Neuroscientistsof the Karolinska Institute (Sweden) have shown thatit is possible for healthy volunteers to experience the sensation of having three arms at the same time. In a new article published in the scientific journalPLoS ONE describe how this illusion can be created under controlled laboratory conditions. The experiment consists of leaving a realistic prosthetic arm next to the participant’s arm, who remains seated on a table. In this way,subject sees his two real arms and the extra prosthetic arm, made of rubber. To produce the sensation of being the owner of the rubber arm, the scientist touches the subject’s right hand and the rubber hand with two small brushes in the same place and with as perfect synchronization as possible in the touches.
? What happens then is that a conflict arises in the brain about which of the right hands belongs to the body of the participants “, clarifies Arvid Guterstam, co-author of the study.” It would be expected that only one of the hands would be experienced as its own, presumably the one with the royal arm. But what we have discovered, surprisingly, is thatthe brain resolves this conflict by accepting both right hands as part of body image, and the subjects experience the sensation of having an extra third arm ?, he adds.
In the study, involving 154 healthy volunteers, to demonstrate that the prosthetic arm was actually experienced as a third arma scientist “threatened” both the prosthetic and the real hand with a kitchen knifeand the degree of palm sweating was measured as a physiological response to this stimulus. The results showed thatsubjects had the same stress response when the prosthetic hand was threatened as when the actual hand was threatenedbut only during the periods when they experienced the third arm illusion.
Study results can benefit patients by creatingnew applications in prosthetic research. ? In the future, a patient with one side of the body paralyzed by a stroke may be offered a prosthetic arm that can be used and experienced as their own, while the paralyzed arm would remain within the patient’s body image? highlights Henrik Ehrsson, who has led the study.
“It is also conceivable that people in demanding jobscould benefit from an additional arm, such as firefightersduring rescue operations orparamedicsin their speeches, “adds Ehrsson.
