People who've stared death in the face and lived to tell about it—mountain climbers who've made a harrowing descent, say, or survivors of the World Trade Center attacks—sometimes report that just when their situation seemed impossible, a ghostly presence appeared. People with schizophrenia and certain types of neurological damage sometimes report similar experiences, which scientists call, aptly, "feeling of presence."

Now a team of neuroscientists says it has identified a set of brain regions that seems to be involved in generating this illusion. Better yet, they've built a robot that can cause ordinary people to experience it in the lab.

The team was led by Olaf Blanke, a neurologist and neuroscientist at the Swiss Federal Institute of Technology in Lausanne. Blanke has a long-standing interest in creepy illusions of bodily perception. Studying these bizarre phenomena, he says, could point to clues about the biology of mental illness and the mechanisms of human consciousness.

In 2006, for example, Blanke and colleagues published a paper in Nature that had one of the best titles you'll ever see in a scientific journal: "Induction of an illusory shadow person." In that study, they stimulated the brain of a young woman who was awaiting brain surgery for severe epilepsy. Surgeons had implanted electrodes on the surface of her brain to monitor her seizures, and when the researchers passed a mild current through the electrodes, stimulating a small region at the intersection of the temporal and parietal lobes of her brain, she experienced what she described as a shadowy presence lurking nearby, mimicking her own posture.

Colored areas indicate regions of overlap in the lesions of neurological patients who experienced feeling of presence illusions. Blanke et al., Current Biology

The new study also implicates this region, the so-called temporoparietal junction, as well as two others. Blanke and colleagues examined 12 people who experienced a feeling of presence after brain damage resulting from epilepsy, stroke, or other causes. Like the epilepsy patient in the earlier study, these patients tended to describe the presence as somewhat menacing, Blanke says. "It's an uncanny feeling … that they're not exactly in danger, but this other presence doesn't want anything good."

Using MRI scans, the researchers identified three brain regions that were most often damaged in these patients: the temporoparietal junction, the insula, and the frontoparietal cortex. All three are thought to play a role in integrating sensory signals from outside the body (like what you see and hear around you) with signals from inside the body (like signals from the joints and muscles that indicate your posture and the position of your limbs).

To see if they could create the feeling-of-presence illusion in healthy people, the used what they call a "master-slave robotic system" built by roboticist Giulio Rognini, who's also based at the Swiss Federal Institute of Technology in Lausanne. To interact with the robot, subjects insert an index finger into a mechanical arm. This arm is the "master" part of the robot. The "slave" component is another arm located directly behind the subject that mimics the motion of the master. So, for example, if the subject makes a poking motion, the robot pokes him or her in the back (you can see it in action starting at about 45 seconds into the video below).

With the robot in this configuration, subjects reported the strange feeling that they were poking themselves in the back. But things got even weirder when the researchers introduced a delay, so that the poke in the back lagged by half a second. This caused some subjects to feel they were being poked by an invisible presence lurking behind them. The illusion was generally weaker in the healthy people than it was in the neurological patients, Blanke says, and some people were far more susceptible to it than others. The researchers report their findings today in Current Biology.

Here's Blanke's hypothesis about what's going on: Normally, the parts of the brain that initiate movements send a signal to sensory regions of the brain to give them a heads up; But when the robot is in delay mode there's a disconnect between the movement signal (initiate poke!) and the sensation (um, not feeling any poking). It's a mismatch that almost never happens in real life. The researchers suggest that the subjects' brains reconcile this mismatch by creating the illusion of another presence that's doing the poking. After all, something is poking them in the back.

This kind of sensory-motor mismatch has been proposed previously to explain some of the symptoms of schizophrenia, most notably by the cognitive neuroscientist Chris Frith at University College London. For example, it's very hard for healthy people to tickle themselves—you don't experience that funny feeling if you know what's coming. But Frith and colleagues have found that people with schizophrenia can tickle themselves, suggesting that they perceive motions that they themselves have initiated as coming from another source. In a similar way, people with the disorder might misattribute their own internal speech to an external agent. It's no wonder paranoia is so common.

"There's convergent evidence that this type of action monitoring is essential for the sense of self, the sense that we are in charge of our own bodies," said neuroscientist Peter Brugger of University Hospital Zurich. Blanke's new study is important, Brugger says, because it makes it possible to study an experience that's very real to many psychiatric and neurological patients, but very foreign to everybody else. "It's a very elegant way to explore something that other people think is just bizarre and a part of their world we can never access."

As to why feeling of presence seems to be especially common in life-threatening situations, Blanke says he can only speculate. In some situations, the lack of oxygen might make the illusion more common, he says. But he also notes that people who are recently widowed often report feeling the presence of their lost spouse. Clearly there are other psychological factors that can come into play, he says.