How about a sixth finger?

Have you noticed what distinguishes this hand from the others you usually see? Count the number of fingers...

Ganesh Gowrishankar, University of Montpellier

The hand carries 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. In fact, we can use an algorithm to isolate the part of the forearm's muscular 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.

The user can manipulate this supernumerary limb with a minimum of training - after less than an hour's use for many people. Handy for playing the piano!

In this way, we study how our body reacts to new limbs - 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 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 space.

We are currently pursuing these studies in order to directly observe potential changes in the brain activity of users usingfunctional magnetic resonance imaging, linked 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 the finger.

In neuroscience, the "embodiment" of a limb refers to the human brain's ability to "accept" this foreign limb and believe it to be part of its body.

Another striking example is the"rubber hand illusion", where a user fears being slapped on the hand when his "real" arm is elsewhere.

The human brain can adopt foreign limbs

This example and other scientific studies carried out over the last few decades, including our own, have shown that it's actually quite easy to fool our brains into believing that other artificial limbs are part of our bodies: our brains are very adaptable and flexible, in what they define and accept as our bodies.

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

This notion of "embodiment" is also what allows us to accept prostheses to replace or complete lost functions.

Limits to accepting new members?

With our studies on supernumerary limbs like the sixth finger, we're looking at 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 one.

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

So, from an application point of view, our results - that additional limbs can be accepted by our brains - 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. Read theoriginal article.