Have you spotted what distinguishes this hand from those you see usually? Count the number of fingers…
The hand has a robotic “sixth finger” which we developed with our collaborator, Prof Yoichi Miyawaki of the University of Electro-Communications in Tokyo.
Users can control this sixth digit independently of the others. In fact, we can pinpoint, with an algorithm, muscle activity in the forearm which doesn’t contribute to normal finger movement, and use this signal to control the robot finger.
It’s also equipped with a haptic sensor (ie, concerned with the sense of touching): this feels what a real finger would feel, and offers “haptic feedback” – that is, light pressures applied on the palm of the hand, giving a tactile sensation.
The user can move around this extra digit with a minimum of training – for many people within less than an hour. One could put it to use by playing the piano!
What we have been studying is how, confronted with new digits, the body reacts. This is also what happens when the body is challenged to accept a prosthesis, for example.
When the representation of the body changes
Drawing on behavioural experiments and brain imagery, our research is focused on the way in which the user’s brain gels with the sixth finger. Changes in users’ bodily perceptions come very quickly.
More specifically, we’ve asked participants to touch a drawn line with their own little finger, without looking at their fingers. This experiment showed that people became uncertain about the positioning of their own little finger in space.
We’re pursuing these studies at the moment to directly observe using magnetic resonance imaging the extent of change to users’ brain activity, as this relates to representation of the robot sixth finger. For example, one could look to find out which zones of the brain are activated when the user moves their finger.
In neuroscience, the term “embodiment” of a limb refers to the human brain’s capacity to accept a prosthesis and believe it is part of one’s body. In French the expression is “incarnation”.
Another striking example is that of the “rubber hand” illusion, where the user thinks someone is tapping their hand, when their real arm is somewhere else.
The human brain can accept foreign body parts
This example and other scientific studies over recent decades, including our own, have shown that it’s actually quite easy to deceive our brain into thinking that artificial limbs are part of our bodies. The brain is very adaptable and flexible about what it defines and accepts as our body.
This flexibility is very useful, because the human body changes as we grow up and get old. Physical changes can also be caused by accidents or through paralysis, which people are potentially capable of adapting to as well.
This notion of “incarnation” is also what allows us to accept prostheses to replace or complete lost bodily functions.
The limits of acceptance for a new limb
In our study of extra body parts, like the sixth finger, we have been interested in the limits of this acceptance. Is it possible to add new integral body parts? And can we feel added elements as if they are part of our body?
A number of previous studies have tried to address these questions by attaching artificial limbs to their subjects, including robot fingers, arms, and a virtual tail for humans.
However, all these investigations are on the basis of a limb replacement, where the added part is animated by movements and haptic feedback of existing body parts – effectively substituting a new artificial limb for a flesh and bones one.
In our study, we’re trying to find out if our brains can accept a truly autonomous extra body part, which can be moved around independently of any other part and from which we can obtain haptic feedback, on which the flesh and bones body has no bearing. It seems that they can.
Thinking of the applications, our finding that additional limbs can be accepted by the brain is encouraging for the future development of wearable artificial limbs.
This article was translated from French by Joshua Neicho.