 Fingerprints key to ability to process fine textures: study
Fingerprints don't exist simply to improve gripping ability or identify cat burglars - they may also act as a type of touch filter that improves perception, according to a new study.
30/01/2009 5:12:12 PM Sympatico msn
A fingerprint from contact with a flat surface. The contact occurs only along the summits of the epidermal ridges. This contact pattern results in a particular filtering of tactile information during active exploration of a surface.
(Picture courtesy of Science/AAAS)
CBC News
The study, published in Friday's edition of Nature, suggests that fingerprints may help perceive very fine textures more efficiently. Specifically, fingerprints can tune out vibrations outside a certain frequency so only very specialized touch receptors can pick them up.
These receptors, called Pacinian fibres, are involved in detecting fine features. But they also have the lowest spatial resolution of any of the receptors involved in touch. This means that, taken alone, they are the least able to distinguish between two closely spaced pressure points, a characteristic that appears to be a contradiction for a receptor involved with fine touch, the study says.
The researchers from the University of Paris think this is where fingerprints come into play. They said there is no way to experimentally measure stress under the skin on a finger, so they created a synthetic mechanical sensor instead. They then placed two types of rubber caps on top of the sensor - one smooth, and the other ridged like a fingerprint.
The sensor was then rubbed across a glass surface covered with patterned textures. They found that the sensor covered with the smooth cap picked up a large variety of vibrations. But in the case of the ridged sensor, it filtered out many of the vibrations picked up by the smooth cap and amplified them.
The researchers concluded that a typical set of fingerprints would produce vibrational frequencies in the range of 200 to 300 Hz. Pacinian fibres respond best to frequencies around 250 Hz, and would therefore be able to pick up these filtered vibrations.
"Fingerprints thus allow for a conditioning of the texture-induced mechanical signal which facilitates its processing by specific [touch receptors]," the study says.
The study authors concede that their sensor offers "a crude version of finger physiology." Amplification of the vibrations depends strongly on the orientation of ridges with respect to the movement of the finger. In the study, the ridges had to be oriented perpendicular to the direction of movement.
But the authors note that fingerprints are arranged in swirls, so that each region of the fingertip can achieve optimal orientation.
"Further studies are needed in order to elucidate how this may reflect on the exploratory procedures [such as fingertip trajectory and contacting zone] used by humans during texture evaluation tasks," the study says.
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