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."
