In a first, Swiss researchers have succeeded in recreating a ghost illusion in the laboratory.
Patients suffering from neurological or psychiatric conditions have often reported feeling a strange “presence” that is felt but unseen, akin to a guardian angel or a demon.
Researcher Olaf Blanke’s team at Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland was able to recreate the illusion of a similar presence in the laboratory and provide a simple explanation.
They showed that the “feeling of a presence” actually results from an alteration of sensorimotor brain signals, which are involved in generating self-awareness by integrating information from our movements and our body’s position in space.
Blanke’s team interfered with the sensorimotor input of participants in such a way that their brains no longer identified such signals as belonging to their own body, but instead interpreted them as those of someone else.
The researchers first analysed the brains of 12 patients with neurological disorders – mostly epilepsy – who have experienced this kind of “apparition.”
MRI analysis of the patients’s brains unveiled interference with three cortical regions: the insular cortex, parietal-frontal cortex, and the temporo-parietal cortex.
These three areas are involved in self-awareness, movement, and the sense of position in space (proprioception).
They contribute to multisensory signal processing, which is important for the perception of one’s own body.
The scientists then carried out a “dissonance” experiment in which blindfolded participants performed movements with their hand in front of their body.
Behind them, a robotic device reproduced their movements, touching them on the back in real time. The result was a kind of spatial discrepancy, but because of the synchronised movement of the robot, the participant’s brain was able to adapt and correct for it.
Next, the neuroscientists introduced a temporal delay between the participant’s movement and the robot’s touch.
Under these asynchronous conditions, distorting temporal and spatial perception, the researchers were able to recreate the ghost illusion.
The participants were unaware of the experiment’s purpose. After about three minutes of the delayed touching, the researchers asked them what they felt.
Instinctively, several subjects reported a strong “feeling of a presence,” even counting up to four “ghosts” where none existed.
“For some, the feeling was even so strong that they asked to stop the experiment,” said Giulio Rognini, who led the study.
“Our experiment induced the sensation of a foreign presence in the laboratory for the first time. It shows that it can arise under normal conditions, simply through conflicting sensory-motor signals,” Blanke added.
The research was published in the journal Current Biology.