What cause hallucinations, vivid perceptions of unreal sights or sounds that appear quite real to the person experiencing them? These mystical experiences have long fascinated psychologists, neuroscientists, and anthropologists alike. In many cultures, shamans, prophets, and seers are marked by their susceptibility to hallucinations. Are hallucinations caused by ghosts or spirits? Are they messages from another world? Although researchers don’t have all the answers, there is some intriguing information on the topic.According to survey, anywhere from 10 to 25 percent of the population has experienced at least one hallucination. Most often, the hallucination comes in the form of some visual experience, but some people report hearing a sound or even voices. Even rarer, but not unheard of, is a hallucination of a particular smell or aroma. It is not known exactly what causes hallucinations, although one commonly accepted theory is that hallucinations occur when the external stimulus received by the senses no longer matches the level of activity occurring in the brain. Sensory deprivation is one of the surest ways to elicit hallucinations.Hallucinations can also be elicited in a number of other ways. Some of the most common experiences of hallucinations happen when a person is in the throes of an epileptic fit or suffering from a high fever. Other methods of bringing about a hallucination include fasting or sleeplessness. Admiral Richard Byrd reported having hallucinations after spending several months alone in the Antarctic. Hallucinations can be so powerful that members of many cultures seek them out, undertaking “vision quests” in the hopes of having a hallucinatory experience. Usually the participants who go on these quests journey out into the elements without food or shelter.Not all hallucinations are the product of extreme physical conditions. Some very complex hallucinations can be triggered by nothing more unusual than everyday memories. People who have lost limbs often report that they continue to feel physical sensations as if the limb were still there. These “phantom limbs” are most likely the result of the brain interpreting signals it receives from severed nerve endings in the context of its memories of the missing limb. An even stranger phenomenon involves hallucinations produced by the memories of recently departed loved ones. Called grief hallucinations, these vivid visions can be simple visual hallucinations or more complex fantasy interactions. The woman claimed that she and her “ghosts” regularly held long and involved conversations. Neuroscientists theorize that grief hallucinations may be the product of vivid memories that last in the mind long after a loved one has passed away. As bizarre as grief hallucinations may sound, the experience is quite common. Some researchers estimate that up to 80 percent of people will experience some form of grief hallucination in their lifetime.Although neuroscientists may not be sure of the exact mechanism in the brain that causes hallucinations, they have isolated activity in the left temporal lobe of the brain that appears to play a part in the phenomenon. Certain drugs that affect this region of the brain are known for their ability to cause hallucinations. Drugs such as LSD, psilocybin, and mescaline gained popularity with the 1960s Western youth culture for their ability to provide vivid hallucinatory experiences.It is also suspected that the brain has its own chemicals designed to produce hallucinations. For example, some patients suffer from delirium tremens, a violent period of hallucinations accompanied by sweating, an increase in heart rate, and a rise in body temperature. Through experience treating episodes such at this, it is also known that certain chemicals can stop hallucinations. The drug Thorazine often used to treat patients suffering from psychotic disorders that involve hallucinations.Regardless of the causes of hallucinations, the effects they have on their subjects are very real. Hallucinations can cause the aforementioned change in heart rate and body temperature, and they can also lead a person to act on the hallucination. Psychologists have found that the memories created by a hallucination are processed by the same part of the brain that handles normal memories. Thus, for the subject of a hallucination, the experience is as real as any other.