A recent study suggests that the adult human brain is incapable of adjusting its sensory map to match the location of bionic sensors. This may make it more difficult to obtain precise sensory feedback from bionic hands.
The human brain retains a sensory map of amputated limbs. When the brain attempts to move a missing limb, it still sends signals to any remaining nerves. Intercepting these signals and translating them into commands is the basis for bionic arm/hand user control systems.
However, this isn’t a one-way system. Electrically stimulating a nerve can send information back to the brain, fooling it into believing it has touched something with the missing limb. This is the basis for sensory feedback.
The most sophisticated way to stimulate a nerve is to surgically connect an electrode to it.
In a perfect world, the stimulation would mimic the anticipated sensory feedback from a bionic device. Touch your bionic thumb and your brain is tricked into thinking that you have touched your natural thumb.
At least that’s the plan!
It is quite difficult for surgeons to implant electrodes for precise communication with a nerve. If the position of the electrode is off even slightly, it can result in imprecise stimulation. If you touch your bionic thumb, it may feel as if you have touched another finger instead.
This could be quite problematic if the purpose of the feedback is to improve user control over individual digits.
Scientists had hoped that repeated visual cues about the correct touchpoint would eventually help the brain compensate by adjusting its sensory map. Unfortunately, this doesn’t seem to be the case:
This diagram represents the results of a study titled “Use of a Sensitized Bionic Hand Does Not Remap the Sense of Touch” 1. In this study, three participants received stimulation from a bionic thumb through an implanted electrode. The perceived location of the stimulation differed from the actual touchpoint.
Despite using their bionic hands continuously for up to three years, none of the participants experienced any adjustment to their sensory maps. Stimulation applied to the thumb continued to be perceived at other locations even though there were repeated, long-term cues that this was not correct, such as visual awareness that the thumb was the true contact point.
The sensory map of an adult brain appears to be quite rigid.
What can bionic hand users learn from this study?
Non-invasive sensory feedback systems using vibrators in the socket or transcutaneous electrical nerve stimulation (TENS) via an arm cuff already exist. There are two disadvantages to these types of systems:
- They’re unable to convey complex sensory feedback such as texture, shape, etc.
- They’re non-intuitive. That is, the brain must learn to correlate crude signals like the strength of vibration to grip force.
The only reason to undergo the added expense and risks (scarring, infection, etc.) of surgery is to address these shortcomings. But if an electrode can’t stimulate a nerve with sufficient precision, the second disadvantage remains, i.e. a user will have to consciously interpret sensations at one location as having occurred at another.
If you think this might be taxing when dealing with simple points of contact, imagine how confusing it might be when trying to sense the shape of an object.
Our verdict? Not every neural interface will be identical to the type referenced in this article. New advances are occurring all the time. But if you are considering having electrodes surgically implanted to improve sensory feedback, you should thoroughly discuss this issue of precision with your doctor, especially for use with a bionic hand.
For a comprehensive description of all current upper-limb technologies, devices, and research, see A Complete Guide to Bionic Arms & Hands.
 Ortiz-Catalan, Max & Mastinu, Enzo & Greenspon, Charles & Bensmaia, Sliman. (2020). Chronic Use of a Sensitized Bionic Hand Does Not Remap the Sense of Touch. Cell Reports. 33. 108539. 10.1016/j.celrep.2020.108539.