You’ve probably heard of phantom limb syndrome. Because of odd wiring in the brain, a majority of amputees get the (frequently painful) sensation of having a limb, even after it’s gone. But a team has shown that you don’t have to be an amputee to have this feeling.
Neuroscientists at the Karolinska Institutet in Sweden created the illusion of a phantom limb in non-amputees like this. First, they had subjects sit down and place their arm behind a screen, so it was out of view. Next, behind the screen, the scientists tickled the unseen hand with a paintbrush. While they did that, the scientists waved a second paintbrush in front of the subjects, in full view. The two paintbrushes–the invisible one tickling the hand and the other just brushing the air in front of the subject–made the same movements.
The scientists discovered that the majority of subjects, within a minute, had the sensation of an invisible hand reaching out toward the paintbrush in mid-air. To test the sensation, researchers placed a knife in the air, where the the subjects reported feeling the third hand (eep). The subjects’ sweat was measured as a way of testing stress, and when the knife was placed across the “invisible” hand, the subjects sweated more. As a control, if the knife was waved in front of the subjects without having the phantom hand induced, they didn’t experience elevated stress.
Another experiment included in the study had subjects close their eyes and quickly point with their left hand to their right hand. After a while, with the phantom limb feeling induced, the subjects would end up pointing to the phantom hand. An fMRI test also showed that participants’ brains lit up when they had the phantom limb in the same spots that light up when a real hand is touched.
The researchers say this could lead to a better understanding of phantom limb syndrome, and maybe, eventually, a way of treating it.