A hallucination (ha-LOO-sin-A-shun) is something that a person perceives as real but that is not actually caused by an outside event. It can involve any of the senses: hearing, smell, sight, taste, or touch.
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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 optical 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. 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 takes place right inside the brain. In her book Mapping the Mind, science writer Rita Carter defines hallucinations as "exceptionally intense self-generated sensory experiences."
Dreams are the most common types of hallucinations that people experience on a regular basis. While dreaming, people may think that what they are seeing and hearing is real, but actually it is all in their heads. One frequently occurring "dream hallucination" is the sensation of falling, followed immediately by a jerking reflex that wakes the person up. Hallucinations also commonly occur when a person experiences extremely high fever or when an anesthetic * starts to wear off after surgery.
How can the brain create sights, sounds, feelings, and even tastes and smells that seem so real? For a variety of reasons, the areas of the brain responsible for interpreting sensory input can become activated on their own. When a person dreams, for example, these areas of the brain are still working even though the person is asleep and not processing stimuli from the environment.
* anesthetic (an-es-THET-ik) is a medicine that decreases the sensation of pain.
* amputation (am-pyu-TAY-shun) is the removal of a limb or other appendage of the body.
Why Does the Brain Hallucinate?
People who have undergone amputation * 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 not uncommon 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. Another theory is that the brain is programmed for a body where everything is 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.
That the brain may compensate for a lack of sensory input helps to explain why people who experience vision or hearing loss or who are placed in solitary confinement often experience hallucinations (they start "seeing things" or "hearing things"). Under such circumstances, the different areas of the brain that were used to receiving signals through the senses start to stimulate themselves into action. This also explains why people tend to "see" 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. In other words, people's minds tend to play more tricks on them at twelve midnight than at twelve noon.
Too much stimulation can also result in hallucinations. Excessive anxiety * , intense emotions, certain drugs, and even some mental disorders can essentially flood the brain with too much sensory input. In these cases, the brain's circuits get jammed and it cannot concentrate on making sense of the person's real environment; instead, the brain starts generating its own sensations. For example, people who experience the death of a loved one often report hallucinations in which they see that person or hear his or her voice. Similarly, people who undergo the terrible trauma of abuse sometimes report later visions of their abuser. Hallucinogenic drugs * such as LSD and Ecstasy are artificial sources of 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. Researchers have found that subjecting people to constant loud noise and bright lights can also produce hallucinations.
* anxiety 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.
* hallucinogenic drugs are substances that cause a person to have hallucinations.
* central nervous system refers to the brain and the spinal cord, which coordinate the activity of the entire nervous system.
* psychotic (sy-KOT-ik) disorders are mental disorders, such as schizophrenia, in which the sense of reality is so impaired that a person can not 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-so-FREE-nee-ah) is a serious mental disorder that causes people to experience hallucinations, delusions, and other confusing thoughts and behaviors, which distort their view of reality.
Perhaps the most disturbing hallucinations are those that can accompany psychotic disorders * such as schizophrenia * . People with schizophrenia lose touch with aspects of reality, which affects their thinking and behavior. They often report hearing voices that tell them that they are bad or that they should act in a certain way. Experts who work with these patients have found evidence that these voices actually belong to the patient; patients may be generating "speech" in one part of their brain then experiencing it as sound in another part. People with schizophrenia also tend to see disturbing visions. Medications can help with hallucinations caused by schizophrenia, and in some cases, people can be trained to recognize and even control their hallucinations. This level of improvement usually requires intensive therapy.
Carter, Rita. Mapping the Mind. Berkeley: University of California Press, 2000. See especially Chapter 5, "A World of One's Own."
Siegel, Ronald K. Fire in the Brain: Clinical Tales of Hallucination. New York: Penguin, 1993.