SUSANNAH WALKER AND FRANCIS MCGLONE
Human skin has two different ways of perceiving its environment: through relatively large, fast discriminative touch nerve fibers that detect objects and through smaller, unmyelinated C fibers that are typically associated with itch or touch. “One is about sensing the world and the other is about feeling,” Susannah Walker of Liverpool Johns Moores University in the U.K. told The Scientist today (November 17) at the Society for Neuroscience (SfN) conference in Washington, DC.
One type of C fiber, known as C-tactile afferents (CTs) is specifically dedicated to the sensation of gentle touch, responding most strongly to strokes around 3 to 10 centimeters/second. In mice, these fibers are found to be most dense on the back and upper extremities, with fewer CTs found on the lower limbs and feet. Similarly, humans do not have CTs on the palms of their hands or the soles for their feet, but the distribution of these fibers throughout the body is largely unknown. Moreover, it is currently unclear how people respond to seeing someone else being gently touched.
To address these questions, Walker and her colleagues asked participants to rate the pleasantness of static, slow, and fast touches as they viewed them being applied to the back, shoulder, forearm, or hand of a person on a video screen. As expected, slow touches to the back and shoulder were rated the most pleasant, while slow touches to the forearm were less so. In all three locations, participants rated slow touches as more pleasurable than fast or static touches. On the palm, however, where no CTs exist, slow strokes were not perceived as more pleasant than static touch.
The results, presented this week at SfN, suggest that people are empathetic towards such gentle touches and that, as in mice, CTs are differentially distributed throughout the body. Walker and her colleagues propose an emotional homunculus, analogous to the classical homunculus that depicts the amount of brain matter dedicated to processing signals from the body's discriminative touch neurons. Instead of big lips, hands, and feet, however, the emotional homunculus would have a big back and shoulders.
Understanding where CTs exist and how they function could inform how these nerve fibers help build and maintain social relationships, Walker said. Preterm babies, for example, benefit greatly from daily stroking, growing faster and showing reduced signs of stress. Conversely, lack of physical contact has been associated with poor outcomes in children at orphanages. It seems that “these CTs are providing that signal of safety and well-being,” Walker said.