Would you like a sixth finger?
Have you noticed what distinguishes this hand from the ones you usually see? Count the number of fingers...
Ganesh Gowrishankar, University of Montpellier
The hand has a robotic "artificial sixth finger" that we developed with our collaborator, Professor Yoichi Miyawaki from Tokyo's University of Electro-Communications in Japan.
Users can control this sixth finger independently of their other fingers. Using an algorithm, we can isolate the part of the forearm's muscle activity 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 refers to the sense of touch): this senses what a finger would feel and calculates "haptic feedback, " i.e., slight deformations that are applied to the palm of the hand and generate tactile sensations.
Users can operate this extra limb with minimal training—after less than an hour of use for many people. Handy for playing the piano!
We are therefore studying how our body reacts to new members—which is also necessary when it has to accept a prosthesis, for example.
When the representation of the body changes
Through behavioral experiments and brain imaging, our work focuses on how the user's brain "adopts" the sixth finger. Changes in users' body perception occur very quickly.
More specifically, we asked users to touch a target line with their own pinky finger (without seeing their fingers), and this experiment showed that users actually become uncertain about the location of their own pinky finger in space.
We are currently continuing these studies in order to directly observe potential changes in users' brain activity usingfunctional magnetic resonance imaging, linked to the representation of their robotic sixth finger. For example, we may seek to determine which areas of the brain "activate" when the user moves the finger.
In neuroscience, the "embodiment" of a limb refers to the human brain's ability to "accept" this foreign limb and believe that it is part of the body – in French, we refer to this as "incarnation."
Another striking example is the"rubber hand illusion," where a user fears that someone will slap their hand even though their "real" arm is elsewhere.
The human brain can adopt foreign members
This example and other scientific studies conducted over the past few decades, including our own, have shown that it is actually quite easy to trick our brains into believing that other artificial limbs are part of our bodies: our brains are highly adaptable and flexible in terms of what they define and accept as our bodies.
This flexibility is very useful, 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 supplement lost functions.
Are there any limits to accepting a new member?
With our studies on supernumerary limbs such as the sixth finger, we are interested in the limits of this acceptance. Is it possible to add new limbs 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, such as robotic fingers, arms, and a virtual tail, to humans.
However, all 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 existing limb with a new artificial limb.
In our study, we seek to determine whether our brain can accept an additional, truly independent limb that can be moved independently of any other limb and from which we can obtain haptic feedback independent of any other limb. It would appear that it can.
Thus, from an application perspective, our findings—namely, that additional limbs can be accepted by our brain—are encouraging for the future development of wearable artificial limbs.
Ganesh Gowrishankar, Researcher at the Montpellier Laboratory of Computer Science, Robotics, and Microelectronics, University of Montpellier
This article is republished from The Conversation under a Creative Commons license. Readthe original article.