October 1, 2013
Touch Receptors In Skin Distinguish Textures Via Vibrations
April Flowers for redOrbit.com - Your Universe Online
Humans can distinguish between fine textures, such as silk or satin, through vibrations, a new study has found. These vibrations are picked up by two separate sets of nerve receptors in the skin and is relayed to the brain.
Previously, studies have shown that course textures like Braille dot patterns are encoded by receptors that are densely packed into the primate fingertip. There is a correspondence between the spatial layout of these receptors and the spatial layout of surface features of a texture. Most natural textures are too fine to be perceived in this way, however.
A new study shows, for the first time, that two other sets of receptors convey information about fine textures by responding to the high-frequency vibrations produced in the skin as it is scanned across a surface. The study is published early online in the Proceedings of the National Academy of Sciences.
"Coarse textures are reflected in the spatial pattern of responses by one set of receptors, but that's only a small part of the story," said Sliman Bensmaia, PhD, assistant professor in the Department of Organismal Biology and Anatomy at the University of Chicago. "Most of what we consider to be natural textures are represented in temporal patterns of activation in the other two groups of receptors."
Most previous studies that have investigated the neural basis of texture perception have used course materials - gratings and Braille patterns. These activate a set of receptors in the skin called slowly adapting type 1 (SA1) afferents.
For the current study, the team used a drum covered with strips of such coarse textures, along with several materials with finer textures, such as sandpaper, fabrics and plastics. While the researchers recorded the neuronal responses, the drum ran the textures across the fingertips of Rhesus macaques whose somatosensory system is similar to humans.
Course textures produced the expected SA1 response, while the majority of finer textures did not cause the SA1 afferents to fire. Two sets of afferents, rapidly adapting (RA) and Pacinian (PC) fibers, which have never before been implicated in texture sensation, responded in a temporal pattern that followed the vibrations produced in the skin by scanning the surface.
"If you relied on SA1 afferents alone for texture perception, you would not be able to discriminate most textures. You couldn't tell silk from satin, or denim from felt and corduroy," Bensmaia said.
Before this study, rapidly adapting afferents were primarily thought to play a role in detecting when an object was slipping from a grasp. Scientists believed that pacinian afferents were to detect vibrations such as those felt after striking something with a hammer.
The study also highlights that touch sensation uses two modes of operation that coexist and interact. One mode is based on spatial patterns of activation, the second on temporal patterns.
There are important implications for the field of neuroprosthetics in this new study. Neuroprosthetics seeks to develop devices that can substitute for motor, sensory or cognitive functions that might have been damaged through injury or disease. Both modes must be engaged to produce realistic tactile sensations, whether the goal is to elicit texture perceptions or any other kind of artificial tactile sensation.
"What we've shown here is that all three sets of afferents contribute to texture perception," Bensmaia said. "In fact, signals from all three populations are integrated to culminate in any kind of tactile perception."