People Come to Grips with Having an Extra Pair of Arms—in VR

What would you do with two hands? It may sound too much of a good thing. A new study shows that people can use additional robotic arms as if they were their own limbs.

Scientists have been studying how the human brain works for decades. When you pick up a wrench or a screwdriver, your brain interprets it as a substitute for your own hand, according to a new theory. When you wield a long stick, your sense of personal space is extended so that you don't accidentally whack someone with it when you turn around. Adding entirely new parts instead of just changing the function, shape or size of existing ones can affect your perception of your own body.

The design of new robotic devices could be influenced by that question. Many roboticists are interested in building systems that would give humans the ability to use additional limbs and possibly enable people to complete tasks that need an extra arm or leg. Virtual reality gives people the chance to try experiences that aren't yet possible in the real world and to act as virtual humans. Real or virtual body parts need to blend in as if they have always been there. It's important to understand if and how this can be used in real-life parts and video games.

Experiments have tried to figure out how humans respond to having more appendages. In these tests, participants were fitted with an extra arm, hand or finger made of rubber, which they could touch at the same time as they touched a real one. Humans can feel the extra limbs in their body. It wasn't yet known if people could use controllable limbs.

New research was published last month. There was an extra pair of virtual robotic arms just below the real ones for researchers to immerse participants in a virtual reality environment. Ken Arai is a roboticist and cognitive scientist at the University of Tokyo. The challenge of getting a pair of robotic arms to move without a delay is what our brain expects from our real body parts. The time between the input from the sensors and the movement of the virtual arms is shorter in virtual reality.

The participants used their feet and waists to control the fake arms. The extra arm on that side of the body would be used to move the lower leg in virtual reality. The virtual hand was told to grab something by bending toes. People using virtual reality were able to feel the movement of their limbs. If the palm of a robotic hand touched something in the virtual reality space, participants felt a vibration against the sole of their foot.

The participants used the extra arms to touch the balls that appeared in random places. Participants rated how much they agreed with statements such as "I felt as if the virtual robot limbs/ arms were my limbs/ arms" and "I felt as if the movements of the virtual robot arm were affecting my own movements." People reported feeling more agency and ownership when they completed the ball-touch task multiple times.

People were tested on how quickly they moved their robotic arms. The participants were told to move their robotic arms away from the virtual objects that were touching their artificial limbs. Sometimes the sensation didn't match where the object was in virtual reality. The same experiment was carried out on people's limbs. The researchers think that this is a sign that participants' sense of personal space expanded to include the area around their robotic arms in virtual reality.

Participants felt like they had acquired whole new body parts, not just like they had extended their existing feet by adding a new tool. This could possibly open up a world of possibilities.

The Virtual Embodiment Lab at Cornell University was not involved in the study. It's possible to give yourself wings and fly around in a virtual space. Researchers will be able to design that experience if they learn more about how people feel about their extra body parts. Won thinks that the question of how people might interpretavatar bodies, which don't exist physically, and react to them as though they were real is an interesting one.

Arai is most excited about the possibility of expanding what humans can do in the physical world. People could get an extra pair of hands. It's possible that the knowledge from the virtual reality system can be used to make the actual robotic system. The feedback loop will be very important to improve the designs of robotic limbs. We want to make impossible things possible. We should be able to add more limbs. Everything should be doable.