There was someone behind her; she was sure of it. A malignant presence: always a man, and always to her right. But whenever she looked around, he was gone.
It was 2006. The woman was in University Hospital, Geneva, awaiting surgery to remove parts of her brain that were causing epileptic seizures. The surgeons, led by neurologist Olaf Blanke, prepared for the operation by electrically stimulating her brain, to identify important regions that they needed to preserve. When they shocked one specific area—the left temporoparietal junction—she immediately felt a phantom menace behind her.
She turned to look and when Blanke asked her why, she said that she felt a “shadow” behind her, silent and still. She did the same thing every time the team stimulated the same region. “It was so real,” says Blanke. “She wasn’t convinced by us telling her that it wasn’t real.”
This sort of feeling isn’t new. Since time immemorial, humans have traded stories about ghosts and wraiths—haunting presences that are strongly felt but never seen. Mountaineers often report feeling an unseen presence keeping in step beside them. And many people with neurological or psychiatric problems have reported similar sensations. Blanke was seeing the same phenomenon at work in his patient, but with one critical difference: he could turn it on and off.
His team asked the woman to change position. She laid flat on the bed or sat up, holding her knees. In each case, the phantom assumed the same posture. That was a huge clue. Blanke suspected that her brain was somehow misplacing her sense of self.
That’s an odd concept. As you read this, you know that you are located inside your own body—a feeling so ingrained that it seems facile and absurd to even state it. But it turns out that your brain continuously constructs a feeling of body ownership, and that this seemingly hard-wired sensation is actually rather brittle. Scientists can easily disrupt it through simple illusions, which convince people that they’ve swapped bodies with a partner or are having an out-of-body experience. Blanke wondered if he could develop an illusion that could make a healthy person feel a ghostly presence.
Together with Giulio Rognini, a biomedical engineer from the EPFL in Switzerland, he designed a set-up involving two robots—a master that sits in front of a volunteer, and a slave that sits behind. You stick your right index finger in the master and move it around. These movements are sent to slave, which prods you in the back, using the same pressure, timing, and pattern. Meanwhile, the master also gives you tactile feedback through your fingertip. As a result, you feel that you’re stroking your own back, even though your arm is stretched out in front of you.
It’s a neat trick. But things got really interesting when the team added a short delay—a half-second difference between the volunteers moving the master, and the slave prodding them in return.
Suddenly, five participants felt that there was someone in the room, standing behind them and touching them—the same feeling that Blanke’s patient had. This is the first time that anyone has been able to deliberately recreate that feeling in a lab.
Now, you could argue that someone actually was standing behind them and touching them—the slave robot! But the volunteers all saw the set-up and they all knew about the robots. That didn’t matter; they genuinely and strongly felt that there was an actual person in the room with them, prodding them in the back. And it was a totally different feeling than when the master and slave robots were moving in sync.
The mismatched sensory and motor information confused their brains. If I prod myself in the chest right now, I can feel my joints moving, see my finger hitting my sternum, feel my chest through my finger, and feel my finger through my chest. Everything agrees, and I know that I’m inside my body. If those bits of information didn’t match up, my brain would revise its perception of reality to account for the discrepancies. Maybe I’m not inside my body at all, it might think. Maybe I’m over there.
Blanke suspects this is what happens in his volunteers, when they fall for the illusion. Their brains create two representations of their bodies: a strong one in the usual place, and a weaker one behind them. It’s a partial out-of-body experience.
The team found support for this idea by asking the volunteers to imagine throwing a ball to the opposite wall, and holding a button for as long as the ball is in the air. This simple task measures where the volunteers think they are. If they hold the button for a longer time, it’s because they intuitively feel that they’re further back in the room. And sure enough, the people who felt a strange presence held the button for longer than those who didn’t.
Blanke now wants to scan the volunteers’ brains to see what happens when they fall for the illusion. He has some hints already, through studying 12 people who had suffered from brain damage and regularly felt an unseen presence. All of them had damage to the same three parts of the brain: the insula, the frontoparietal cortex (FPC), and the temporoparietal cortex (TPC). Of these, only the FPC seems to be specifically damaged in people who feel a presence, and not in those who experience other types of hallucination.
This makes sense. These areas are involved in combining information from our senses and our movements, and in controlling our bodies. And other scientists have found that people who fall for body ownership illusions have stronger activity in these same areas. “It makes sense that lesions in these areas could lead to a breakdown in the normal integration of bodily signals,” says Henrik Ehrsson from the Karolinska Institute, who also studies body ownership. “Sensory information gets misplaced in space, leading to the feeling of a presence.”
These kinds of mismatches might occur spontaneously. Blanke thinks this is why so many mountaineers have experienced a presence beside them. “You’re exhausted. You have low oxygen. You don’t see anything except white and grey, and you just put one foot in front of another again and again,” he says. “So, you have a brain in an altered state of consciousness and also a robot-like repetitive state of motion.” There’s a lot of potential for the sensory and motor information in your brain to slip out of sync.
It’s possible that social isolation and extreme stress might so something similar, which might explain why people often see apparitions in times of loneliness or anxiety. They aren’t witnessing supernatural entities. They’re just feeling themselves, but relocated several feet outside of their bodies by a confused and misinformed brain.
This is a hypothesis, but one that the team can now test in their lab. “We think if you take people into an exhaustive state on a treadmill, their ability to feel a presence would be enhanced,” says Blanke.
Ehrsson adds that although the phantom presences are usually anonymous “shadows”, three of the patients in the study felt that actual family members were behind them. “What are the brain mechanisms that kick in when unknown shadow becomes a real person with a mind of its own?” he asks.
Reference: Blanke, Pozeg, Hara, Heydrich, Serino, Yamamoto, Higuchi, Salomon, Seeck, Landis, Arzy, Herbelin, Bleuler & Rognini. 2014. Neurological and Robot-Controlled Induction of an Apparition, Current Biology http://dx.doi.org/10.1016/j.cub.2014.09.049