Ghostly presences 鈥� the feeling of someone near you when there鈥檚 no one there 鈥� could be down to your brain trying to make sense of conflicting information. For the first time, the brain regions involved in such hallucinations have been identified 鈥� and a ghost presence induced in healthy people.
The work sheds light on why some people with conditions such as schizophrenia and epilepsy feel an alien presence nearby, and may also explain why mountain climbers often report being accompanied by the presence of what鈥檚 called 鈥渢he third man鈥�.
In 1933, when British explorer Frank Smythe came close to conquering Mount Everest all by himself, he couldn鈥檛 shake off the feeling that someone else was climbing with him. But he was alone, having left his team far behind. Smythe was hallucinating. He even broke off a piece of cake and offered it to his invisible climbing partner.
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This condition, called feeling of presence (FoP), is different from other bodily hallucinations, such as out-of-body experiences, in which you feel you are outside your body looking at it, or the doppelg盲nger effect, in which you see and interact with your double. Such hallucinations have a visual component.
Mystical presence
That鈥檚 not the case with FoP. 鈥淸It鈥檚] more mystical,鈥� says neurologist of the Swiss Federal Institute of Technology in Lausanne. 鈥淵ou are convinced that there is something, but you don鈥檛 see anything, you don鈥檛 hear anything.鈥�
To identify the potential neural mechanisms behind FoP, Blanke鈥檚 team first studied 12 people with epilepsy and other sensory-motor problems, all of whom had reported feeling a presence nearby. Their analysis pointed to damage in three brain regions: the temporoparietal junction (TPJ), the insula and the frontal-parietal cortex.
In previous studies, Blanke鈥檚 team had linked the TPJ with out-of-body experiences and the insula with the doppelg盲nger hallucination. Normally, these brain regions integrate sensory signals from outside and inside the body, to create the sense of an embodied self. In out-of-body experiences and other such conditions, the integration of these multisensory signals is compromised, leading to hallucinations.
The new study shows that FoP involves disruptions not just in the integration of external and internal sensations in the TPJ and insula, but also signals related to movement (which are processed in the frontal-parietal cortex).
Armed with this knowledge, Blanke鈥檚 team turned to a robot to see if they could use it to disrupt the normal brain processes and induce a feeling of presence.
Master and slave
The robot had two components: a master and a slave. The volunteers were blindfolded and asked to move an arm of the master robot with one hand. This caused the slave robot, behind them and touching their back, to move. In essence, they were able to make the slave robot stroke their own backs. The volunteers also received tactile feedback as they moved the master robot, meaning that when they moved the robot, it applied a force back which the volunteers could feel in their fingers.
The presence made itself felt when the slave robot moved with a delay of 500 milliseconds, so that the volunteers鈥� hand movements were out of sync with the touch on their backs. Five of 17 people spontaneously reported feeling a presence behind them 鈥� without even being asked about it. 鈥淚t means the illusion is pretty striking,鈥� says team member and robotics expert .
Next, the team removed the tactile feedback from the master robot to the fingers of their volunteers. This induced the illusion more consistently. Of the 19 subjects, 14 reported a stronger feeling of presence when the stroking of their backs was not in synchrony with their arm movements than when it was.
It seems that the brain, confounded by the mismatch between internal bodily signals related to the movements of their arms and the out-of-sync sensation of touch on their backs, attributed the touch to the presence of someone standing behind.
Blanke speculates that such disintegration of signals may lie behind the third man phenomenon experienced by mountaineers. It usually happens above altitudes of 6000 metres, under conditions of extreme fatigue and oxygen deprivation, and possibly sensory deprivation in a grey and white landscape. 鈥淵ou don鈥檛 see other human beings, you don鈥檛 see animals, you don鈥檛 see colours,鈥� says Blanke. 鈥淭he brain is probably in a situation in which it is prone to induce altered states of consciousness; if this comes together with physical fatigue due to continuous motor activity, the potential to have multisensory motor errors is probably increased.鈥�
Altered states
The robotic experiment also shows that the illusion occurs when people have a disturbed sense of 鈥渁gency鈥� 鈥� the sense that they are initiating bodily movements 鈥� because of the delay in the response of the slave robot.
An altered sense of agency has been implicated in people with schizophrenia, and has been used to explain why they attribute, for instance, their own actions to other people, often leading to paranoid delusions. A lot of people with schizophrenia claim to feel a presence, says , a schizophrenia expert at the University of California, San Francisco.
Blanke suggests that both a disturbed sense of agency and a mismatch between sensory signals and motor signals could be contributing to the alien presences felt by people with schizophrenia. The hypothesis is 鈥渙n target鈥�, says Ford.
, a neuropsychologist at University Hospital Zurich, Switzerland, says the argument is convincing. 鈥淭he leap from the robotic experiments to explain complex psychiatric phenomenology is certainly not too big,鈥� he says.
The finding could one day help patients with schizophrenia. 鈥淭he same way that you can trick the brain into creating an alien presence, you could train the psychotic brain to relearn the difference between self and other,鈥� says Rognini. 鈥淭he dream would be to down-regulate psychosis. But we are far from that.鈥�
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