Fingerprints are for more than a good grip; they also allow fingers to feel fine textures, according to a new study. As fingers move across a surface, the intricate geography of the finger tips, known as epidermal ridges, help select and amplify just the right vibrations to convey information from the skin to the brain. Neuroscientist Ellen Lumkin compares the ridges on fingers to the cochlea in the ear. “Like the cochlea is a frequency analyzer for sounds, the fingertips are frequency analyzers for fingers,” says Lumpkin [Science News] Fingerprints help filter out the tactile equivalent of white noise.
When a finger sweeps over a finely textured surface, such as a cotton sleeve or a wooden coffee table, the interaction sends a large range of vibrations into the skin. Specialized sensors called Pacinian fibers, the tips of nerve fibers, detect only a select few of the vibrations — those right around 250 hertz — before sending the signal to the brain, where the touch sensation is processed [Science News]. But since Pacinian fibers are located relatively deep—about 2 millimeters—under the skin, researchers guessed that fingerprints help magnify the vibrations.
To study the effect of fingerprints, they created a tactile sensor that closely mimics the actions of our fingers. Their sensor can be equipped with either a smooth fingertip or a “fingerprinted” one. The researchers compared how these two types of fingertips performed on different textures by measuring their pressure variations with a microforce device [Ars Technica]. The “fingerprinted” sensor, which has parallel ridges about half a millimeter apart, imitates the human finger. Like sunglasses that filter out UV light and let the useful visible light through, the artificial fingerprints filtered out vibrations above and below 250 hertz, leaving only the vibrations that could be detected by Pacinian fibers [Science News]. The smooth sensor created less friction and picked up a wider range of unamplified signals, researchers reported in Science [subscription required]. Lead researcher Georges Debrégeas says, “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,” [Nature News].
The researchers noted that their artificial fingerprints worked only if the direction of motion was perpendicular to the direction of the ridges. Thankfully, the whorls, arches, and loops on real human fingertips mean that swiping in any direction will activate the filtering effect. This could imply that the contours of our fingerprints are patterned to optimize texture perception [Ars Technica]. Understanding the physics behind touch could help scientists build better prosthetics for amputees or robots capable of a sensitive touch and delicate handiwork. But the first challenge, the researchers say, is to replicate the results from the elastic faux fingers using real human fingers.