Have you spotted what distinguishes this hand from the ones you usually see? Count the number of fingers…

The hand wears a robotic “artificial sixth finger” that we developed with our collaborator , Professor Yoichi Miyawaki of Tokyo University of Electro-communication in Japan.

Users can control this sixth finger independently of their other fingers. Indeed, we can isolate, with an algorithm, the part of the muscle activity of the forearm that does not contribute to the movements of our usual fingers, and use this signal to control the robotic finger.

It is also equipped with a “haptic” sensor (which designates the sense of touch): this senses what a finger would feel and calculates “haptic feedback” , i.e. slight deformations which are applied to the palm of the hand and generate tactile sensations.

The user can manipulate this supernumerary limb with minimal learning – after less than an hour of use for many people. Handy for playing the piano!

We thus study how our body reacts to new limbs – this is also what is necessary when it has to accept a prosthesis, for example.

When the representation of the body changes

Through behavioral and brain imaging experiments, our work focuses on how the user’s brain “adopts” the sixth finger . Changes in users’ body perception happen very quickly.

Specifically, we asked users to touch a target line with their own little finger (without seeing their fingers), and this experiment showed that users actually become uncertain about the location of their own little finger in the space.

We are currently pursuing these studies in order to directly observe potential changes in the brain activity of users using functional magnetic resonance imaging , related to the representation of their robotic sixth finger. For example, we can try to determine which areas of the brain “activate” when the user moves their finger.

In neuroscience, the “embodiment” of a limb refers to the capacity of the human brain to “accept” this foreign limb and to believe that it is part of its body – we speak in French of “incarnation”.

Another vivid example is the “rubber hand illusion” , where a user fears being tapped on their hand while their “real” arm is elsewhere.

The human brain can adopt foreign limbs

This example and other scientific studies over the past few decades , including our own , have shown that it’s actually quite easy to trick our brains into thinking that other artificial limbs are part of our bodies: it is very adaptable and flexible in what it defines and accepts as our body.

This flexibility comes in handy because the human body changes as we grow and age. Physical changes can also be caused by accidents or paralysis, to which we are potentially able to adapt.

This notion of “incarnation” is also what allows us to accept prostheses to replace or complete lost functions.

Limits to the acceptance of a new member?

With our studies on supernumerary limbs like the sixth finger, we are interested in the limits of this acceptance. Is it possible to add new members to our innate body? And can we still feel the added limbs as part of our body?

Several previous studies have attempted to answer this question by attaching additional artificial limbs, for example robotic fingers , arms , and a virtual tail to humans.

However, all of these attempts have relied on “limb replacement” where the added limb is actuated by the movements of an existing limb and any haptic feedback on the added limb is provided to the existing limb – effectively replacing that limb. existing by a new artificial limb.

In our study, we are investigating whether our brain can accept a truly independent additional limb, which can be moved independently of any other limb, and from which we can obtain haptic feedback, independent of any other limb. It would seem so.

Thus, from an application point of view, our results, that additional limbs can be accepted by our brain, are encouraging for the future development of wearable artificial limbs.

Author Bio: Ganesh Gowrishankar is a Researcher at the Computing, Robotics and Microelectronics Laboratory of Montpellier at the University of Montpellier