Hallucinations are false perceptions that occur without an external stimulus, unlike illusions which are a mistaken perception of real things that are caused by an external stimulus. Hallucinations may be caused by a wide variety of things and occur within all senses of the body. However, a person must be awake to experience them, unlike dreams which occur while someone is sleeping.
People who suffer from mood disorders such as schizophrenia and depression frequently hallucinate. Schizophrenics commonly hear voices and sounds which are auditory hallucinations that are believed to be caused by high levels of dopamine in a person’s brain. Although schizophrenics also experience visual hallucinations, seeing things which aren’t really there, they are most common in manic depressives, especially elderly people.
Another common cause is drug use. Regardless of the legality of the drug in question, hallucinogens cause false perceptions because they disrupt the normal balance of neurotransmitters in a person’s brain. People who use cocaine, crystal meth or other amphetamines may hallucinate because of an overproduction of dopamine in the brain. Lysergic acid diethylamide, or LSD, is responsible for blocking the functions of the neurotransmitters serotonin and norepinephrine. Phencyclidine (PCP) causes people to hallucinate by blocking glutamate. Haptic hallucinations, which occur when a person feels something which is not there, are rare in general, but are seen mostly in drug users.
Medications prescribed for depression, sleep aids and certain anesthetics may cause someone to hallucinate. Medications including ketamine, paroxetine, mirtazapine and zolpidem have hallucinogenic side effects. Antiretroviral medications used to treat HIV and antipsychotics used for Alzheimer’s patients are also believed to cause hallucinations in some people.
In addition to drug use, alcohol use may also cause people to hallucinate. More specifically, the abrupt cessation of alcohol use may cause auditory hallucinations in people. After a few days of withdrawal, when the delirium tremens (DTs) begin, it is common for people to visually hallucinate.
Disorders caused by the imbalance of neurotransmitters in the brain commonly cause people to hallucinate. One example is narcolepsy, where people uncontrollably sleep for brief periods. In addition to auditory and visual hallucinating, it is common for narcoleptics to experience gustatory and olfactory hallucinations. A gustatory hallucination occurs when a person tastes something that is not present, while an olfactory hallucination occurs when a person smells something that is not there.