucinationconsortium.org/papers-announcements (accessed June 7, 2016).


Alzheimer's Association. 225 N. Michigan Ave., Fl. 17, Chicago, IL 60601. Toll-free: 800-272-3900. Website: http://www.alz.org (accessed July 15, 2015).

American Psychiatric Association. 1000 Wilson Blvd., Suite 1825, Arlington, VA 22209. Toll-free: 888-35-PSYCH. Website: http://www.healthyminds.org (accessed July 15, 2015).

* stimulus (STIM-yoo-lus) is an agent in the environment that excites a response or reaction. A stimulus might cause a person to function, become active, or respond. The plural form is stimuli.

* psychotic (sy-KOT-IK) disorders are mental disorders in which the sense of reality is so impaired that a person cannot function normally. People with psychotic disorders may experience delusions, hallucinations, incoherent speech, and agitated behavior, but they usually are not aware of their altered mental state.

* schizophrenia (skit-suh-FREE-neeuh) is a serious mental disorder that causes people to experience hallucinations, delusions, and other confusing thoughts and behaviors, which distort their view of reality.

* anxiety (ang-ZY-uh-tee) can be experienced as a troubled feeling, a sense of dread, fear of the future, or distress over a possible threat to a person's physical or mental well-being.

* post-traumatic stress disorder (post-traw-MAT-ik STRES dis-ORder) is a mental disorder that interferes with everyday living and occurs in people who survive a terrifying event, such as school violence, military combat, or a natural disaster.

* flashbacks are intensely vivid, recurring mental images of a past traumatic event. People may feel or act as if they are reliving the experience.

* hallucinogenic drugs are substances that cause a person to have hallucinations.

* central nervous system (SEN-trul NER-vus SIS-tem) is the part of the nervous system that includes the brain and spinal cord.

* Alzheimer's (ALTS-hymerz) disease is a condition that leads to gradually worsening loss of mental abilities, including memory, judgment, and abstract thinking, as well as changes in personality.

* lesions (LEE-zhuns) is a general term referring to a sore or a damaged or irregular area of tissue.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)