Feeling of a presence is a curious phenomenon reported by people with neurological and psychiatric disorders where the presence of something strange nearby, like a ghost or angel, is not seen, but somehow felt. FoP has also been reported by explorers and mountaineers -- and now a robot has helped reveal how it happens.
The study was conducted on 12 individuals with various neurological conditions such as epilepsy, stroke, migraine and tumours, each of them previously experiencing FoP for a period of seconds to minutes as a result of their conditions. The team, led by Professor Olaf Blanke, MD, PhD, of Ecole Polytechnique Fédérale de Lausanne, Switzerland, was able to trace the patients' FoP to damage in any of three brain regions: the temporoparietal, the insular, and -- most especially -- the frontoparietal cortex. These cortices are involved with self-awareness, movement, and spatial positioning.
Yet with otherwise healthy individuals under extreme circumstances, brain damage isn't necessarily the cause. Examining the patients' symptoms further -- such as sensorimotor deficits -- the researchers surmised that, in addition to brain lesions, FoP can be caused by confusion over the source of sensorimotor signals -- that is, individuals mistake their own bodily movements as belonging to something else.
Armed with this knowledge, the team created the robot, designed to mix up the sensorimotor input of healthy individuals in such a way that their brains no longer recognised those signals as belonging to their own body -- rather, they interpreted them as belonging to a mysterious, invisible presence.
"In our first experiment [with the robot], thirty percent of the healthy participants spontaneously reported the feeling of having somebody behind them, touching them," Professor Blanke said. "Such spontaneous reports are quite unusual in the field of bodily illusions."
In fact, two of the healthy study participants were so perturbed that they requested the experiment to stop.
The participants were connected to a master-slave robot and blindfolded. As they made movements with their hands in front of their body, the robot reproduced the movement in sync, touching the participant's back. Because of the synchronisation, the brain was able to correct for the spatial discrepancy.
Next, the researchers introduced a delay between the participant's movements and the robot's copy of those movements. The spatial and temporal discrepancy together created the illusion of a ghost: after about three minutes of the delayed touching, the study participants reported a strong FoP, even counting up to four ghosts.
"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," Blanke. "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."
This not only helps to explain a phenomenon reported around the world, but aims to offer insight into some of the symptoms experienced by schizophrenia patients, such as the presence of an alien, demonic or angelic entity.
The full paper, "Neurological and Robot-Controlled Induction of an Apparition", can be found online in the journal Current Biology.