Why do some people see ghosts?

For most people ghosts and spirits are part of the imaginary, but a few are truly convinced they can sometimes feel a strange presence near them. These individuals are not experiencing a paranormal phenomenon—they’re having an illusion. Schizophrenics, for instance, consistently report hearing voices or feeling someone—a ‘shadow’ or a ‘man’—close to them. Scientists have long known that illusions have a neurological cause, but they haven’t managed to pinpoint exactly how they are triggered by the brain.

Now, Olaf Blanke and colleagues have not only mapped the brain regions responsible for the ‘feeling of a presence’ illusion in neurological patients, but they have also developed a robot that tricks healthy people into sensing a ‘ghostly’ apparition. This work may shed light into what causes hallucinations in schizophrenia, and help design new therapeutic approaches to treat this psychosis.

 

Credit: Alain Herzog, EPFL.

In 2006, Blanke showed that he could induce the feeling of a presence in an epileptic patient by electrically stimulating a particular brain area—the temporoparietal junction. This region is involved in integrating body-related information from our senses and movements, and is often overactive in schizophrenic patients. But he found something even more interesting: the presence always mirrored the patient’s body position and movements; if the patient was sitting, the presence was also sitting and so on. “The presence was a duplicate of the patient, as if the patient’s body was recognised as another agent”, says Giulio Rognini, a collaborator at the Ecole Polytechnique Fédérale de Lausanne. “The body sensory information, which is not well integrated by the brain, is attributed to someone else.”

The researchers suspected that electrical stimulation of the temporoparietal region somehow disturbed integration of the patient’s sensory and motor information—her brain got confused and misplaced the bodily signals to the presence. To test this hypothesis, the team needed to be creative.

“The patient studies show that when there is no appropriate integration of the body sensory signals, then the feeling of a presence can occur, so we tried to do the reverse process: we perturbed the sensory motor system to see whether we could induce the presence”, says Rognini. And what better way to do this than with… a robot.

In their new study, Blanke and colleagues asked 12 blindfolded healthy participants to stick their finger into a ‘master’ robot and then move it around. The ‘slave’ robot, which was touching the participants’ back, mimicked the movements of the master robot either simultaneously, or with a slight delay. In the first condition (simultaneous touch), the participants felt as though they were touching their own back. This is already a strange illusion, but what happened when the slave robot poked them with a slight delay relative to the master robot is even weirder. About a third of the participants felt like someone else was touching them. Not the robot, but just ‘someone’, a presence. This illusion was short lived, but according to the participants’ description, it was very vivid and also a bit creepy.

“30% [of the participants] reported without asking them that they had a feeling of a presence. This is already very strong because in this field of body illusions, it’s very rare to find somebody that reports the illusion without being asked” says Rognini, who is senior author in the study.

The team also mapped the brain regions that trigger the illusions in several neurological patients. As expected, electrical stimulation of the temporoparietal, but especially the frontoparietal brain regions, induced the illusion. And again, most patients reported that the presence mimicked their movements.

 

Lesion overlap analyses revealed three brain regions involved in the feeling of a presence illusion: temporo-parietal 
and fronto-parietal cortex (© Current Biology)

The feeling of a presence is mostly associated with epilepsy and schizophrenia, but healthy people can also feel ‘ghosts’, especially during periods of extreme stress or physical exhaustion. Many mountaineers report they sometimes feel someone climbing with them, even though there was no one around. “If you’re walking and doing repetitive movements over and over again, your brain loses control over your movements because they’re not informative anymore”, says Rognini. “Your actions and the consequences of your actions can be misinterpreted, and together with low oxygen conditions in high altitude, this could give rise to feeling of a presence. But this is completely speculative.” The researchers are planning to test this hypothesis by trying to exhaust people in treadmills, and then check whether they are more prone to experiencing the illusion. They are also developing an fMRI-compatible robot to induce the illusion while the participants are being scanned.

“The next steps are about understanding the brain mechanisms by putting the subjects in the scanner, and then try to investigate how this phenomenon is perceived in schizophrenic patients to try to set out a therapeutic strategy or a way to better understand this psychosis,” says Rognini.

Herta Flor, director of the Institute of Cognitive and Clinical Neuroscience of the University of Heidelberg (Germany) says “Disturbed body perception is a core feature in several mental disorders, such as schizophrenia or borderline-personality disorder. To be aware of the underlying neural mechanisms might not only help to understand clinically altered behaviour in patients, but may lead to innovative treatment approaches.”

Reference:

Blanke O., Masayuki Hara, Lukas Heydrich, Andrea Serino, Akio Yamamoto, Toshiro Higuchi, Roy Salomon, Margitta Seeck, Theodor Landis & Shahar Arzy & (2014). Neurological and Robot-Controlled Induction of an Apparition, Current Biology, 24 (22) 2681-2686. DOI: http://dx.doi.org/10.1016/j.cub.2014.09.049

An edited version of this article was published in Lab Times on the 19-12-2014. You can red it here.