Don’t turn around. He’s standing right behind you.

Nature takes a second look at the neurology of feeling a presence right next to you:

Some people with relatively rare types of brain injury also experience this ‘feeling of a presence’. A study involving a dozen such patients, published today in Current Biology, suggests that the eerie feeling may arise when a person’s brain fails to integrate properly the different signals it receives from the limbs, such as those generated by touch and information about their position in space. The researchers also reproduced the illusion in healthy volunteers in the lab, with the help of purpose-built robots.

The results show that not all types of spookiness emerge in the same way from the brain. “They show that the neural networks involved in the feeling of a presence are not the same as those involved in out-of-body experiences or in seeing a doppelgänger,” says the lead author of the study, cognitive neuroscientist Olaf Blanke of the Swiss Federal Institute of Technology of Lausanne (EPFL).

Blanke suspected that the effect could be due to the brain misperceiving the source and identity of signals from the body generated by touch, perception of one’s position in space (also known as proprioception) and movement. He designed experiments to see whether it was possible to induce the illusion in healthy volunteers….

The team set up a master–slave robotic system to test the proposition. In a series of experiments, participants — who were unaware of the researchers’ purposes — had to move a handle on the master robot in front of them with their right index finger; a slave robot behind them would then touch them on the back with a similar movement, either right away or with a half-second delay.

That touch was the only sensory information about their environment that the participants received: they were blindfolded and wore headphones delivering white noise to mask the sounds of the robots’ movements.

When the robot’s feedback was immediate, most participants soon began to feel that they were touching themselves, even though they were reaching forwards. But when there was a delay, they felt more often that the touch was coming from someone, or something, else. Many had a spooky feeling that a presence was close behind them doing the touching. They also felt that their bodies were farther back in the room than they really were, and closer to the invisible presence.