A perception that occurs when there is no external stimulus. Auditory hallucinations (the hearing of voices) are a major symptom of schizophrenia but may also be caused by manic–depressive illness and certain brain disorders. Visual hallucinations are most often found in states of delirium brought on by a physical illness (such as pneumonia) or alcohol withdrawal (delirium tremens). Hallucinogenic drugs are another common cause of visual hallucinations. Hallucinations of smell are associated with temporal lobe epilepsy. Those of touch and taste are rare, however, and occur mainly in people with schizophrenia. People subjected to sensory deprivation or overwhelming physical stress sometimes suffer from temporary hallucinations.
n. a false sensory perception of something that is not really there; the perception is subjectively convincing but lacks an external stimulus. Hallucinations may be visual, auditory, tactile, gustatory (of taste), or olfactory (of smell). They may be provoked by mental illness (such as *schizophrenia or severe anxiety disorders), *personality disorders, or physical disorders affecting the brain (such as temporal lobe *epilepsy, sepsis, acute organic syndrome, or stroke) or they may be caused by drugs, alcohol, or sensory deprivation. Hallucinations should be distinguished from dreams and from *illusions (which are misinterpretations of real stimuli). A substantial minority of the population experiences hallucinations not caused by mental illness. Some hallucinations are not always pathological: hypnagogic hallucinations occur in the state between wakefulness and sleep, and the images are often very distinct; these hallucinations occur in 30–60% of patients with *narcolepsy. Hypnopompic hallucinations occur in the state between sleep and full wakefulness; like hypnagogic hallucinations, the experiences may be very vivid.
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