hallucination, false perception characterized by a distortion of real sensory stimuli. Common types of hallucination are auditory, i.e., hearing voices or noises and visual, i.e., seeing people that are not actually present. Hallucinations play a prominent role in schizophrenia and in the mania stage of bipolar disorder (see depression). They are also significant during withdrawal from various drugs, particularly depressants such as barbiturates, heroin, and alcohol (see delirium tremens), and under the influence of hallucinogenic drugs such as LSD, mescaline, and psylocybin. Hallucinations may occur in normal people under conditions of sensory deprivation, emotional stress, religious exaltation, or great fatigue.