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. These definitions distinguish hallucinations from the related phenomena of dreaming, which does not involve consciousness; 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.[1] Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted genuine perception is given some additional (and typically bizarre) significance.
Hallucinations may 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.
Hypnagogic hallucinations and hypnopompic hallucinations are considered normal phenomena. Hypnagogic hallucinations can occur as one is falling asleep and hypnopompic hallucinations occur when one is waking up. Hallucinations may also be associated with drug use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens. |