With a tap on the back, researchers create ghostly sensation

Using well-timed back taps supplied by an odd contraption, scientists have figured out how to induce the weird sensation that someone is standing nearby when not a soul is there.

This eerie experimental effect represents a way to explore how some people supposedly experience the unseen presence of ghosts and other apparitions, say neuroscientist Olaf Blanke of École Polytechnique Fédérale de Lausanne in Switzerland and his colleagues. The new findings, to be published in the Nov. 17 Current Biology, support the idea that in schizophrenia, a breakdown in monitoring sensations produced by one’s own actions contributes to hallucinations and delusions.

“If certain sensory signals are not properly synchronized, the brain mistakes the self for another person,” Blanke says.

Experimentally induced illusions of an unseen entity don’t have the same urgency as real-life feelings that a ghostly entity lurks nearby, says neuropsychologist Peter Brugger of University Hospital Zurich. But similar brain processes may underlie both experiences, he suggests.

A sense of being accompanied by an invisible presence occurs most often during especially taxing tasks, as in mountain climbers’ accounts of having been trailed by climbers they couldn’t see, says Stanford University anthropologist Tanya Luhrmann. Ghosts are sometimes sensed as unseen spirits, but people in various cultures often say that they have actually seen or heard ghosts, she adds.

Blanke’s team first took brain scans of 24 individuals with epilepsy and other neural conditions, 12 of whom reported a strong sense of an unseen person just behind or to the side of their bodies. Brain comparisons identified three regions, which together help integrate movement and touch sensations, as critical to feeling an invisible presence nearby.

The researchers then tested healthy volunteers who stood between two interconnected machines. Each participant inserted an index finger into a slot in the machine in front of them. The finger slot enabled an individual to reach out with a mechanical arm to touch a virtual person’s back, which was seen in 3-D through a head-mounted device. On some trials, the machine delivered fingertip pressure that coincided with a series of virtual touches. During three-minute trials, a mechanical arm from the second machine touched volunteers’ backs either at the same moment they touched the virtual back or with a half-second lag.

In one experiment with the two experimental touches occurring simultaneously as fingertip pressure was delivered, 17 young adults reported feeling that they had touched their own backs despite having extended their arms forward. Participants also felt that their bodies had drifted forward toward where they felt their outstretched hands. Similar manipulations cause people to perceive a rubber hand or another person’s body as their own (SN: 12/6/08, p. 16).

After the experiment, five volunteers spontaneously told researchers that, when feeling touches on their back just after their own virtual touches, they had sensed the presence of another person and felt that person’s touch. These individuals also felt that their bodies had drifted back toward the position of the invisible presence.

In a follow-up, 14 of 19 volunteers reported sensing and feeling an invisible presence nearby when their own touches that included fingertip pressure and the machine’s touches were out of sync.

That illusion doesn’t totally correspond to the experience of people with schizophrenia, some of whom report feeling controlled by space aliens or other distant forces, says neuropsychologist Chris Frith of University College London. “But techniques Blanke is developing will soon enable a much better understanding of these strange experiences,” Frith says.