Study Shows That Virtual Robotic Limbs Can Feel Like Parts of Our Own Bodies

We do plenty of what we need to do in life through the manipulation of our own bodies. To do what we cannot, we rely on tools — bags carry loads we cannot hold, scissors cut a piece of paper more neatly than we can rip. But what about ways to extend the body’s own functions? Supernumerary, or extra, robotic limbs are designed to do just this, paired with appropriate human-computer interaction (HCI) systems. The goal of these limbs is that they move naturally, just like the user’s own arms and legs, though it is yet unclear whether they can be truly embodied and considered part of a user’s body.

Researchers from the University of Tokyo, Keio University, and Toyohashi University have developed a supernumerary robotic limb system that can be operated by the user’s feet in a virtual environment. In large part, this research was undertaken as a way to understand and explore the limits of human “plasticity” — just how much can our brains alter and adapt to internal and external changes?

Humans can already learn to use new tools, and sometimes we even come to see these tools as extensions of the self, a concept referred to as “tool embodiment.” The team, including University of Tokyo doctoral student Ken Arai, set out to investigate whether virtual robotic arms could be perceived as supernumerary limb that is part of a user’s body and whether and how perceptual changes occurred in the space around the robotic arm.

Participants in the study wore a head-mounted display that gave them a first-person view of their own arms in virtual reality. They were then asked to perform tasks using only the virtual supernumerary limbs, which they controlled by moving their toes. When they touched an object with these limbs, the sensations were returned to the tops and soles of their feet with a tactile device.

After familiarizing themselves with how to use the virtual system, participants reported feeling like the virtual arms had become their own extra arms, rather than perceiving them as extensions of their arms or feet. In subjective evaluations, the system resulted in high marks in categories like “sense of body ownership,” “sense of agency,” and “sense of location.” It was also found that the participants’ “peripersonal space” — the area around our bodies we perceive as personal space — extended to include the area around the robotic arms. The full study and findings can be found in a paper published in Scientific Reports.

The team wants to continue their research by exploring the potential for cooperative behavior between the participants’ own arms and the virtual robotic limbs. They see this direction as important for continuing to explore the limits of human plasticity as well as the design of supernumerary robotic limb systems. Continuing to explore these systems and the perceptual changes and cognitive effort required to operate them in VR will aid in designing real-life systems in the future.