Where are you right now? Maybe you are at home, the office or a coffee shop—but such responses provide only a partial answer to the question at hand. Asked another way, what is the location of your "self" as you read this sentence? Like most people, you probably have a strong sense that your conscious self is housed within your physical body, regardless of your surroundings.



But sometimes this spatial self-location goes awry. During a so-called out-of-body experience, for example, one's self seems to be transported outside the physical body into a surreal perspective—some people even believe they are viewing their bodies from above, as though their true selves were floating. In a related experience, people with a delusion known as somatoparaphrenia disown one of their limbs or confuse another person's limb for their own. Such warped perceptions help researchers understand the neuroscience of selfhood.



A new paper offers examples of rare bodily illusions that are not confined to a single limb, nor are they complete out-of-body experiences—they are somewhere in between. These illusory body perceptions, described in the September issue of Consciousness and Cognition, could offer novel clues about how the brain maintains a link between the physical and conscious selves, or what the researchers call "bodily self-consciousness."



"These reports could be interesting for us to better understand how the brain produces ownership of the entire body—a sense that we have a body in the first place," says Henrik Ehrsson, a neuroscientist at the Karolinska Institute in Sweden who was not involved in the new study.



Lukas Heydrich, a cognitive neuroscientist at the Brain–Mind Institute (B.M.I.) in Lausanne, Switzerland (which is part of the Ecole Polytechnique Fédérale de Lausanne), and his colleagues recruited two epileptic patients from the University Hospital of Geneva. The researchers gave the patients a full diagnostic workup, including neurological and psychiatric examinations, various brain scans using electroencephalography (EEG) and magnetic resonance imaging (MRI), along with structured interviews focusing on aspects of bodily self-consciousness.



Patient 1 was a 55-year-old man who had suffered from epilepsy since he was 14 years old. For nine years the man also endured reoccurring attacks of strange bodily sensations that always followed the same pattern. First, without warning, he would feel an increasing pressure all along his left side, which escalated to the point that he was convinced a stranger had invaded the left region of his body. He would suddenly feel that the left half of his body no longer belonged to him—that the left half of his head, the upper part of his left trunk, and his left arm and leg were divided from the rest of his body. During an episode, the man believed himself to exist only in the right side of his body, although he remained calm and continued to function normally. Most people around him never noticed anything unusual, even if he was giving a lecture.



Patient 2 was a 30-year-old man suffering from epilepsy that resisted medication. Since age 11 the man experienced seizures characterized by an overwhelming sense of numbness in his legs, chest and neck. The numbness consistently became so intense that he lost awareness of everything below his chin, felt his head was detached from the rest of his body and experienced himself as simultaneously an observer of his body and the subject of observation.



"Clinical observations of disorders like these are very rare," says Olaf Blanke, a neurologist and cognitive neuroscientist at the Brain-Mind Institute (B.M.I.) and a co-author of the paper. "It's difficult even for a novelist or a fiction writer to come up with this."



Heydrich says that these patients provide new clinical evidence for the idea that bodily self-consciousness has three major components: self-location (where in physical space we experience ourselves to be located); first-person perspective (our primary viewpoint of the outside world from a place within the body); and self-identification (the degree to which we feel our bodies are part of us).



An out-of-body experience, Heydrich explains, warps all three aspects of bodily self-consciousness. In contrast, the two patients in the new study maintained normal self-location and first-person perspective even during an illusion. "They still perceived the world from their normal perspective, and they still felt they were in their bodies. But they had strong problem of self-identification. Patient 1 felt that…[the left]…half of him was a stranger and patient 2 felt that everything below his chin was no longer his."



Individuals who have trouble with only one aspect of bodily self-consciousness suggest that the three aspects can be dissociated, offering researchers an opportunity to determine which brain regions or networks underlie which components of self-perception.



An MRI revealed that patient 1 had a brain lesion in the right posterior intraparietal sulcus. In patient 2's brain, the researchers identified a concentration of aberrant electrical activity (the epileptogenic focus) in the right supplementary motor area (SMA) and right superior frontal gyrus. Surgery that removed patient 2's SMA and parts of his superior frontal gyrus cured the seizures and strange bodily perceptions, according to a checkup 15 months later. Heydrich says this implicates the SMA and premotor cortex specifically in the self-identification component of bodily self-consciousness.



"What we found is that the damage in these patients is different than what we find in other illusions, like full-body illusions," Blanke says. "We found damage in high-level motor cortex areas and in the intraparietal sulcus region—both are very multisensory regions," possibly explaining why they are implicated in the sense of self, which integrates many different bodily inputs.



Peter Brugger, a neuropsychologist at University Hospital in Zurich, is cautious about linking specific brain regions to particular kinds of self-perception. "If you operate in a certain region and observe a subsequent change in the behavior, you are very much seduced to think that the behavior resides in this location," Brugger says. "But because the brain primarily consists of connections, you have to think about whether you disrupted some kind of communication or cut faulty connections, not just a region."



Brugger agrees, however, that researchers need to pursue similar studies to better understand bodily self-consciousness. "We can learn and should learn much more from disturbances of bodily perceptions," he says.