How an extra thumb changes the way your brain perceives the hand
A few years ago a London-based designer named Dani Clode introduced the world to the Third Thumb, a novel robotic finger controlled using pressure sensors under one’s feet. The project was more a novelty than a prototype, exploring futurist ideas surrounding body augmentation.
A team of neuroscientists working in The Plasticity Lab at University College London saw the Third Thumb on the news and immediately contacted Clode. They were studying how the human brain adapts to operate augmentative technology, and the Third Thumb was a perfect device to incorporate into their research.
"Body augmentation is a growing field aimed at extending our physical abilities, yet we lack a clear understanding of how our brains can adapt to it,” explains Tamir Makin, head of the Plasticity Lab and lead author on the new research. “By studying people using Dani's cleverly-designed Third Thumb, we sought to answer key questions around whether the human brain can support an extra body part, and how the technology might impact our brain."
The researchers recruited 20 subjects who each spent five days training in the lab to use the Third Thumb. They were encouraged to take the device home with them each evening and try to use it for between two and six hours in total per day.
At the beginning and end of the study all participants were scanned using MRI to track brain activity while moving their fingers. A control group of ten participants were also recruited. They completed the same five day training protocol but used a static version of the Third Thumb.
The research discovered after just five days of using the Third Thumb there were significant changes to activity in the brain’s sensorimotor cortex. The biggest change came in areas of the brain responsible for hand representation.
Generally, when we use tools our brain doesn’t alter its biological representation of a hand. So to use a hammer, for example, we simply grip the tool tightly with our hand. Our brain still understands the shape of our hand and five fingers.
But the striking finding in this new study was how quickly that fundamental representation of the hand changed in the brain. After just five days of using the Third Thumb, brain activity associated with each finger had become less distinct. This means neural hand representation in the sensiromotor cortex had reduced.
“In our lab, we have been looking at hand representations for a while now, and we usually see hand representation remain very stable, very consistent within and across participants,” explains Paulina Kieliba, first author on the study. “For example, even after the most profound change to the hand – arm amputation, the representation of the amputated hand remains stable in the brain. So, to see it change, after only 5 days of using the Third Thumb is not trivial to us!”
A follow-up MRI conducted one week later revealed these sensorimotor changes had returned to normal. Kieliba says this swift change back to normal neural hand representation could be related to the short time frame of the study. This particular aspect of the research needs much more investigation as there could be major implications for safety if body augmentation devices permanently change the brain’s ability to control the body.
“Would people wearing extra arms for a prolonged period of time, for example, while working in the factory, be able to efficiently re-adapt to their natural body movements when driving home afterwards?” asks Kieliba. “We must ensure that these devices are safe to use, even after the user takes them off.”
And it is here the research quickly turns from an academic curiosity to something with real-world implications. Industrial exoskeletons or devices offering extra limbs are no longer the realm of science fiction. So it is crucial we understand how our brains respond and adapt to these devices.
“… what would happen if we give these devices to children or adolescents - how would augmentation impact their vastly more plastic brains?” asks Clode. “We need to ensure that by learning to control an augmentation device, we are not negatively impacting the motor capabilities of the biological hand and body.”
The new study was published in Science Robotics.