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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 schizophreniaschizophrenia
, group of severe mental disorders characterized by reality distortions resulting in unusual thought patterns and behaviors. Because there is often little or no logical relationship between the thoughts and feelings of a person with schizophrenia, the disorder has
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 and in the mania stage of bipolar disorder (see depressiondepression,
in psychiatry, a symptom of mood disorder characterized by intense feelings of loss, sadness, hopelessness, failure, and rejection. The two major types of mood disorder are unipolar disorder, also called major depression, and bipolar disorder, whose sufferers are
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). They are also significant during withdrawal from various drugs, particularly depressants such as barbiturates, heroin, and alcohol (see delirium tremensdelirium tremens
, hallucinatory episodes that may occur during withdrawal from chronic alcoholism, popularly known as the DTs. An episode of delirium tremens is usually preceded by disturbed sleep and irritability, and generally takes several days to develop.
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), and under the influence of hallucinogenic drugshallucinogenic drug
, any of a group of substances that alter consciousness; also called psychotomimetic (i.e., mimicking psychosis), mind-expanding, or psychedelic drug.
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 such as LSDLSD
or lysergic acid diethylamide
, alkaloid synthesized from lysergic acid, which is found in the fungus ergot (Claviceps purpurea). It is a hallucinogenic drug that intensifies sense perceptions and produces hallucinations, mood changes, and changes in the
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, mescaline, and psylocybin. Hallucinations may occur in normal people under conditions of sensory deprivation, emotional stress, religious exaltation, or great fatigue.
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A perceptual experience in the absence of external stimulation. Hallucinations differ from illusions, which are changes in the perception of a real object. Hallucinations tend to fade with fixation or with attention to the content. Except for afterimages, which lie like a film over objects, hallucinations replace objects and object space. A hallucination is not objectlike in its realness. The conviction of reality is due to the loss of an object for comparison and the inability to disprove the image through other sensory modalities.

Hallucinations that are recognized as such by the experiencer include those resulting from sensory deprivation, drug use, and the phantom limb state. See Schizophrenia

Hallucinations may occur in a range of neurologic and psychiatric conditions, although they are usually considered hallmarks of schizophrenia. Delusional misidentification syndromes are a subtype of hallucinations and may also occur in neurological and psychiatric disease. For example, Capgras syndrome, which is commonly seen in schizophrenia, causes the individual to replace a familiar person (usually the spouse) with an imposter with the same or similar physical appearance. Frégoli syndrome is the delusional confusion of an individual as a familiar person in disguise.

Neurotransmitters are directly involved in the regulation of drug-induced and schizophrenic hallucinations, with many accounts pointing to the involvement of serotonin and dopamine. Therefore, it is possible to treat individuals with antipsychotic drugs that stabilize the chemical systems involved.

With localized damage to the brain, hallucinations are usually brief and intermittent, though in some cases, especially neurologic damage involving the brainstem, hallucinations can be chronic and sustained.

Physical input to the eyes and ears constrains and guides the construction of mental images, but the final result—the perception of an object or sound as a meaningful event occurring in the external world—also reflects very complex physiological processes. They begin in the brainstem, pass to the limbic system of the brain, and finally involve the temporal, parietal, and occipital areas of the cerebral cortex. Various types of hallucination are caused by disruptions that occur at different levels along that sequence of brain processes. See Cognition

At its earliest phase, damage to the upper brainstem produces peduncular (crepuscular) hallucinations of faces, torsos, and occasionally geometric patterns or landscapes near the viewer at the close of day. The images may be static and immobile or may change in content and affective tonality while being viewed. A smiling young boy, for example, may change into a scowling old woman. The hallucinations are often vivid and chromatic, and tend to be multimodal: they are seen, heard, and even touched, and occur over the entire visual field. Olfactory and gustatory images have also been described. Peduncular hallucinations are similar to the hypnagogic hallucinations that are experienced when falling asleep. See Sleep and dreaming

Neurologic damage involving limbic and temporal-lobe structures yields hallucinations of faces or formed scenes laden with meaning and affect. Changes in size (micropsia, macropsia) and shape (metamorphopsia) may occur. Déjà vu, derealization, and dreamy states are common. Auditory hallucinations are usually of speech or music. Microscopic (Lilliputian) and autoscopic (out-of-the-body) hallucinations also occur with temporal-lobe lesions. Exposure to a wide range of drugs and many psychiatric disorders, especially schizophrenia, can lead to hallucinations whose form suggests dysfunction involving limbic or temporal-lobe structures. See Psychotomimetic drug

Damage to the parietal lobe leads to illusory distortions of shape, size, and motion, whereas occipital lesions or stimulation—or migraine—gives elementary hallucinations of sparks, flames, lines, or simple patterns. These hallucinations share features with afterimages. Palinopsia, the hallucinatory persistence of an object after the viewer has turned away, is a form of pathological afterimagery. See Perception

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.


(religion, spiritualism, and occult)

Lewis Spence defined hallucination as, “A false perception of sensory vividness arising without the stimulus of a corresponding senseimpression.” He went on to say that it differed from illusion, which is merely the misinterpretation of an actual sense-perception. Visual hallucinations are the most common, with auditory ones a close second. Nandor Fodor observed, “In the years following the foundation of the SPR [Society for Psychical Research] the hallucination theory of psychic phenomena was in great vogue. If no other explanation were available the man who had a supernormal experience was told that he was hallucinated, and if several people testified to the same occurrence, that the hallucination of one was communicated to the other.” Unfortunately this is often still the case today. Rather than accept the possibility of a supernormal happening, many will dismiss the witnessing of phenomena as “hallucination.”

Sir William Crookes made a wonderful comment on the subject, saying, “The supposition that there is a sort of mania or delusion which suddenly attacks a whole roomful of intelligent persons who are quite sane elsewhere, and that they all concur, to the minutest particulars, in the details of the occurrences of which they suppose themselves to be witnesses, seems to my mind more incredible than even the facts which they attest.” (Researches In the Phenomena of Spiritualism London, 1874)

Fodor gives the example of Sir John Heschel, who had been watching the demolition of an ancient building. The day after its demolition, in the evening, Sir John was passing the site and saw, in its entirety, the building standing as before. “Great was my amazement,” he said, “to see it as if still standing, projected against the dull sky. I walked on and the perspective of the form and disposition of the parts appeared to change as they would have done as real.” In other words, as he walked past, the building appeared as it would have done if viewed from various angles.

Fodor concludes by saying much the same as Spence, “The difference between hallucination and illusion is that there is an objective basis for the illusion, which is falsely interpreted. In hallucination, though more than one sense may be affected, there is no external basis for the perception.”


Fodor, Nandor: Encyclopedia of Psychic Science. London: Arthurs Press, 1933
Spence, Lewis: An Encyclopedia of the Occult. London: George Routledge & Sons, 1920
Halo see Aura
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A perception without an appropriate stimulus.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


the alleged perception of an object when no object is present, occurring under hypnosis, in some mental disorders, etc.
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005