Participants using an extra thumb to go about their daily tasks readily adapted to the robotic appendage over five days, but the relationship between their brain and their biological hand changed over this period. The new study, published in the May 19 issue of Science Robotics, addresses key unresolved questions regarding human brain adaptation to augmentative devices.
The study found that people quickly learned to control the extra thumb in daily life, but behavioral tests and MRI scans showed that the brain space devoted to representing the biological hand shrank over the same period.
"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, [and] we need to better understand these potential changes," said Paulina Kieliba, research assistant at University College London and first author of this study.
The researchers say that their hand-augmenting device has the potential to benefit both able-bodied individuals and those with hand impairments, in many different areas from factory work to performing surgery. But before introducing augmentative technologies into real-world applications, a greater understanding of how they impact the brain is crucial. If the brain cannot adapt to these devices in a safe and productive manner, it can restrict widespread implementation and acceptance of augmentative devices by society.
"Motor augmentation has the potential to transform our lives, from how we interact with our immediate environment to how we care for patients with motor disabilities," said Dani Clode, a co-author of the paper and senior research technician at University College London. "And so, we need to know whether our brains can handle and support that."
The Third Thumb
Clode, formerly a design student at Royal College of Art, designed the "Third Thumb" as part of her master's project. The 3D-printed thumb is driven by motors strapped to the wrist and controlled using sensors under both big toes, with the right toe able to control device flexion and extension while the left toe drives adduction and abduction.
"As an upper-limb prosthetics designer, I wanted to understand the unique relationship between a person and a prosthetic," said Clode. "It's a relationship unlike any other product, and I wanted to explore that."
She and her robotic thumb were recruited by a neuroscience lab studying human augmentation at University College London. Clode and her colleagues investigated the neurological effect of the Third Thumb in 36 able-bodied, right-handed participants recruited from internet ads and randomly assigned to either augmentation or control groups. Professional musicians were excluded from the study, as previous studies suggested musicians' brains approach finger movement differently than the average non-musician.
Participants wore the Third Thumb at the end of their dominant hand next to the pinkie. After a five-hour preliminary training session to familiarize themselves with the thumb, the participants used their extra finger over five days in lab settings and while out and about. The researchers monitored Third Thumb use outside of the lab by recording when the Third Thumb's motors were turned on and off. The control group wore a static, "dud" version of the thumb.
"The thing that is really unique about our study is that this is the first time we have actually been able to allow people to use an augmentative device outside of the lab and see how taking advantage of the extra thumb changes the way people use their hands," said Kieliba.
Each of the participants underwent MRI and behavioral tests before Third Thumb use to establish a baseline, as well as after the five-day period to evaluate neurological changes. The tests revealed great improvements in motor control, dexterity, and hand-robot coordination of the extra thumb, even when the brain was preoccupied or vision was obstructed.
However, the tests also revealed that Third Thumb use altered the way the brain represented coordinated joint movements of the natural hand. The researchers observed a shrinkage of the biological hand representation in the sensorimotor cortex. This trade-off effect was observed even while participants were not using or wearing the Thumb.
"Augmentation devices require you to edit the way you use your biological body, in order to gain function," said Tamar Makin, professor of cognitive neuroscience at University College London. "The Third Thumb does not occupy your hand-space — it extends it — but in order to do more, you need to first learn new finger coordination patterns, and that can require changes to your body representation in the brain."
Toward a Better Understanding of Augmentation
In follow-up MRI scans in 12 of the participants conducted seven to 10 days after ceasing Third Thumb use, the researchers found no significant difference between the augmented hand's representation in scans conducted immediately after Third Thumb use and in follow-up scans. This suggests that changes in the brain's perception of the hand could last up to a few days even after participants took off their extra thumb.
Before drawing conclusions, the researchers note that the altered representation could be either adaptive or maladaptive. They aim to evaluate this question in future studies, as well as explore the effect of augmentative tech on the minds of children and teens, and whether users can safely re-adapt to biological hands after prolonged use of the Third Thumb.
Clode has also developed an MRI-safe Third Thumb, which could allow for a deeper look into the neural activity of the participants while they are using the robotic finger.
"The success of our study shows the value of neuroscientists working closely together with designers and engineers, to ensure that augmentation devices make the most of our brains' ability to learn and adapt, while also ensuring that augmentation devices can be used safely," said Clode.