2023-04-20 20:11:38
Have you noticed what distinguishes that hand from those you usually see? Count the number of fingers…
The hand has a robotic “artificial sixth finger” that we have developed with our collaborator, Professor Yoichi Miyawaki of the Tokyo University of Electrocommunications, Japan. Users can control this sixth finger independently of the rest of their hand. In fact, we can isolate, with an algorithm, the part of the forearm muscle activity that does not contribute to the usual movements of our fingers, and use this signal to control the robotic finger.
It is also equipped with a haptic sensor: it detects how a finger would feel and calculates a haptic feedback, that is, slight deformations that are applied to the palm of the hand and generate tactile sensations.
One of its main advantages is that the user can manipulate this extra finger with a minimum of training: less than an hour is enough for many people. Very handy for playing the piano!
Thanks to this sixth finger we can study how our body reacts to new members, something that also happens when we get a prosthesis, for example.
When the representation of the body changes
Starting from the fact that changes in the perception of the user’s body occur very quickly, our work has focused on understanding how the user’s brain integrates the sixth finger.
In early behavioral experiments, we asked users to touch a finish line with their own little finger (without seeing their fingers). This experience showed that users do not clearly perceive the location of their own little finger in space.
We are currently continuing these studies to directly observe, through functional magnetic resonance imaging, possible changes in users’ brain activity related to the representation of their robotic sixth finger. For example, it can be observed which areas of the brain are activated when the user moves their finger.
In neuroscience, we speak of the “personification” of a limb to refer to the ability of the human brain to “accept” the foreign limb and believe that it is part of its body.
Another striking example is the “rubber hand illusion”, in which a user fears that their hand will be hit while their “real” arm is elsewhere.
The human brain can integrate foreign limbs
This and other scientific studies carried out in the last decades, including our own, have shown that it is actually quite easy to trick our body into thinking that other artificial limbs are part of it. The human brain is very adaptable and flexible in what it defines and accepts as our body.
This flexibility is helpful because the human body changes as we grow and age. In addition, it helps us adapt to physical changes as a result of accidents or paralysis.
On the other hand, this notion of “incarnation” allows us to accept prostheses to replace or complement lost functions.
Are there limits to accepting a new member?
With our studies on supernumerary members such as the sixth finger, we wonder about the limits of this acceptance. Is it possible to add new members to our body? Can we feel those additional members as part of it?
Several previous studies have attempted to answer this question by adding artificial limbs such as robotic fingers, arms, and a virtual tail to humans.
However, all these attempts have been based on “limb replacement”. In them, the added limb is actuated by movements of an existing limb and any haptic feedback from the added limb is carried over to the existing one.
What is innovative about our study is that we are investigating whether our brain can accept a truly independent extra limb, capable of moving unrelated to the rest, and from which we can obtain haptic feedback, independent of any other limb. And it seems that yes.
From a practical point of view, the results, according to which our brain can accept additional limbs such as a sixth finger, are encouraging for the future development of wearable artificial limbs.
#sixth #finger