Like ‘A Part of Their Body’: People Adapt to an Extra Thumb in Fascinating Experiment

Like ‘A Part of Their Body’: People Adapt to an Extra Thumb in Fascinating Experiment

An experiment in which 36 people were fitted with a robotic third thumb has demonstrated the brain’s uncanny ability to adapt and leverage an entirely new body part, and in ways the researchers are still trying to understand.

The Third Thumb started as an award-winning graduate project at the Royal College of Art in London, England, and it was done to reframe the traditional view of prosthetics. “The project began as a way to better understand what it was like to control something extra attached to my body,” Dani Clode, designer of the Third Thumb, explained in an email. “As a prosthetic arm designer, I wanted to understand the unique relationship between a person and a prosthesis. It’s a relationship unlike any other product, and I wanted to explore that.”

Indeed, the Third Thumb represents an augmentation of the human body, as opposed to the replacement or restoration of “normal” human functionality. It’s a very transhumanist concept, but scientists don’t actually know if the human brain can meaningfully support an added body part or the long-term consequences of the extra cognitive load.

A user's view of the Third Thumb.  (Image: Dani Clode)
A user’s view of the Third Thumb. (Image: Dani Clode)

“These questions are complex and require the collaboration of experts from different fields,” Tamar Makin, professor of cognitive neuroscience at University College London and head of the Plasticity Lab, said in an email. “In our study, we used Dani’s cleverly designed Third Thumb to explore how the human brain can support an extra body part, and how the augmentative technology might impact our brain.”

The answers are important, as an additional thumb could lead to a host of benefits. It could help with repetitive, difficult, and physically demanding tasks, while also being of assistance to people who have either permanently or temporarily lost the use of one hand. It could also result in entirely new capabilities and activities, whether it be a new way of playing a musical instrument (or enabling the invention of a new type of musical instrument!) or the advent of an entirely new sporting activity.

“On a more day-to-day level, some of our participants reported using the Thumb to flip pages of a book, holding a banana while peeling it with the rest of their fingers, or opening a bottle one-handed,” wrote UCL neuroscientist Paulina Kieliba, an author of the new Science Robotics study, in an email. “Most of them used it to pick up and carry small objects while their hand was occupied with other tasks. Personally, I found the Thumb really useful for getting the keys out of a pocket, while simultaneously browsing the internet on my phone.”

The Third Thumb being used to hold multiple balls.  (Image: Dani Clode)
The Third Thumb being used to hold multiple balls. (Image: Dani Clode)

A group of 36 healthy participants were trained on the device (professional musicians were excluded) and used it to perform a variety of tasks, such as building a tower of blocks, manipulating multiple balls simultaneously, or stirring a coffee cup with a spoon. They even managed to perform some of these tasks while blindfolded or distracted with maths problems.

“The Third Thumb is a flexible 3D-printed thumb extension for your hand, controlled by your feet,” said Clode, a designer at UCL. “It is worn on the hand, next to your little finger, and dynamically moves like a thumb by using two motors controlled by two pressure sensors retrofitted into your shoes, under your toes, and communicating with the Thumb piece wirelessly.”

The use of feet to control this device might seem weird, but Clode compared it to driving a car, using a sewing machine, or playing the piano — all activities that make use of foot pedals.

The participants had access to the device for five days, and their brains were fMRI scanned both before and after the experiment. They learned quickly, using the device to pick up wine glasses, sort objects, and go about their daily routines. The participants were encouraged to take the Thumb home each day, allowing them to wear the device between two and six hours each day. A control group was also created, in which participants used a static third thumb with no robotic capability.

A person supporting a coffee cup with the Third Thumb, while stirring a spoon with their natural fingers. (Image: Dani Clode)
A person supporting a coffee cup with the Third Thumb, while stirring a spoon with their natural fingers. (Image: Dani Clode)

By the end of training, “some of our participants even reported that they started to feel like the Thumb was becoming a part of their body,” said Clode. “We were also surprised to see people forming such strong bonds with the Thumb.” Some participants “needed a little bit of time to say goodbye” to the prosthetic, and some even said they felt like “they were missing something after the training had finished,” she added.

As the training progressed, the participants changed the way they used the device, which resulted in new finger coordination patterns. This was recorded in their hand movements as well as in their brains.

In the brain’s sensorimotor cortex, “each individual finger is represented distinctly from the others, forming what we call a hand representation,” Kieliba explained. After using the device, this hand representation shrunk in the participants’ brains, in that the neural activity patterns corresponding to the individual fingers became less distinct and more alike.

“This is a very important message for everyone interested in safe and successful motor augmentation — augmentation may incur changes to how the brain represents our bodies — those changes need to be understood and explored further before this technology can be widely implemented,” said Kieliba.

At the same time, scientists have seen evidence for brain plasticity when studying how prosthetic devices are represented in the brains of users and amputees, added Makin. Brain plasticity can be understood as a “bidirectional process,” in which the brain will adapt both the representation of the prosthesis and the user’s body to improve adaptability, she said.

Looking ahead, the team would like to develop a Third Thumb that’s easy to use while walking (a problem with the current design) and also a prosthetic that’s safe to use in an fMRI scanner, which would allow the team to study the brain while the device is in use. We wish them the best of luck, as our augmented transhuman future awaits.

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