Some people with polydactyly can use just one hand to perform tasks for which others would need both their hands.
Image credit: Wilhelmy
An estimated one in every 700–1,000 babies is born with polydactyly, which means they have extra fingers on their hands or extra toes on their feet or both.
Because polydactyly is so unusual, some people may consider it a malformation or anomaly. Many doctors may surgically remove any extra fingers or toes at birth, as they do not consider these digits useful. They may also have concerns about the individual's self-image later in life.
But while its otherworldly aesthetic may be what stands out at first, polydactyly may bring individuals some practical benefits.
This, at least, is what a study from the University of Freiburg in Germany has concluded. The research — which appears in the journal Nature Communications — suggests that people with polydactyly have more dexterity of movement than their counterparts with fewer digits.
In this small study, the investigators worked with two volunteers who both had six fully developed fingers on each hand: a 52-year-old woman, and her 17-year-old son.
"We wanted to know if the subjects have motor skills that go beyond people with five fingers and how the brain is able to control the additional degrees of freedom," says study coauthor Prof. Carsten Mehring.
Are 6 fingers as good as 2 hands?
The researchers asked the two volunteers to engage in various tasks while recording their brain activity through functional MRI. This revealed that the extra digits work independently from the other fingers, moved by their own muscles.
"Our subjects can use their extra fingers independently, similar to an additional thumb, either alone or together with the other five fingers, which makes manipulation extraordinary versatile and skillful," explains Prof. Mehring.
"For instance, in our experiments, subjects can carry out a task with one hand, for which we normally need two hands."
Prof. Carsten Mehring
Moreover, the researchers noticed that even though individuals with polydactyly must control extra digits, this did not seem to place an additional strain on the brain, which they found surprising.
"Despite the extra finger increasing the number of degrees of freedom that the brain has to control, we found no disadvantages relative to five-finger people. In a nutshell, it is amazing that the brain has enough capacity to do it without sacrificing elsewhere. That's exactly what our subjects do," observes another of the study authors, Prof. Etienne Burdet.
'Dedicated neural resources' in the brain
Another study finding indicates that the brains of the two study volunteers had organized specific resources to control the sixth fingers.
"We found dedicated neural resources that control the sixth finger, and the somatosensory and motor cortex are organized exactly to allow for the additional motor skills observed," explain Prof. Andrea Serino and Michael Akselrod, Ph.D., who were in charge of the functional MRI analyses.
These findings offer not just a better understanding of polydactyly, but also allow scientists to see how people's brains adapt to controlling body parts that are not part of the "original template."
"The additional extremities have been trained in the subjects since birth. This does not necessarily mean that similar functionality can be achieved when artificial limbs are supplemented later in life," caution Serino and Akselrod.
"Yet, people with polydactyly provide a unique opportunity to analyze the neuronal control of extra limbs and the possibilities of sensorimotor skills," they add.
In the future, a better understanding of the brain's ability to rewire to accommodate new bodily features could come to the aid of scientists developing wearable robotic limbs that would be able to integrate with a person's nervous system.