A hallucination is a false perception occurring without any identifiable external stimulus and indicates an abnormality in perception. The false perceptions can occur in any of the five sensory modalities. Therefore, a hallucination essentially is seeing, hearing, tasting, feeling, or smelling something that is not there. The false perceptions are not accounted for by the person's religious or cultural background, and the person experiencing hallucinations may or may not have insight into them. Therefore, some people experiencing hallucinations may be aware that the perceptions are false, whereas others may truly believe that what they are seeing, hearing, tasting, feeling, or smelling is real. In cases when the person truly believes the hallucination is real, the individual may also have a delusional interpretation of the hallucination.
Hallucinations must be distinguished from illusions, which are misperceptions of actual external stimuli. In other words, an illusion is essentially seeing, hearing, tasting, feeling, or smelling something that is there, but perceiving it or interpreting it incorrectly. An example of an illusion might be hearing one's name called when the radio is playing. There is an external auditory stimulus, but it is misperceived. True hallucinations do not include false perceptions that occur while dreaming, while falling asleep, or while waking up. Unusual perceptual experiences one may have while falling asleep are referred to as hypnagogic experiences. Unusual perceptual experiences one may have while waking up are referred to as hypnopompic experiences. Hallucinations also do not include very vivid experiences one may have while fully awake (such as especially vivid daydreaming or imaginative play).
Hallucinations are a symptom of either a medical (e.g., epilepsy), neurological, or mental disorder. Hallucinations may be present in any of the following mental disorders: psychotic disorders (including schizophrenia, schizoaffective disorder, schizophreniform disorder, shared psychotic disorder, brief psychotic disorder, substance-induced psychotic disorder), bipolar disorder, major depression with psychotic features, delirium, or dementia. Auditory hallucinations, in particular, are common in psychotic disorders such as schizophrenia.
Use of certain recreational drugs may induce hallucinations, including amphetamines and cocaine, hallucinogens (such as lysergic acid diethylamide or LSD), phencyclidine (PCP), and cannabis or marijuana. For example, visual hallucinations are commonly associated with substance use. Individuals may report false perceptions of little people or animals (sometimes referred to as Lilliputian hallucinations). In addition, withdrawal from some recreational drugs can produce hallucinations, including withdrawal from alcohol, sedatives, hypnotics, or anxiolytics. Withdrawal from alcohol, for instance, commonly causes visual hallucinations, especially at nighttime.
Hallucinations are categorized according to which sensory modality is involved and, in addition, are categorized as either mood-congruent or mood-incongruent. The types of hallucinations are:
• Auditory: The false perception of sound, music, noises, or voices. Hearing voices when there is no auditory stimulus is the most common type of auditory hallucination in mental disorders. The voice may be heard either inside or outside one's head and is generally considered more severe when coming from outside one's head. The voices may be male or female, recognized as
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