An occasional, disturbing feeling that someone is behind us, watching -- what neuroscientists call Feeling of Presence or FoP -- may seem a minor mystery to most but can be a true torment for people who suffer from visual or auditory hallucinations.
While the phenomenon may be discounted as something for a ghost story, it is a real symptom experienced by people with some neurological conditions, including schizophrenia and Alzheimer's disease, researchers say.
And now some researchers at the Ecole Polytechnique Federale de Lausanne in Switzerland say they've managed to create the illusion of such a presence in a laboratory setting, and offered a simple neurological explanation.
A "feeling of a presence" can result from an disruption of "sensorimotor" brain signals, which we utilize to create self-awareness by bringing together data from our movements and from our body's position in space, they suggest.
In a laboratory experiment using a custom-built robot, scientists led by EPFL researcher Olaf Blanke were able to interfere with study participants' sensorimotor input to the extent that their brains couldn't identify the signals as belonging to their own body, but rather interpreted them as those of a ghostly "someone else."
The researchers analyzed the brains of 12 patients with neurological disorders, focusing on areas involved in multisensory signal processing, important for the perception and awareness of one's own body.
In an experiments designed to create signal "dissonance," blindfolded study participants were told to make hand motions in front of their body; the custom-built robot reproduced the movements in real time, touching the backs of the participants.
That created a sort of spatial discrepancy, but since the robot's moves were synchronized with the participants', their brains were able to adapt and understand it.
However, when the researchers introduced a time delay between the participants' movements and the robot's touches, it distorted the participants' spacial and temporal perception, and many would suddenly report feelings of a "ghostly presence."
"For some, the feeling was even so strong that they asked to stop the experiment," said researcher Giulio Rognini.
The neuroscientists say their finding could lead to better understanding of the symptoms experience by people with schizophrenia, who often have delusions or hallucinations of the presence of an entity they can hear or feel.
Many scientists have theorized that such perceptions are down to a malfunction of the circuits in the brain that pull together sensory information linked to our body movements or its position.
Rognini say the new study strongly supports that.
"Our brain possesses several representations of our body in space," he says, explaining that under normal conditions, our brains are 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.'"
