Martijn van Mensvoort - © Hand Research
 


JANUARY 31, 2009

The 2th function of fingerprints: the 'touch filter'!
What used to be sort of a mystery is now confirmed by French researchers from 'École Normale Supérieure' in Paris: fingerprint ridges 'filter' vibrations created by finger touch!

Author: (compiled from various news reports)


As you probably know, the word 'fingerprints' refers to the impressions made by the skin ridges on the fingers & thumbs (palms can leave the same impressions: 'palmprints'). These major function of these skin ridges is to provide friction, or traction, when we grasp objects. So, because of the presence of skin ridges on our fingers & palms, objects held in our hands do not slip through our fingers.


Fingerprints filter vibrations & touch



Fingerprint ridges




For decades scientists also believed that the fingerprints (skin ridges) have a second function: the enhancement of our sense of touch. Nevertheless, untill recentely there was not much hard evidence for this assumption. But in january 2009 new research has been presented by a team of French physicists led by Georges Debrégeas at the 'École Normale Supérieure' in Paris. They found that the skin ridges also amplify / filter vibrations triggered when fingertips brush across an uneven surface. These processes help transmiting the signals of 'touch' to deeply embedded nerves involved in fine texture perception.

As you probably know, the word 'finger print' refers to the impression made by the skin ridges on the fingers & thumbs (palms can leave the same impressions: 'palmprint'). The major function of these skin ridges is to provide friction, or traction, when we grasp objects. So, because of the presence of skin ridges on our fingers & palms, objects held in our hands do not slip through our fingers. For decades scientists also believed that the fingerprints (skin ridges) may enhance our sense of touch. For example, in 1943 Cummins & Midlo wrote in their famous work 'Finger prints, palms & soles' (p.23): "Abundant nerve endings in the skin of the palmar & plantar surfaces serve the sense of touch." Nevertheless, there was not much hard evidence for how the underlying mechanism behind this assumption exactly works.

However, recently new research was presented by a team of French physicists led by Georges Debrégeas at the 'École Normale Supérieure' in Paris. They found that the skin ridges also amplify / filter vibrations triggered when fingertips brush across an uneven surface. These processes help transmiting the signals of 'touch' to deeply embedded nerves involved in fine texture perception.






ARTIFICIAL FINGERTIP

Fingerprints may be essential for detecting fine textures. This is what the scientists (Debrégas, et.al.) have found by rubbing an artificial fingertip (see the left picture above) across rough surfaces. The use of an artificial device to determine the role of fingerprints is "very novel," says Mark Hollins a psychologist at the University of North Carolina at Chapel Hill.

PATTERNED VS. SMOOTH FINGERTIPS

Debrégeas and his colleagues hypothesize that fingerprints can act as a 'filter' by reinforcing the friction between our finger pads and objects. In order to test their hypothesis, they created a tactile sensor that closely mimics the actions of our fingers. Their sensor can be equipped with either a "smooth artificial fingertip" or a "fingerprinted artificial fingertip". The researchers report in Science Magazine that certain vibrations from the patterned fingertip are 100 times stronger than those from the smooth fingertip.

The results suggest that the ridged skin acts as a filter that selects which vibrations to transmit to the underlying finger nerves. For a typical human fingertip that has ridges half a millimetre apart and scans a surface at about 10 to 15 centimetres per second, the amplified frequencies would be about between 200 and 300 hertz, the team calculated. Like sunglasses can filter out UV light and let the useful visible light through, the artificial fingerprints filtered out all vibrations above and below 250 hertz, leaving only the vibrations that could be detected by Pacinian fibers:

"Fingerprints might actually improve the sensitivity of perception by enhancing the skin vibrations at a frequency that matches the best frequency of these Pacinian corpuscles," says Debrégeas.

On their own, Pacinian corpuscles have the lowest spatial resolution of the four main types of mechanoreceptors in the body because they have the most extended receptive field. Even vibrations from centimeters away can stimulate them, so how can they pick out minute changes in texture? In an advanced publication in Science Magazine, French scientists report that our fingerprints play a crucial role in boosting the resolution of our touch.






FINGERPRINT ARCHES, LOOPS & WHORLS

The researchers also found that fingerprints only do their vibration-filtering job when the finger motion is perpendicular (at right angles) to the fingerprint ridges. But not to worry: Human fingerprints are patterned in various 'swirls' (the so-called: 'triradii', 'loops' and 'whorls') — every direction of swiping will activate some filtering properties.

"The nice thing is that pattern doesn’t matter," Debrégeas says. The distinctiveness of fingerprint patterns from one person to the next doesn’t seem to have an effect on filtering capabilities.

ROBOT FINGERTIP

Creating an artificial fingertip was a major aspect of the MONAT project, which began in 2006 and is scheduled to end later this year. The project aims to understand how people use their senses of sight and touch to distinguish between natural and synthetic materials.

"The findings could help engineers to design robots with a more refined sense of touch", says Richard Crowder, a robotics engineer at the University of Southampton, UK. Most robots have fairly crude tactile senses that can't distinguish different textures, but such capabilities might eventually be useful for medical robots performing delicate surgery. "It certainly gives us an extra tool in the tool kit," he says.

Lumpkin points out that the study was conducted on artificial tissue, not the real thing, which will be far more difficult. "What we have is a plausible model," Lumpkin says. "This could be the way it works in biology. Now, biologists need to verify that." If the verification process leads to a confirmation of the 'artificial' results, another milestone is ahead for the bionic hand: the inclusion of a true 'finger-tip-touch'!


SUMMARY OF THE PUBLICATION IN SCIENCE MAGAZINE

Title: The Role of Fingerprints in the Coding of Tactile Information Probed with a Biomimetic Sensor

Authors: J. Scheibert, S. Leurent, A. Prevost, G. Debrégeas*

* To whom correspondence should be addressed: G. Debrégeas, E-mail: georges.debregeas@lps.ens.fr

In humans, the tactile perception of fine textures (spatial scale <200 µm) is mediated by skin vibrations generated as the finger scans the surface. To establish the relationship between texture characteristics and subcutaneous vibrations, a biomimetic tactile sensor has been designed whose dimensions match those of the fingertip. When the sensor surface is patterned with parallel ridges mimicking the fingerprints, the spectrum of vibrations elicited by randomly textured substrates is dominated by one frequency set by the ratio of the scanning speed to the inter-ridge distance. For human touch, this frequency falls within the optimal range of sensitivity of Pacinian afferents which mediate the coding of fine textures. Thus, fingerprints may perform spectral selection and amplification of tactile information that facilitate its processing by specific mechanoreceptors.


Sources: Nature, Science News & ARS Technica



FINGERPRINTS

Related sources:
Fingerprints reveal more
A fingerprint characteristic associated with the early prenatal environment
Fingerprint method breakthrough
How fingerprinting works
Dermatoglyphics: a review
Another milestone for the bionic hand
The function of fingerprints: skin ridge friction + a touch filter


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