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Scientists Develop Real-World Robotic Supernumerary Limb to Help Perform Tasks

A virtual robotic limb system that can be controlled by users’ feet in a virtual world as additional or supernumerary limbs has been created by research teams at the University of Tokyo, Keio University, and the Toyohashi University of Technology in Japan. Users said that after training, they felt like their virtual robotic arms had merged with their bodies.

Scientists Develop Real-World Robotic Supernumerary Limb may Help Performing Tasks.
VR supernumerary robotic system. In this diagram of the system, the dotted lines represent wireless connections and solid lines represent wired connections. Image Credit: 2022 Ken Arai.

This study concentrated on the participants’ changes in perception because a better understanding of these changes can aid in developing genuine, physical robotic systems for extra limbs that individuals can use naturally and freely, exactly like their own bodies.

What would a human do if they had a second arm or four more like Spider-foe Man’s Doctor Octopus? Research on robotic limbs with extra or supernumerary limbs examines how humans could emotionally and physically adjust to having more limbs added to their bodies.

Ken Arai, a doctoral student at the University of Tokyo’s Research Center for Advanced Science and Technology (RCAST), became interested in this study as a way to investigate the limits of human “plasticity,” or, more specifically, the human brain’s capacity to change and adapt to internal and external changes.

Researchers may learn to utilize new tools and occasionally even start to think of them as extensions of ourselves, a process known as “tool embodiment,” whether it is a paintbrush used by an artist or scissors used by a hairdresser.

Teams from the University of Tokyo, Keio University, and the Toyohashi University of Technology in Japan worked together to construct a virtual robotic limb system to test these ideas in practice. Then, utilizing the virtual limbs, they gave the subjects VR tasks to complete.

We investigated whether virtual robotic arms, as supernumerary limbs, could be perceived as part of one's own body, and whether perceptual changes would occur regarding the proximal space around the robotic arm.

Ken Arai, Doctoral Student, Research Center for Advanced Science and Technology, University of Tokyo

Participants wore a head-mounted display that provided a first-person perspective of both their virtual robotic arms and the extra virtual arms. Users were then limited to utilizing the simulated robotic arms, which they moved with their toes to manipulate. When participants touched an item, such as a virtual ball, tactile devices sent feelings from the virtual robotic arms to the tops and soles of their feet.

The participants said that after becoming familiar with the virtual system, they felt as though the robotic limbs had taken on a life of their own and were no longer merely extensions of their own arms or feet.

The scores of subjective evaluation statistically became significantly higher for ‘sense of body ownership,’ ‘sense of agency’ and ‘sense of self-location,’ which are important measures of embodiment, where the supernumerary robotic limb is able to become part of the body.

Ken Arai, Doctoral Student, Research Center for Advanced Science and Technology, University of Tokyo

The research team also discovered that the region around the virtual robotic arms fell within the participant’s “peripersonal space,” which refers to the space around the human body that people perceive as belonging to the individual’s personal space.

Arai stated, “We succeeded in capturing the positive association between the perceptual change in visuo-tactile integration around the supernumerary robotic limbs (peripersonal space), and the score change of subjective evaluation of feeling the number of one’s arms increased (supernumerary limb sensation).

The team’s next goal is to investigate the possibility of cooperative behavior between the virtual robotic arms and the participants’ own arms in virtual reality.

Investigating the mechanisms and dynamics of the supernumerary limb sensation reported here from the standpoint of cognitive neuroscience will be important in exploring human plasticity limits and the design of supernumerary robotic limb systems.

Ken Arai, Doctoral Student, Research Center for Advanced Science and Technology, University of Tokyo

In order to build real-life systems that people can use as part of their own body, it is essential to study the perceptual alterations and cognitive effort needed to operate a supernumerary robotic limb system in virtual reality.

Journal Reference:

Arai, K., et al. (2022) Embodiment of supernumerary robotic limbs in virtual reality. Scientific Reports.


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