Artificial "ghosts" have been conjured up by scientists in an experiment so spooky that two participants asked for it to stop.
By having signals mixed up in their brains, volunteers were made to feel that a creepy "presence" was behind them.
They counted up to four phantoms positioned where no-one was standing - and touching their backs with invisible fingers.
The illusion was created using a robotic device that confuses people's inner perceptions of "self" and "other".
It suggests that "feelings of presence" (FOPs), often interpreted as spirits, angels or demons, are really all in the mind, say the researchers.
Such experiences are frequently reported by people in extreme physical or emotional situations, such as mountaineers and explorers, or those grieving for a lost loved one.
They are also associated with medical conditions that affect the brain, including epilepsy, stroke, migraine and cancer.
Professor Olaf Blanke, from the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland, said: "Our experiment induced the sensation of a foreign presence in the laboratory for the first time. It shows that it can arise under normal conditions, simply through conflicting sensory-motor signals.
"The robotic system mimics the sensations of some patients with mental disorders or of healthy individuals under extreme circumstances. This confirms that it is caused by an altered perception of their own bodies in the brain."
To manifest their ghosts, the scientists set up a slave robot device that allowed volunteers to control the movements of a jointed mechanical arm with their index fingers.
The movements were translated to another robot arm behind them which touched their backs.
When both the finger-pushing and back-touching occurred at the same time, it created the illusion that the volunteers were caressing their own backs.
That felt weird enough to the blindfolded participants. But something a lot stranger happened when the back-touching was delayed and about 500 milliseconds out of sync with the finger movements.
Suddenly the volunteers felt as if they were being watched, and touched, by one or more ghostly presences.
At the same time, they had the disconcerting sensation of drifting backwards, towards the unseen hand.
When questioned, several reported a strong feeling of invisible people being close to them. On average, they counted two, with up to four being reported.
Two of the 12 healthy participants were so disturbed by the experience that they asked the scientists to halt the experiment, the results of which appear in the journal Current Biology.
Co-author Dr Giulio Rognini, also from the EPFL, said:
"Our brain possesses several representations of our body in space. Under normal conditions, it is able to assemble a unified self-perception of the self from these representations.
But when the system malfunctions because of disease - or, in this case, a robot - this can sometimes create a second representation of one's own body, which is no longer perceived as 'me' but as someone else, a 'presence'."
In their paper, the researchers describe the case of mountaineer, Reinhold Messner, who had an FOP experience while descending from the summit of the Himalayan peak Nanga Parbat in June, 1970.
Accompanied by his brother, he was freezing, exhausted and oxygen-starved. He recalled becoming aware of a third climber "descending with us, keeping a regular distance, a little to my right and a few steps away from me, just outside my field of vision".
Before conducting the experiment, the researchers carried out brain scans of 12 patients with neurological disorders who had encountered FOPs in the past.
They identified disturbances in three specific brain regions, the insular cortex, parietal-frontal cortex, and temporo-parietal cortex. All were involved in self-awareness, movement, and sense of position in space.