The rubber hand illusion is an impressive dinner party trick. It persuades an unwitting guest that a rubber hand on the table is actually his or her own. For the victim, the illusion is rapid, dramatic, and convincing—the person “feels” the hand and experiences it being stroked.
The illusion reveals how easily the human brain can be tricked into feeling ownership of otherwise inanimate objects. Psychologists have repeated the illusion not just with limbs but with entire bodies. Take part in one of these experiments and you can experience the remarkable illusion of having a different body form—perhaps one of the opposite sex, or even of a nonhuman animal such as a gorilla.
For a long time, researchers believed that full body illusions could only be triggered in specially controlled laboratory conditions using virtual-reality equipment.
But recently, they have found ways to trigger the illusion more easily. And that raises some interesting questions. Researchers would dearly love to know how extreme body shapes can become before the human brain rejects them. Could a human “own” the body of a spider, a lobster, or even a table, for example? And given the ease with which the illusions can be triggered, how widespread can this type of illusion become?
Today we get a partial answer thanks to the work of Andrey Krekhov and colleagues at the University of Duisburg-Essen in Germany. These guys have compared the way humans take on ownership of human versus nonhuman bodies such as those of tigers, bats, and spiders.
They say that in certain situations, the experience of owning a nonhumanoid body is more convincing than that of owning a humanoid body. And this result paves the way for virtual body ownership to play a greater role in applications such as training, education, and of course video-gaming, where the potential is significant.
First some background. The rubber hand illusion begins by covering a person’s real hand and placing a rubber hand next to it. The subject can see the rubber hand but not their real hand. The illusionist then lightly touches or strokes both hands in the same place and in the same way.
The subject, seeing the rubber hand being stroked, feels the sensation as if it were their real hand, which is also being stroked but in a way they cannot see. In that instant, the illusion is created: the subject feels that the rubber hand is part of their own body. Many people find this a surprising and dramatic sensation.
The illusion was first described in the 1990s, but since then researchers have gone much further. The next generation of experiments used early forms of virtual reality. The subject wears a VR headset and looks down at their body to find it is that of a gorilla, for example.
While the subject watches, the virtual body receives a poke in the chest. But at the same instant, the the subject’s real body is poked in the same place. In that instant, the illusion is created: the subject feels ownership of the virtual body.
The limitations of this technique were immediately clear. The illusion requires visual and tactile stimuli simultaneously. That’s straightforward for a rubber hand but becomes more difficult for an entire body, particularly when the virtual limbs start moving in a way that the subject’s real body does not match.
That seemed to limit the application of the illusion until a breakthrough in 2010. At that time, researchers discovered how to trigger the body ownership illusion using visual cues alone. They did this by tracking the subject’s body movements in microfine detail, the level of hand and finger movements. They then reproduced these movements exactly in the virtual body.
This immediately makes the illusion easier to achieve. And this is what Krekhov and co study for a wider range of body ownership models. Their work focuses on three types of virtual bodies—a four-legged animal in the form of a tiger, a flying animal in the form of a bat, and a creature with an entirely different exoskeleton in the form of a spider.
Their goal is to explore the limits of body ownership with challenging examples that differ from human bodies in posture, shape, and even entire skeleton. Their goal is to understand how well the human brain copes with the body ownership illusion in these cases, and whether it is possible at all in the most extreme cases.
Their method was straightforward. They subjected 37 volunteers to the illusion, measuring how they coped and asking them to rate their experiences compared with humanoid body ownership. In each case, they mapped the participant’s body movements precisely to the movements of the virtual body to trigger the illusion.
The results make for interesting reading. Krekhov and co say that humans are surprisingly good at adopting alien forms. “Our experiment shows that even spiders, despite having a skeleton that significantly differs from ours, offer a similar degree of the illusion of virtual body ownership compared to humanoid avatars,” they say.
What’s more, some body types are better than humanoid ones in certain circumstances. For example, bat bodies are better at reproducing the sensation of flying than humanoid bodies. “Our empirical results demonstrate that virtual body ownership is also applicable for nonhumanoids and can even outperform human-like avatars in certain cases,” they say.
The technique is clearly popular with the participants. The researchers asked them what other body types they’d like to try, with suggestions ranging from cats and dogs to sea animals such as whales and dolphins. But there was a clear winner in this respect. “We suppose that flying creatures have the largest potential to fascinate users as embodiment targets in VR,” say Krekhov and co.
That’s interesting work that has the potential for significant applications. The most obvious is in the gaming world, where the illusion of owning another character’s body has obvious charms. But there are also educational and training applications, plus the inevitable applications in the world of pornography.
Of course, there are limitations to overcome before this kind of illusion can become widespread. Virtual-reality kits are becoming increasingly common, but this technology has to be coupled with accurate body tracking equipment. That will be an enabling step. A broader challenge will be to overcome the problems with VR, which often leaves users feeling nauseated.
Nevertheless, the startling fact is that virtual body ownership is both easier than ever and possible with a wider variety of bodies than anyone imagined. Expect to hear more about it.
Ref: arxiv.org/abs/1907.05220 : The Illusion of Animal Body Ownership and Its Potential for Virtual Reality Games