By blocking the view of a limb from able-bodied participants, researchers were able to evoke the illusion of having a phantom hand, according to recent study results.
More than 230 volunteers underwent 10 behavioral experiments and one functional magnetic resonance imaging (fMRI) experiment. Researchers from the Karolinska Institutet, Sweden, instructed participants to sit at a table with their right arm hidden from their view behind a screen. The researchers would touch 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. Most participants transferred the sensation of touch to the empty space where they saw the paintbrush move, and experienced an invisible hand in that place, according to the researchers.
Study results showed that when the researchers made a stabbing motion with a knife toward the empty space occupied by the invisible hand stress responses were elevated. However, when the illusion was broken, the participants had no stress response. When asked to close their eyes and point with their left hand to their right hand, after experiencing the illusion for a while, participants would point to the location of the invisible hand.
In the fMRI experiment, researchers found that the invisible hand illusion led to increased activity in the parts of the brain that are normally active when the participants saw their real hand being touched or when participants experienced a prosthetic hand as their own. Overall, the researchers hope that the results of their study will offer insight into future research on phantom pain in amputees.
“Previous research has shown that non-bodily objects, such as a block of wood, cannot be experienced as one’s own hand, so we were extremely surprised to find that the brain can accept an invisible hand as part of the body,” Arvid Guterstam, lead author of the study, stated in a press release. “Taken together, our results show that the sight of a physical hand is remarkably unimportant to the brain for creating the experience of one’s physical self.”
For more information:
Guterstam A. J Cogn Neurosci. 2013; doi:10.1162/jocn_a_00393.
Disclosures: The researchers had no relevant financial disclosures.