In 2006, cognitive neuroscientist Olaf Blanke of the University of Geneva in Switzerland was testing a patient’s brain functions before her epilepsy surgery when he noticed something strange. Every time he electrically stimulated the region of her brain responsible for integrating different sensory signals from the body, the patient would look back behind her back as if a person was there, even when she knew full well that no one was actually present.
Now, with the help of robots, Blanke and colleagues have not only found a neurological explanation for this illusion, but also tricked healthy people into sensing “ghosts,” they report online today in Current Biology. The study could help explain why schizophrenia patients sometimes hallucinate that aliens control their movements.
“It’s very difficult to try to understand the mechanisms involved in something so strange,” says cognitive neuroscientist Henrik Ehrsson of the Karolinska Institute in Stockholm, who was not involved with the study. “It’s very encouraging, very impressive, the way this team is making science out of this question.”
Ghosts and apparitions are a common theme in literature and religion. In real life, patients suffering from schizophrenia and epilepsy sometimes report sensing a presence near them. After studying such cases, Blanke found some striking similarities in how epilepsy patients perceive these eerie “apparitions,” he says. Almost all patients said the presence felt like a human being positioned right behind their back, almost touching them, with malicious intentions. Patients with brain damage on the left hemisphere felt the ghost at their right side, and vice versa.
To pinpoint the brain regions responsible for such illusions, Blanke and colleagues compared brain damage in two groups of patients. The first group, mostly epilepsy patients, all reported feeling ghostly presences near them. The other group matched them in terms of the severity of their neurological illnesses and hallucinations, but didn’t perceive any ghostly presence. Brain imaging revealed that patients who sensed the “ghosts” had lesions in their frontoparietal cortex, a brain region that controls movements and integrates sensorimotor signals from the body—such as the “smack” and pain accompanying a punch—into a coherent picture.
The researchers suspected that damage to this region could have disrupted how the brain integrates various sensory and motor signals to create a coherent representation of the body. That may have led the patients to mistakenly feel that someone else, not themselves, were creating sensations like touch.
So the team built a robot to test their theory on healthy people. The machine consisted of two electrically interconnected robotic arms positioned in front of and behind a participant, respectively. The smaller arm in front had a slot where participants could insert their right index fingers and poke around. The poking motion triggered the bigger arm at the back to poke the participants at different positions on their backs, following the movement of their fingers. During the experiments, the participants wore blindfolds and headphones so that they would concentrate on what they felt. They were told that only the robot was poking them at the back, but unbeknownst to them, the back-poking was sometimes synchronized with their finger movements, and sometimes delayed by half a second.
When the participants reported how they felt, a clear pattern emerged. If the back-poking was in sync with the participants’ finger movements, they felt as if they were touching their backs with their own fingers. But when the back-poking was out of sync, a third of the participants felt as if someone else was touching them. The sensation was so spooky that two participants actually asked the researchers to stop the experiment.
To verify the response, the researchers conducted another study in which four researchers stood in the room. Participants were told that while they were blindfolded and operating the machine, some experimenters might approach them without actually touching them. The researchers told participants to estimate the number of people close to them at regular intervals. In reality, no researcher ever approached the participants. Yet people who experienced a delayed touch on their back felt more strongly that other people were close to them, counting up to four people when none existed.
The researchers suspect that when participants poked their fingers in the finger slot, their brains expected to feel a touch on the back right away. The delay created a mismatch between the brain’s expectations and the actual sensory signals it received, which disrupted how the brain integrated the signals to create a representation of the body, and thus created the illusion that another human being was touching them.
The findings could help scientists understand the hallucinations of schizophrenia patients, Blanke says. Scientists have long hypothesized that patients hear alien voices or feel that they are not controlling their own bodies because their brains fail to integrate bodily signals properly.
The researchers are now building an MRI-equipped robot system to study what exactly happens in healthy people’s brains when they feel the ghostly presence and to test how schizophrenia patients would react to the mismatched pokes.
(Video credit: EPFL)