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Scientists report creating illusion of having third arm

Feb. 28, 2011
Courtesy of the Karolinska Institute
and World Science staff

Sci­en­tists say they have fig­ured out a way to give healthy vol­un­teers the il­lu­sion of hav­ing three arms.

The trick helps clar­i­fy an old ques­tion in psy­chol­o­gy and neu­ro­sci­ence: ex­actly how we ex­pe­ri­ence our own bod­ies, re­search­ers not­ed. It has long been be­lieved that our nat­u­ral body plan lim­its our body im­age, that is, that we can’t ex­pe­ri­ence hav­ing more than one head, two arms and two legs. The new ex­pe­ri­ments un­der­mine that no­tion, say the re­search­ers with the Karolin­ska In­sti­tute, a med­i­cal uni­vers­ity in Swe­den.

A volun­teer tries out the third-arm-il­lu­sion with re­search­er Ar­vid Gu­ter­stam. (Im­age © Hen­rik Ehrs­son)


In a pa­per pub­lished in the on­line sci­en­tif­ic jour­nal PLoS One, they de­scribe ex­pe­ri­ments in which they put a realistic-looking rub­ber arm down on a ta­ble next to the right arm of a vol­un­teer par­ti­ci­pant. 

The sci­ent­ist then touches the sub­jec­t’s right hand and the rub­ber hand with two small brushes at cor­re­spond­ing loca­t­ion, syn­chro­niz­ing the strokes as per­fectly as pos­si­ble. The re­sult: the sub­ject de­vel­ops a feel­ing of own­ing both the rub­ber arm and the real arm.

“A con­flict arises in the brain con­cern­ing which of the right hands be­longs to the par­ti­ci­pan­t’s body,” said Ar­vid Guter­stam, one of the sci­ent­ists be­hind the stu­dy. “What one could ex­pect is that only one of the hands is ex­pe­ri­enced as one’s own, pre­sumably the real arm. But what we found, sur­pris­ing­ly, is that the brain solves this con­flict by ac­cept­ing both right hands as part of the body im­age.”

In all 154 vol­un­teers were tested, the sci­ent­ists said. To prove that the pros­thetic arm was really ex­pe­ri­enced as a third arm, a sci­ent­ist “threat­ened” ei­ther the false hand or the real one with a kitch­en knife, and meas­ur­ed the re­sult­ing amount of sweat­ing of the palm. It was the same, though only dur­ing the pe­ri­od when the subjects ex­pe­ri­enced the third-arm il­lu­sion, in­ves­ti­ga­tors re­ported.

The re­sults of the study may ben­e­fit pa­tients by cre­at­ing new ap­plica­t­ions in pros­thetics re­search, they added.

“It may be pos­si­ble in the fu­ture to of­fer a stroke pa­tient, who has be­come par­a­lysed on one side of the body, a pros­thetic arm that can be used and ex­pe­ri­enced as his own, while the par­a­lysed arm re­mains with­in the pa­tient’s body im­age,” said in­sti­tute neu­ro­sci­ent­ist Hen­rik Ehrs­son, who led the stu­dy. “It is al­so con­ceiv­a­ble that peo­ple with de­mand­ing work situa­t­ions could ben­e­fit [from] an ex­tra arm, such as fire­men dur­ing res­cue opera­t­ions, or paramedics in the field.”

In 2008, Ehrs­son and colleagues reported making peo­ple per­ceive the bod­ies of man­nequins and oth­er peo­ple as their own.

* * *

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Scientists say they have figured out a way to give healthy volunteers the illusion of having three arms. The trick helps clarify an old question in psychology and neuroscience: exactly how we experience our own bodies, researchers noted. It has long been believed that our natural body plan limits our body image, that is, that we can’t experience having more than one head, two arms and two legs. The new experiments undermine that notion, say the researchers with the Karolinska Institute, a medical university in Sweden. In a paper published in the online scientific journal PLoS ONE they described experiments in which they put a realistic-looking rubber arm down on a table next to the right arm of a volunteer participant. The scientist then touches the subject’s right hand and the rubber hand with two small brushes at corresponding location, synchronizing the strokes as perfectly as possible. The result: the subject develops a feeling of owning both the rubber arm and the real arm. “A conflict arises in the brain concerning which of the right hands belongs to the participant’s body,” said Arvid Guterstam, one of the scientists behind the study. “What one could expect is that only one of the hands is experienced as one’s own, presumably the real arm. But what we found, surprisingly, is that the brain solves this conflict by accepting both right hands as part of the body image, and the subjects experience having an extra third arm.” In all 154 volunteers were tested, the scientists said. To prove that the prosthetic arm was really experienced as a third arm, a scientist “threatened” either the false hand or the real one with a kitchen knife, and measuring the degree of sweating of the palm as a physiological response. The subjects had the same stress response, though only during the period when they experienced the third-arm illusion, investigators reported. The results of the study may benefit patients by creating new applications in prosthetics research, they added. “It may be possible in the future to offer a stroke patient, who has become paralysed on one side of the body, a prosthetic arm that can be used and experienced as his own, while the paralysed arm remains within the patient’s body image,” said institute neuroscientist Henrik Ehrsson, who led the study. “It is also conceivable that people with demanding work situations could benefit [from] an extra arm, such as firemen during rescue operations, or paramedics in the field.”