A hallucination, in the broadest sense, is a perception in the absence of a stimulus. In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid, substantial, and located in external objective space. The latter definition distinguishes hallucinations from the related phenomena of dreaming, which does not involve wakefulness; illusion, which involves distorted or misinterpreted real perception; imagery, which does not mimic real perception and is under voluntary control; and pseudohallucination, which does not mimic real perception, but is not under voluntary control. Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted genuine perception is given some additional (and typically bizarre) significance.
Hallucinations can occur in any sensory modality — visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, and thermoceptive.
A mild form of hallucination is known as a disturbance, and can occur in any of the senses above. These may be things like seeing movement in peripheral vision, or hearing faint noises and voices.
Hallucinations can also be associated with drug or alcohol use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.
Hallucinations typically occur in the context of neurological or mental disorder, although they are not tied to any particular disease and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders and particularly in schizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.
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