The hallucination of a ghostly presence is common among people with psychiatric disorders such as schizophrenia and sometimes occurs in healthy people under extreme conditions. To investigate the cause of this “feeling of presence” (FoP), researchers at the École Polytechnique Fédérale de Lausanne in Switzerland and their colleagues observed study participants’ reactions to a specially designed robot that mimicked their movements. The findings, reported last week (November 6) in Current Biology, suggest that FoP results from a mismatch between perceptions of the self and of external signals.
As part of the study, the team scanned the brains of 12 patients who had reported FoP and discovered lesions in the frontoparietal cortex, which integrates sensory and motor signals and is involved in self-awareness. This led to the idea that FoP is prompted by conflict between external and internal inputs. To test this hypothesis, the researchers created a “master-slave” robot to recreate the phenomenon in healthy people. Blindfolded participants used their index fingers to move a robotic arm in front of them, while a robot behind them made the same movements on their backs. When the movements were simultaneous, people felt as if they were touching their own backs, but delaying the robot’s motions by a half-second produced the illusion of another person behind them.
“Our brain possesses several representations of our body in space,” study coauthor Giulio Rognini told BBC News. “Under normal conditions, it is able to assemble a unified self-perception of the self from these representations,” he explained. “But when the system malfunctions because of disease—or, in this case, a robot—this can create a second representation of one’s own body, which is no longer perceived as ‘me’ but as someone else, a ‘presence’.”
In the future, Rognini and his team hope to create a robot that can help schizophrenic patients to differentiate their own actions from those of others.