The report of a sensory experience in the absence of an actual external stimulus appropriate to the reported experience is called a hallucination. A mental patient, for example, may report that he hears voices, but no voices or sounds that could be mistaken for voices can be heard by those who are with the patient.
Hallucinations should be distinguished from illusions and delusions. An illusion is a false interpretation of an actual external stimulus. A patient may hear the wind blowing and report that he hears a host of heavenly ghosts beckoning him to come to the top of the building. There actually is an external sound, but the patient has misinterpreted it. The term "illusion" is applied to many misinterpretations of observation, as in optical illusions. A delusion is a false idea, such as when a patient announces that he is Napoleon or that he holds the future of the world in his left hand. Otherwise normal people exhibit delusions and illusions, but hallucinations are rare in normal individuals.
Psychotic Versus Normal Experiences. Among mental patients, hallucinations, illusions, and delusions are very commonly reported. They are, most often, of a bizarre quality and are easily distinguished from misperceptions exhibited by normal individuals (except for the often logical delusions of the paranoiac). Hallucinations commonly occur among schizophrenics, manic-depressives, alcoholics, drug addicts, and in persons with certain types of brain damage.
Normal persons suffering from extreme fatigue may experience hallucinations. Sometimes delirium accompanying very high fevers produces hallucinations. In laboratory experiments with sensory deprivation, in which subjects are isolated from external stimuli, normal subjects will often report hallucinatory experiences.
Types and Causes
Auditory hallucinations, hearing voices or other sounds in the absence of any external stimuli to account for these sounds, are the most commonly observed hallucinations among mental patients. Visual hallucinations are the second most often encountered among psychotic patients—the patients see images that they believe to be real. There are also olfactory hallucinations, as when a patient claims that he smells poison gas in his room, and tactual hallucinations, as when the patient reports the sensation of bugs crawling over his body.
There is evidence that hallucinations are influenced by cultural factors. For example, the voices heard typically speak in the patient's native tongue. Often hallucinations are autobiographical, as when a patient thinks he hears his mother scolding him for sinful behavior.
Among mental patients, hallucinations are related to restricted thinking and preoccupation with conflict, wishful thinking, and projection. The patient seems to become unable to distinguish between his own thoughts and preoccupations and the events in the outside world.
Both illusions and hallucinations can be induced by drugs such as the psychedelics.