Our fingertips have an extraordinarily high sensitivity to touch – and now it looks like that sensitivity might be largely confined to the ridges of our fingerprints.
“They really help us get very detailed information about what we touch,” says Ewa Jarocka at Umeå University in Sweden.
Scientists have suspected that our circular, winding fingerprints might have evolved to improve our ability to grip objects by creating better friction, says Jarocka. But she says others have suggested they might contribute to our “very refined sense of touch”.
Because current models can’t explain the high levels of sensitivity people have shown in past scientific studies, Jarocka and her colleagues decided to investigate.
They asked six men and six women between the ages of 20 and 30 to lie comfortably in a dentist chair while their fingers were held in place. The researchers then ran a card covered in tiny, flat-tipped cones, each less than half a millimetre high, over their fingertips at different speeds and in different directions. Meanwhile, they recorded electrical activity of a single nerve cell using tungsten electrodes inserted into a main nerve in each participant’s upper arm.
The results allowed the researchers to map out exactly where on the fingertips the information that was sent to the nerve was collected. These sensitivity hotspots turned out to be very small, each only about 0.4 millimetres wide.
What’s more, these hotspots followed specific patterns on the fingertips – the same ones as the fingerprint ridges. Regardless of how the researchers moved the dotted card over a finger, its hotspot map stayed the same, suggesting the sensitivity zones were “anchored in the very stable structure” of the ridges themselves, says Jarocka.
“We have all those multiple hotspots, and each one responds to the details of 0.4 millimetres, which is the approximate width of the [fingerprint] ridge,” she says. “Then our brain receives all that information. This really offers an explanation to how it’s possible that we’re so dexterous and have such a high sensitivity in our fingertips.”
This doesn’t mean fingerprints might not have other functions as well, however – perhaps including improving grip, says Jarocka. But it does reveal the important role that the ridges play in touch.
“Now that we know that the single neuron can be so sensitive on such a [precise] scale, we can finally explain how people can be so detail-sensitive,” she says.
Journal reference: JNeurosci, DOI: http://dx.doi.org/10.1523/JNEUROSCI.1716-20.2021