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Scientists produce illusion of body-swapping

Dec. 2, 2008
Courtesy Karolinska Institute
and World Science staff

Cog­ni­tive neu­ro­sci­en­tists at the Swed­ish med­i­cal uni­ver­s­ity Karolin­ska In­sti­tute say they have made peo­ple per­ceive the bod­ies of man­nequins and oth­er peo­ple as their own. The find­ings are to ap­pear Dec. 3 in the on­line re­search jour­nal PLoS One.

“This shows how easy it is to change the brain’s per­cep­tion of the phys­i­cal self,” said the in­sti­tute’s Hen­rik Ehrs­son, who led the proj­ect. “By ma­ni­pu­lat­ing sen­so­ry im­pres­sions, it’s pos­si­ble to fool the self not only out of its body but in­to oth­er bod­ies too.”

The re­search was aimed at learn­ing more about how the brain con­structs an in­ter­nal im­age of the body. Ma­ni­pu­lation of bodily iden­ti­fica­t­ion and self-per­cep­tion may be use­ful in vir­tu­al real­ity ap­plica­t­ions and ro­bot tech­nol­o­gy, ac­cord­ing to the re­search­ers.

In a first ex­pe­ri­ment, they fit­ted the head of a shop dum­my with two cam­er­as con­nect­ed to two small screens placed in front of the sub­jects’ eyes, so that they saw what the dum­my “saw.” When the dum­my’s cam­era eyes and a sub­jec­t’s head were di­rect­ed down­wards, the sub­ject saw the dum­my’s body where he or she would nor­mally have seen his or her own.

The bod­y-swapping il­lu­sion arose, investigators said, when a sci­ent­ist touched the stom­ach of both with two sticks. The sub­ject could then see that the man­nequin’s stom­ach was be­ing touched while feel­ing, but not see­ing, a si­m­i­lar sensa­t­ion on his or her own stom­ach. As a re­sult, the sub­ject de­vel­oped a pow­er­ful sensa­t­ion that the man­nequin’s body was his or her own, re­search­ers said.

In anoth­er ex­pe­ri­ment, the cam­era was mount­ed on­to anoth­er per­son’s head. When this per­son and the sub­ject turned to­wards each oth­er to shake hands, the sub­ject per­ceived the cam­er­a-wear­er’s body as his or her own.

“The sub­jects see them­selves shak­ing hands from the out­side, but ex­perience it as anoth­er per­son,” said Va­le­ria Petkova, who con­ducted the study with Ehrs­son. “The sen­so­ry im­pres­sion from the hand-shake is per­ceived as though com­ing from the new body, rath­er than the sub­jec­t’s own.”

The strength of the il­lu­sion was con­firmed by the sub­jects’ ex­hibit­ing stress re­ac­tions when a knife was held to the cam­era wear­er’s arm but not when it was held to their own, in­ves­ti­ga­tors said. The il­lu­sion al­so worked even when the two peo­ple dif­fered in ap­pearance or were of dif­fer­ent sexes. How­ev­er, it was­n’t pos­si­ble to fool people in­to iden­ti­fy­ing with a non-humanoid ob­ject, such as a chair or a large block, the re­search­ers not­ed.


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Cognitive neuroscientists at the Swedish medical university Karolinska Institute say they have made people perceive the bodies of mannequins and other people as their own. The findings are to appear Dec. 3 in the online research journal PLoS One. “This shows how easy it is to change the brain’s perception of the physical self,” said the institute’s Henrik Ehrsson, who led the project. “By manipulating sensory impressions, it’s possible to fool the self not only out of its body but into other bodies too.” The research was aimed at learning more about how the brain constructs an internal image of the body. The knowledge that the sense of bodily identification and self-perception can be manipulated to make people think that they have a new body may be useful in virtual reality applications and robot technology, according to the researchers. In a first experiment, scientists fitted the head of a shop dummy with two cameras connected to two small screens placed in front of the subjects’ eyes, so that they saw what the dummy “saw.” When the dummy’s camera eyes and a subject’s head were directed downwards, the subject saw the dummy’s body where he/she would normally have seen his/her own. The illusion of body-swapping was created when the scientist touched the stomach of both with two sticks. The subject could then see that the mannequin’s stomach was being touched while feeling, but not seeing, a similar sensation on his/her own stomach. As a result, the subject developed a powerful sensation that the mannequin’s body was his or her own, researchers said. In another experiment, the camera was mounted onto another person’s head. When this person and the subject turned towards each other to shake hands, the subject perceived the camera-wearer’s body as his or her own. “The subjects see themselves shaking hands from the outside, but experience it as another person,” said Valeria Petkova, who conducted the study with Ehrsson. “The sensory impression from the hand-shake is perceived as though coming from the new body, rather than the subject’s own.” The strength of the illusion was confirmed by the subjects’ exhibiting stress reactions when a knife was held to the camera wearer’s arm but not when it was held to their own, investigators said. The illusion also worked even when the two people differed in appearance or were of different sexes. However, it wasn’t possible to fool the self into identifying with a non-humanoid object, such as a chair or a large block, the researchers noted.