For most of us, it’s an uncomfortable tingling sensation—that occasional, disturbing feeling that someone is behind us, watching. But for millions of people around the world who suffer from visual and auditory hallucinations, this minor annoyance develops into a frequent torment.  
Feeling of Presence, or FoP, is the disconcerting notion that someone else is hovering nearby, walking alongside you or even touching you. It’s the stuff of ghost stories, but also a real symptom of several neurologic conditions, including schizophrenia and Alzheimer’s disease. Scientists know so little about the underlying causes of FoP that long-term treatments and cures remain illusive.
Now, researchers are chipping away at the neurobiology behind that uncanny feeling. In a paper published November 6 in Current Biology, a team of scientists described how they used a custom-built robot to induce an eerie Feeling of Presence in healthy participants. Their findings confirm that sensorimotor conflict, a neurologic imbalance between what the mind perceives and what the body feels, lies at the root of some FoP illusions.
“You really need a sensorimotor mismatch for Feeling of Presence,” says Giulio Rognini, a doctoral candidate at the Ecole Polytechnique Federale de Lausanne in Switzerland, and coauthor on the paper. “This asynchronous condition makes the subjects more prone to that feeling of somebody behind them.”
The scientists began by examining 12 patients who had reported Feelings of Presence in the past. Virtually all of the subjects described similar hallucinations—the distinct feeling that somebody was directly behind them and uncomfortably close. Brain scans also revealed lesions that dotted the subjects’ frontoparietal cortexes, an area of the brain associated with awareness of “self” and integration of sensorimotor signals.
Disturbing hallucinations followed them like shadows, at times. “When the patient was standing, the presence was standing,” Rognini says. “When the patient was sitting, the presence was sitting. When the patient was moving, the presence was moving.”
Based on these common brain lesions and experiences, the researchers suspected that it was not just any disharmony between movement and sensation that might trigger FoP, but a specific disharmony, perfectly coordinated with the body’s movements. To test this theory in healthy participants, they designed a robot that could precisely match each subject’s motions, while still delivering a confusing, mismatched sensation.   
When a healthy, blindfolded subject would reach forward, the robot would copy his or her exact motion and tap the participant from behind. If the robot-human interaction was perfectly synchronized, the participants simply reported feeling like they were reaching forward and touching their own backs—a disturbing, but not hallucinatory, sensation.
But when the scientists delayed the robotic reaction even slightly, by a half-second, the participants become disoriented. Many said that they felt like someone else was touching their backs, and estimated that the mostly-empty room was full of people. Some added that they felt as though they were drifting backward, toward The Presence; two participants were so disturbed that they asked to stop the experiment.
“What we are doing is manipulating those sensorimotor signals,” Rognini says. “We found that FoP is only experienced when there is a delay between what participants do and a feeling on their backs.”
Rognini suspects that the findings may have broad implications for neurologic and psychiatric patients who suffer from auditory and visual hallucinations. “If you are able to perturb the system, to induce a Feeling of Presence, maybe you could also tune the sensorimotor system to down-regulate those symptoms,” he says.
Nonetheless, there is a major difference between causing a hallucination and curing it. “Inducing a phenomenon is relatively easy—to control it, and stop it temporarily by electrical stimulation of the brain, is also relatively easy,” says Sophia Frangou, chief of the Psychosis Research Program at Mount Sinai’s Icahn School of Medicine. “The problem we have is making sure that this phenomenon doesn’t return.“
Still, Frangou was impressed by the researchers’ willingness to explore the connection between structure and function in the brain, and to apply that to studying the practical symptoms of psychosis. “It’s really an innovative idea in probing the brain in ways that can be meaningful,” she says. “In that respect, I can see an immediate relevance.”
Judith Ford, a neuroscientist at the University of California San Francisco, who specializes in schizophrenia, added that the study might help researchers pin down the mechanism behind Feelings of Presence. “This kind of study is essential to our efforts to understand the neurobiological basis of such symptoms,” she said in a prepared statement.
For Rognini and his team, the next step will be attempting to design a robot that might work with schizophrenic patients to help them distinguish between their own actions and someone else’s actions.
“That would be the dream,” he says. “To, by some robotic simulation, down-regulate psychotic symptoms.”