Fingerprints: Signal Processors for Touch

They may seem little more than digital decoration, but biomechanics have long known that fingerprints have at least one use: they increase friction, thereby improving grip. There have also been hints that fingerprints play a role in the sensation of touch. One possibility is that when a finger is moved across a surface, each ridge in a fingerprint acts like a tiny lever, magnifying the subsurface strain for the nerve endings beneath.

Today, Georges Debregeas et amis at the University of Paris 6 and 7 say that’s only part of the story. In fact the role that fingerprints play in touch is far more important and subtle than anyone imagined.

Debregeas et amis say it looks as if the ridges and whorls in fingerprints filter mechanical vibrations in a way that best allows nerve endings to sense them.

The mechanoreceptors that do this job are called Pacinian corpuscles. They sit at the ends of nerves and are responsible for sensing pressure and pain. These devices can sense vibrations over a wide area of skin but are sensitive only to a limited range of vibrations. In fact biologists have known for some time that Pacinian corpuscles are most sensitive to vibrations at 250Hz.

So how do fingers generate this kind vibration? Biologists have always assumed that humans can control the frequency of vibrations in the skin by changing the speed at which a finger moves across a surface. But there’s little evidence that people actually do this and the Paris team’s discovery should make this view obsolete.

Debregeas and co have investigated this problem using a “cyber finger” that they built in their lab, complete with synthetic fingerprints on the same scale as human ones and a microelectromechanical sensor that measures force with a spatial resolution of millimetres. What they’ve found is astonishing.

They say that fingerprints resonate at certain frequencies and so tend to filter mechanical vibrations. It turns out that their resonant frequency is around 250 Hz. What an astonishing coincidence!

That means that fingerprints act like signal processors, conditioning the mechanical vibrations so that the Pacinian corpuscles can best interpret them. It’s this optimisation process that allows us to sense textures with a spatial resolution far smaller than the distance between Pacinian corpuscles in the skin.

That shouldn’t surprise anybody who has been following developments in robotics in the last few years. There is a growing awareness that the processing power of the nervous system, including the brain, simply cannot handle the volume of number crunching that has to be done to keep a living body on the road.

Instead, it looks increasingly clear that the brain outsources much of this work to the body itself: to the joints, ligaments, muscles, skin etc. Understanding how these materials do all this processing is turning materials science into a branch of computer science. It’s even got a name: morphological computing.

This work of Debregeas et amis is another example of morphological computing and it’d be interesting to see a treatment of this problem from the information theoretic point of view.

This work on fingerprints should have important implications for our understanding of touch. It should also help in the development of better prosthesis and may even help to give robots a better tactile sense of their own.

Useful things then, fingerprints.

Ref: arxiv.org/abs/0911.4885: The Role of Fingerprints in the Coding of Tactile Information Probed With a Biomimetic Sensor