A tickle on the back of your neck, a sudden feeling that you are not alone: some believers in the paranormal attribute these sensations to the presence of a ghost, angel or otherworldly being.
Now, a team of Swiss neuroscientists have figured out how to conjure those spirits—or at least the perception of them—in the lab. The sensation of an otherworldly presence, they found, actually derives from garbled sensorimotor brain signals, in which a person's self awareness of their own body is projected into a seemingly disconnected space. In such cases, the researchers explained in a release, the brain mis-assigns its own life signals as belonging to someone or something else.
The team first studied the brains of a dozen patients who all suffered from such apparitions—a common symptom for people with some forms of epilepsy, schizophrenia or other neurological disorders. MRI analyses showed that they all had abnormal activity in three regions of the brain involved in self-awareness, movement and proprioception (the sense of one's position in space), the researchers reported.
Next, they attempted to recreate this neurological experience in healthy volunteers. They blindfolded their subjects and asked them to perform a series of movements with their arm while hooked up to the master portion of a "master-slave" robot. At the same time, the slave part of the robot, situated behind the person, perfectly mimicked the participant's movements and traced that pattern on his or her back. Eventually, the robot introduced a time delay between the actual and mimicked movement, creating a wrinkle in temporal and spatial perception.
After three minutes, the researchers asked the participants—who had no idea what the aim of the study was—how they felt. Several reported "a feeling of a presence," and some counted up to four ghosts in the room, the researchers reported. Others were so creeped out that they asked to stop the experiment.
When our brains inaccurately represent our bodies in space, whether due to disease or an uncanny robot, the researchers concluded in the release, "this can sometimes create a second representation of one's own body, which is no longer perceived as 'me' but as someone else, a 'presence.'"
Here, you can see more about how the experiment worked: