Sensing an invisible hand

The ability to recognise your own body is more complicated than you think, suggest Swedish scientists who have demonstrated that is possible to evoke the illusion of having a phantom hand in non-amputees.

In a study detailed in Journal of Cognitive Neuroscience, a perceptual illusion experiment is described where volunteers experience having an invisible hand. The experiment involves the participant siting at table with their right arm hidden from their view behind a screen. To evoke the illusion, the scientist touches the right hand of the participant with a small paintbrush while imitating the exact movements with another paintbrush in mid-air within full view of the participant.

“We discovered that most participants, within less than a minute, transfer the sensation of touch to that region of empty space where they see the paintbrush move, and experience an invisible hand in that position” said Arvid Guterstam from the Karolinska Institutet who led the study.

The experiment was inspired by the classic rubber-hand illusion devised by Princeton scientists in the 1990s – where a fake hand is stroked in time with the participant’s real one.  This study is the first to demonstrate that empty space can be accepted by the brain as a part of the body.

Eleven experiments explored in detail the illusory experience, and 234 volunteers were enrolled. To further demonstrate the illusion, the researchers would make a stabbing motion with a knife towards the empty space occupied by the invisible hand. Measuring participants sweat response to the perceived threat, they discovered that a stress response was elevated while experiencing the illusion, but absent when it was broken.

The researchers also measured the brain activity of the volunteers using functional magnetic resonance imaging (fMRI). When volunteers perceived the invisible hand, increased activity was seen in the same parts of the brain that are usually active when people see their real hand being touched or when participants experience a prosthetic hand as their own.

The researchers hope the work will give insight into the feeling of invisible limbs that the majority of amputees experience.

“These results add to understanding of how phantom sensations are produced by the brain, which can contribute to future research on alleviating phantom pain in amputees,” said Dr Henrik Ehrsson, principal investigator.