Breakthrough investigation of people with a sixth finger has implications for infant medicine and cyborgs

Picture in your mind a futuristic, technologically enhanced human. Perhaps you imagined them with a subcutaneous device in their arm for phone calls and browsing the internet. Maybe they are wearing smart glasses for augmented reality. What I'd wager you didn't think of is the presence of an artificial sixth digit attached to each hand.

However, a breakthrough open-access study in Nature Communications – the first to study the physiology and sensorimotor mechanics of polydactyly volunteers (people born with extra fingers) – shows the feasibility and practical advantages that would be gained from such an extra appendage. The results also have implications for the medical treatment of polydactyl people, who often have their extra finger removed at birth on the presumption that it will be of no benefit to them.

Carsten Mehring and his colleagues conducted various tests with two polydactyly volunteers, a 17-year-old boy and his mother, both born with an extra fully formed finger between their thumb and index finger (known as preaxial polydactyly). The researchers note that polydactyly is "not rare", with an incidence of around 0.2 per cent in the population. However, fully formed preaxial polydactyly is a rarer subset of that group.

Using MRI of the volunteers' hands, the researchers established that the extra finger has a saddle joint, similar to a typical thumb, and that it is innervated by its own dedicated nerves. Further tests established that the volunteers had independent control of their extra finger and that they were able to use it to perform a pinch grip with each of their other fingers.

An MRI of the volunteers' brains further showed that the extra finger was represented in the brain independently of the other fingers. Another test, that involved concealing the extra finger and asking the volunteers' to identify landmarks on it, showed that they had an accurate mental representation of their extra digit.

Next, the researchers used video motion capture to observe the volunteers manipulating various objects. This showed that the volunteers engaged in a "rich ensemble of movement patterns" and that they frequently used their extra finger in coordination with both their thumb and index finger (it was not simply used as a substitute for these digits). "Taken together these results demonstrate that the movements of the six fingers of our two subjects had increased complexity relative to common five-fingered hands," the researchers said.

But do these extra movement capabilities provide any functional advantage? Mehring and his team devised a video game that required coordinating key presses to respond to six boxes oscillating progressively faster up and down onscreen. A different key press was required to respond to each of the six boxes, so people with normal five-fingered hands would need two hands to succeed at the task, the researchers note. Critically, the polydactyl volunteers were able to achieve the same impressive game performance with one hand as with two.

Neuroscience and psychology have studied extensively the profound neural consequences for humans of losing a limb or other appendage, including documenting the pain caused by the phantom limb effect (usually explained as due to reorganisation of the brain's representation of missing part). However, this new study represents the first neuroscientific exploration of having an extra body part, finding "… that the human nervous system is able to develop, embody and control multiple extra degrees of freedom and integrate them into coordinated movements with the other limbs, without any apparent deficits or conflicts in the sensorimotor or mental representations."

This has immediate implications for the medical response to polydactyl, suggesting the need to "…thoroughly evaluate the functionality of [the extra digit] in polydactyl infants before deciding whether to remove it." Also, from a cyborg perspective, the results "…suggest that it may be of value to augment normal five-fingered hands with an artificial supernumerary finger," the researchers said. In fact this new research paves the wave for an entire new research endeavor.

"Polydactyl individuals with functional [extra fingers] offer a unique opportunity to investigate the neural control of supernumerary limbs, analyse internal representations of body and the limits of sensorimotor capabilities in humans."

Further reading

—Augmented manipulation ability in humans with six-fingered hands

About the author

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest