Fingerprints help amplify vibrations detected by nerves deep under the skin.
Fingerprints may be essential for detecting fine textures, scientists have found by rubbing an artificial fingertip across rough surfaces.
The tiny ridges on our fingers help us to grip objects by increasing friction. Now a team of physicists led by Georges Debrégeas at the École Normale Supérieure in Paris has found that these ridges also amplify vibrations triggered when fingertips brush across an uneven surface, helping transmit the signals to deeply embedded nerves involved in fine texture perception.
The physicists covered a pressure-detecting sensor, meant to represent a nerve fibre, with an elastic material that was either smooth, or patterned with fingerprint-like ridges. Then they moved the 'fingertip' over a glass slide that had been covered with raised lines of uneven spacing and thickness, causing the artificial skin to vibrate at varying frequencies that could be detected by the sensor.
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.
Studies on real fingertips by Hollins and his colleagues had shown that humans feel fine textures by moving their fingers over a surface and triggering vibrations in the skin1. The vibrations are picked up by nerves. But some of the nerve endings, called Pacinian corpuscles, are relatively deep — about 2 millimetres — under the skin, raising questions about how they could detect such subtle vibrations.
Debrégeas and his colleagues report in Science2 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 nerves. For a typical human fingertip that has ridges half a millimetre apart and scans a surface at 10–15 centimetres per second, the amplified frequencies would be about 200–300 hertz, the team calculated. That frequency range is significant because the embedded Pacinian nerve fibres are most sensitive to vibrations at 250 hertz.
"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.
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.
The team moved the artificial fingertip at just a fifth of a millimetre per second but people tend to scan surfaces more quickly, Hollins notes. The next step is to test the artificial fingertip at different speeds "to make sure everything is still working the same at higher velocities", he says.
Bensmaia, S. & Hollins, M. Percept. Psychophys. 67, 842–854 (2005).
Scheibert, J., Leurent, S., Prevost, A. & Debrégeas, G. Science advance online publication doi:10.1126/science.1166467 (2009).
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Kwok, R. Fake finger reveals the secrets of touch. Nature (2009). https://doi.org/10.1038/news.2009.68