The word also is used to describe the sensation of having glimpsed oneself in peripheral vision, in a position where there is no chance that it could have been a reflection. Doppelgängers often are perceived as a sinister form of bilocation and are regarded by some to be harbingers of bad luck. In some traditions, a doppelgänger seen by a person's friends or relatives portends illness or danger, while seeing one's own doppelgänger is an omen of death.
In Norse mythology, a vardøger is a ghostly double who precedes a living person and is seen performing their actions in advance. In Finnish mythology, this is called having an etiäinen, i.e., "a firstcomer". In Ancient Egyptian mythology, a ka was a tangible "spirit double" having the same memories and feeling as the original person. In one Egyptian myth titled "The Greek Princess," an Egyptian view of the Trojan War, a ka of Helen was used to mislead Paris of Troy, helping to stop the war. In some myths, the doppelgänger is a version of the Ankou, a personification of death.
Scientific and philosophical investigations
Left temporoparietal junction
In September 2006, it was reported in Nature that Shahar Arzy and colleagues of the University Hospital, Geneva, Switzerland, unexpectedly had reproduced an effect strongly reminiscent of the doppelgänger phenomenon via the electromagnetic stimulation of a patient's brain. They applied focal electrical stimulation to a patient's left temporoparietal junction while she lay flat on a bed. The patient immediately felt the presence of another person in her "extrapersonal space." Other than epilepsy, for which the patient was being treated, she was psychologically fit.
The other person was described as young, of indeterminate sex, silent, motionless, and with a body posture identical to her own. The other person was located exactly behind her, almost touching and therefore within the bed on which the patient was lying.
A second electrical stimulation was applied with slightly more intensity, while the patient was sitting up with her arms folded. This time the patient felt the presence of a "man" who had his arms wrapped around her. She described the sensation as highly unpleasant and electrical stimulation was stopped.