The findings could show us how to tap into the brain's resources to control robotic extra limbs and digits.
Worldwide, about one in every 500 babies are born with extra fingers or toes - a condition known as polydactyly. Often seen as a 'birth defect', extra digits are usually removed shortly after birth.
Research into polydactyly has focused mostly on the genetic mutation behind it - but until now, nobody has studied how the brain and body makes up for the extra workload when the extra digits are functional.
However, new research from the University of Freiburg in Germany, Imperial College London and Université de Lausanne / EPFL in Switzerland has found that the brain allocates dedicated areas to the extra digits, making them as useful as standard digits.
The authors say this means polydactyl brains could teach us how our brains adapt to extra workloads. The findings present an argument for keeping the extra toes or fingers with which some people are born, if they are well-formed and functional.
Each of our fingers is joined to the hand by dedicated tendons, moved by dedicated muscles, and linked to dedicated nerves - all of which are specific to each finger. These are controlled by dedicated brain areas specific to each finger in the motor cortex - the brain region responsible for movement.
The researchers wanted to find out how extra digits fit into this arrangement. Senior author Professor Etienne Burdet, of Imperial's Department of Bioengineering, who carried out this study with his colleagues in Germany and Switzerland, said: "Extra fingers and toes are traditionally seen as a birth defect, so nobody has thought to study how useful they might really be."
The authors of the paper, published in Nature Communications, studied two people - a 52-year-old woman and her 17-year-old son - who both have six fingers on each of their hands, with a well-formed extra finger between the thumb and forefingers.
To study the potential benefits of their extra fingers, the researchers had the subjects explore objects with their hands, tie shoelaces, type on their phones, and play custom-made video games - movements classed an 'manipulation'.
They analysed and compared the movements to the movements of control subjects with five fingers on each hand. During manipulation, high-resolution functional magnetic resonance imaging (fMRI) monitored their brain activity.
The researchers found that, like non-polydactyl fingers, the extra digits had their own dedicated tendons, muscles, and nerves, as well as extra corresponding brain regions in the motor cortex.
Polydactyl participants also performed better at many tasks than their non-polydactyl counterparts. For instance, they were able to perform some tasks, like tying shoelaces, with only one hand, where two are usually needed.
Although controlling the extra fingers requires extra work for the brain, the two subjects suffered no obvious cognitive disadvantages.
Professor Burdet said: "The polydactyl individual's brains were well adapted to controlling extra workload, and even had dedicated areas for the extra fingers. It's amazing that the brain has the capacity to do this seemingly without borrowing resources from elsewhere."
The authors say the findings might serve as blueprint for the developing artificial limbs and digits to expand our natural movement abilities. For example, giving a surgeon control over an extra robotic arm could enable them to operate without an assistant.
Lead author Professor Carsten Mehring of Freiburg University said: "Perhaps we can tap into the brain resources demonstrated in this study to make this possible."
However, he also warned that people with robotic extra limbs may not achieve as good control as observed in the two polydactyl subjects. Any robotic digits or limbs wouldn't have dedicated bone structure, muscles, tendons or nerves. In addition, subjects would need to learn to use extra fingers or limbs, much like how an amputee learns how to use a prosthetic arm.
Professor Mehring added: "In our study, the extra digits have been trained in the subjects since birth. This does not necessarily mean that similar functionality can be achieved when artificial limbs are added on later in life.
"Yet, people with polydactyly provide a unique opportunity to analyse the neural control of extra limbs and the possibilities of to boost sensorimotor control."
The research was funded in part by the German Research Foundation (DFG), the European Commission, the UK EPSRC and the Swiss National Science Foundation.