"We also found that the hallucinated presence was usually in the same position as the subject -- if the patient was standing, so was the sensed presence, and the same if the patient was sitting," said lead author Giulio Rognini, a post doctoral researcher at the Swiss Federal Institute of Technology in Lausanne. "So we felt that the feeling of a presence was being caused by a misperception of ones own bodily signals."

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Rognini and his collaborators, who included Olaf Blanke -- known for his experiments in inducing out-of-body experiences -- set about to create this same mind-body confusion in healthy patients.

Their robot was a great success: While blindfolded, subjects insert their finger into a mechanism and push it around. This prompts the robot to mimic the motion with a pointer pressed against the subject's back.

None of the subjects knew they were meant to experience anything spooky, and they all knew how the machine worked. When the movement of their finger was synced to the robot's poking and prodding, they didn't report any unusual sensations.

But when the robot was placed out of sync with the controller -- meaning that the robot prodder's movements, while driven by the study subject's own finger, happened slightly later -- things got creepy.

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After these out-of-sync tests, subjects who were asked how they felt reported feeling like someone was standing behind them. Many of them began drifting backwards, towards the presence they felt. Two of them were so uneasy during the experiment that they asked if they could stop.

"Several of them really did say the sensation was spooky and creepy, even though consciously they knew where the touching came from," Rognini said.

And the sense of an extra person went farther than just an eerie sensation. When asked to list how many people had been nearby, the study subjects who'd been paired with an out-of-sync robot counted extra people -- one more, on average, than those who'd had direct control over the robot.