More than two decades ago, Peter Brugger, as a PhD student in neuropsychology at the University Hospital Zurich in Switzerland, was developing a reputation as someone interested in scientific explanations of so-called paranormal experiences. A fellow neurologist, who had been treating a twenty-one-year-old man for seizures, sent the patient to Brugger. The young man, who worked as a waiter and lived in the canton of Zurich, had very nearly killed himself one day, when he found himself face-to-face with his doppelgänger.

The incident happened when the young man had stopped taking some of his anticonvulsant medication. One morning, instead of going to work, he drank copious amounts of beer and stayed in bed. But it turned out to be a harrowing lie-in. He felt dizzy, stood up, turned around, and saw himself still lying in bed. He was aware that the person in bed was him, and was not willing to get up and would thus make himself late for work. Furious at the prone self, the man shouted at it, shook it, and even jumped on it, all to no avail. To complicate things further, his awareness of being in a body would shift from one body to the other. When he was inhabiting the supine body in bed, he’d see his duplicate bending over and shaking him.

That’s when fear and confusion took hold: Who was he? Was he the man standing up or the man lying in bed? Unable to take it, he jumped out the window.

When I visited Brugger in the autumn of 2011, he showed me a photograph of the building from which the man had jumped: he had been extremely lucky. He had leapt from a window on the fourth floor and landed on a large hazel bush, which had broken his fall. But he had not really wanted to commit suicide, said Brugger. He had jumped to “find a match between body and self”. After getting treatment for his fall-related injuries, the young man underwent surgery to remove a tumour in his left temporal lobe, and both the seizures and the bizarre experiences stopped.

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Doppelgängers are the stuff of literature: from Edgar Allan Poe’s “William Wilson,” in which William, tormented by his double, stabs him, only to realise that he himself is bleeding, to Guy de Maupassant’s short story “Le Horla”, in which the main character murders his double, but laments at the end, “No...no...of course not...of course he is not dead...So then – it’s me, it’s me I have to kill!,” fictional doubles abound.

Broadly, such hallucinations are classified as autoscopic phenomena (from “autoscopy”; in Greek, autos means “self” and skopeo means “looking at”). The simplest form of an autoscopic phenomenon involves feeling the presence of someone next to you without actually seeing a double – a sensed presence. Olaf Blanke, a neurologist at the Swiss Federal Institute of Technology in Lausanne, Switzerland, told me that a sensed presence is like experiencing a full-body phantom: if a phantom limb is the continued sensation of having a limb that has been amputated, then a sensed presence of a body is its full-body analogue.

TS Eliot immortalised such an extracorporeal presence in his poem “The Waste Land”: “Who is the third who walks always beside you? / When I count, there are only you and I together.”

As it turns out, Eliot was inspired by accounts of the Antarctic explorer Ernest Shackleton, who wrote in his diaries that he and expedition team members Frank Worsley and Tom Crean, on the last leg of an unimaginably dangerous and difficult journey to find help to save the other stranded members of their trans-Antarctic expedition, began feeling the presence of a fourth person. Shackleton wrote, “I know that during that long and racking march of thirty-six hours over the unnamed mountains and glaciers of South Georgia it seemed to me often that we were four, not three. I said nothing to my companions on the point, but afterwards Worsley said to me, ‘Boss, I had a curious feeling on the march that there was another person with us.’ Crean confessed to the same idea. One feels ‘the dearth of human words, the roughness of mortal speech’ in trying to describe things intangible, but a record of our journeys would be incomplete without a reference to a subject very near to our hearts.” We now know that it’s not uncommon for oxygen-deprived mountaineers to report sensing the presence of another.

Autoscopic phenomena can go beyond just a sensed presence. There is the doppelgänger effect, in which a person may hallucinate that they are actually seeing another “me” – a visual double. Often, the hallucination is very emotional, and the person’s sense of location and identity switches between the real and the illusory bodies, as experienced by Brugger’s twenty-one-year-old patient.

Probably the most widely experienced and best-known form of autoscopic phenomena is the out-of-body experience (OBE). During a classic full-blown OBE, people report leaving their physical body and seeing it from an outside perspective, say from the ceiling looking down at the body lying in bed.

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Many people who have such experiences are reluctant to talk about it. OBEs give the person a strong sense of dualism of body and mind: your centre of awareness, which is usually anchored in your body, seems to float free of it.

We saw earlier how the bodily self is the foundation for our sense of self, and disruptions of the bodily self can cause BIID, schizophrenia, and perhaps even autism. In all these cases, however, the centre of awareness remains anchored to the body, however impaired the perception of it may be. OBEs mess with this centre of awareness – suggesting a Cartesian duality. But if you examine OBEs closely, it turns out that the duality is an illusion, a product of a brain that fails to correctly integrate all the signals from the body. Despite their vividness, OBEs are hallucinations caused by malfunctions in brain mechanisms; elucidating these mechanisms gets us closer to understanding how the brain constructs the self.

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Back at the University Hospital Zurich, Peter Brugger tried gamely to induce in me an out-of-body illusion. We were wandering the corridors of the hospital. I was wearing virtual-reality goggles. Brugger was walking about three feet behind me, filming me using my notebook computer’s webcam and feeding the video into the goggles I was wear- ing. So, instead of seeing where I was going, I was seeing myself from behind, walking about three feet in front of me. We must have been a sight as we walked past curious interns and hospital staff. Brugger, looking like an absentminded professor with his white lab coat and wild, greying hair, holding aloft an open notebook computer, and me walking in front, blind but for what I was seeing in the VR goggles.

The setup didn’t quite work. We should have been using a good video camera, which we didn’t have at the time, and longer wires so that Brugger could have been farther behind me. But I did feel weird walking around watching myself from behind.

In 1998, when Brugger first tried the experiment, he wore such goggles for an entire day, and had someone walk about twelve feet behind him, filming him with a video camera. So, if Brugger was picking a flower, or putting a letter in a mailbox, he’d see himself doing the act from an outside perspective. “ is was extremely strange. I lost the sense of where I actually was,” he told me. “I was where I saw the action, rather than where I was actually executing the action.” Brugger was having an out-of-body illusion: the sense of where he was located had shifted several feet, from being in his physical body to being in the virtual body.

But Brugger never actually performed the experiment in a rigorous laboratory setting and so never published the results, though it did get mentioned in an article in Science.

He credits American psychologist George Malcolm Stratton (1865-1957) as the inspiration for this experiment. Stratton had spent a good part of his career at the University of California, Berkeley. He is best known for “perhaps the most famous experiment in the whole of experimental psychology.” Stratton fashioned a contraption that allowed him to see upside down. He walked around with this device on his right eye. He blacked out the left eye, because seeing upside down with both eyes was extremely disorienting. For three days and a total of 21.5 hours, he did nothing but use this device. When he went to bed, he strapped his eyes shut. While the primary motivation for the experiment was to understand visual perception, Stratton experienced other subtle changes in bodily perception. For instance, if he stretched out his hand to touch something, because he was seeing everything upside down, the hand would enter the visual field from above rather than below. Soon, “parts of my body...were seen to be in another position.”

Stratton realised that he was onto something. In 1899, he published another paper, in which he described a crazier experiment, this time with mirrors.

He built a frame that he affixed to his waist and shoulders. The frame held a mirror horizontally above his head. He used the frame to position another mirror at a forty-five-degree angle in front of his eyes, so that it reflected the image from the overhead horizontal mirror directly into his eyes. The net effect was that Stratton was seeing himself and the space around him from the perspective of someone looking down at his head from above. He made sure that no other light entered his eyes. Again, he walked around with this contraption for three days, for a total of twenty-four hours, with his eyes blindfolded when he was not experimenting and when he slept. Doing so, he was able to create a disharmony between sight and touch: when he’d reach out to touch something, his hands felt the touch, but his eyes told him that the touch was somewhere else entirely. It was now up to the brain to bring everything back into harmony, with interesting consequences.

Because Stratton was seeing his own body from above and nothing else, he had to pay close attention to this visual image to guide his actions and movements. By afternoon on the second day he began to notice that the reflected image sometimes felt like his body. is feeling became more persistent on the third day, especially when he was walking with ease and speed, not making any special effort to differentiate between where he was perceiving his body to be and where he “knew” his body to be. “In the more languidly receptive attitude during my walk, I had the feeling that I was mentally outside my own body,” he wrote. Stratton had induced in himself an out-of-body experience.

Excerpted with permission from The Man Who Wasn’t There: Tales from the Edge of the Self, Anil Ananthaswamy, Penguin Random House India.