Using an Oculus Rift virtual reality headset and a motion sensor, Dr. Kitazaki’s team performed a series of experiments in which volunteers watched disembodied hands and feet move two meters in front of them in a virtual room. In one experiment, when the hands and feet mirrored the participants’ own movements, people reported feeling as if the space between the appendages were their own bodies.
This demonstrates the power of synchronized actions and our brain’s ability to fill in missing information, said V.S. Ramachandran, a professor at the University of California, San Diego and rubber-hand illusion pioneer who did not participate in the new study. The “improbability of synchrony occurring by chance” overrides all other information, he said, even knowledge that an invisible body cannot be yours.
In another experiment, the scientists induced illusory ownership of an invisible body, then blacked out the headset display, effectively blindfolding the subjects. The researchers then pulled them a random distance back and asked them to return to their original position, still virtually blindfolded. Consistently, the participants overshot their starting point, suggesting that their sense of body had drifted or “projected” forward, toward the transparent avatar.
Antonella Maselli, a researcher at the Santa Lucia Foundation, a neurological rehabilitation hospital in Italy, noted that the subjects in the study did not show significant conscious responses to seeing their invisible avatars being cut by a knife or colliding with a table.
Rather than an example of illusory body ownership, she said, the drifting effect may be more related to out-of-body experiences, in which people simply feel their bodies “displaced in space.” She added that the researchers might have found effects from the threats had they measured physiological responses, like changes in skin conductance or brain activity.
Dr. Kitazaki replied that the exact difference between out-of-body experiences and illusory body ownership is an open question, but agreed that future research should include such measurements.
Moving forward, he wants to investigate if it’s possible to get rid of even the virtual hands and feet in this study and see what it means, he said, to be totally “free from the current body.”