Autoscopy is the experience in which an individual perceives the surrounding environment from a different perspective, from a position outside of his or her own body. Autoscopy comes from the ancient Greek ("self") and ("watcher").

Autoscopy has been of interest to humankind from time immemorial and is abundant in the folklore, mythology, and spiritual narratives of most ancient and modern societies. Cases of autoscopy are commonly encountered in modern psychiatric practice. According to neurological research, autoscopic experiences are hallucinations.

Experiences - are characterized by the presence of the following three factors:

- disembodiment, an apparent location of the self outside one's body;

- impression of seeing the world from an elevated and distanced visuo-spatial perspective or extracorporeal, but egocentric visuo-spatial perspective;

- impression of seeing one's own body from this perspective (autoscopy).

Laboratory of Cognitive Neuroscience, École Polytechnique Fédérale de Lausanne, Lausanne, and Department of Neurology, University Hospital, Geneva, Switzerland, have reviewed some of the classical precipitating factors of autoscopy. These are sleep, drug abuse, and general anesthesia as well as neurobiology. They have compared them with recent findings on neurological and neurocognitive mechanisms of the autoscopy. The reviewed data suggest that autoscopic experiences are due to functional disintegration of lower-level multisensory processing and abnormal higher-level self-processing at the temporoparietal junction.

A hallucination is a perception in the absence of external stimulus that has qualities of real perception. Hallucinations are vivid, substantial, and are perceived to be located in external objective space. They are distinguishable from several related phenomena, such as dreaming, which does not involve wakefulness; pseudohallucination, which does not mimic real perception, and is accurately perceived as unreal; illusion, which involves distorted or misinterpreted real perception; and imagery, which does not mimic real perception and is under voluntary control. Hallucinations also differ from "delusional perceptions", in which a correctly sensed and interpreted stimulus (i.e., a real perception) is given some additional (and typically absurd) significance.

Hallucinations can occur in any sensory modality—visual, auditory, olfactory, gustatory, tactile, proprioceptive, equilibrioceptive, nociceptive, thermoceptive and chronoceptive.

A mild form of hallucination is known as a "disturbance", and can occur in most of the senses above. These may be things like seeing movement in peripheral vision, or hearing faint noises and/or voices. Auditory hallucinations are very common in schizophrenia. They may be benevolent (telling the subject good things about themselves) or malicious, cursing the subject, etc. Auditory hallucinations of the malicious type are frequently heard, for example people talking about the subject behind his/her back. Like auditory hallucinations, the source of the visual counterpart can also be behind the subject's back. Their visual counterpart is the feeling of being looked or stared at, usually with malicious intent. Frequently, auditory hallucinations and their visual counterpart are experienced by the subject together.

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 can be associated with drug use (particularly deliriants), sleep deprivation, psychosis, neurological disorders, and delirium tremens.

The word "hallucination" itself was introduced into the English language by the 17th century physician Sir Thomas Browne in 1646 from the derivation of the Latin word "alucinari" meaning to wander in the mind. For Browne, hallucination means a sort of vision that is "depraved and receive[s] its objects erroneously".

Hallucinations may be manifested in a variety of forms. Various forms of hallucinations affect different senses, sometimes occurring simultaneously, creating multiple sensory hallucinations for those experiencing them.