Type an email on your computer. Raise a glass to your lips. Feel for the light switch in a dark room. Simple, everyday tasks can demand subtle interactions between our hands and our surroundings, but, surprisingly, we still don’t know a lot about the mechanics of the sense of touch.
“Most people don’t have a very clear picture of how touch sensation actually arises,” says Yon Visell, assistant professor in the electrical and computer engineering department at the University of California, Santa Barbara.
While people are familiar with touch when it comes to the interaction between two surfaces—the skin and whatever it is in contact with—they are less aware of the subtle ways that touch sensing helps us to identify and navigate our surroundings, he says.
For instance, if your fingers are numb, you may still be able to move them, but be hesitant to pick up an object or send a text message because of the lack of sensation—think of what happens when your foot or arm falls asleep.
[Robot fingers touch with fiber optic sensors]
Our hands in particular have access to rich tactile information that travels far beyond the tips of our fingers. This may help to explain some remarkable capabilities of the sense of touch—why, for example, people whose fingers have been anesthetized are still able to feel fine surface detail, as has been demonstrated in prior research.
“Most people don’t have a very clear picture of how touch sensation actually arises.”
“The way they seem to be able to do this is by using mechanical signals, or vibrations, that travel beyond the fingers, farther up the arm,” Visell says. “The hand has specialized sensory end organs distributed widely in it that can capture such mechanical vibrations at a distance.”
Published in the Proceedings of the National Academy of Sciences, the study used a specialized array of tiny accelerometers, or vibration sensors, worn on the sides and backs of the fingers and hands. The device allowed researchers to, for the first time, capture, catalog, and analyze patterns of vibration in the skin of the whole hand that were produced during active touch.
Actions such as tapping and sliding one or several fingers over different types of material, as well grasping, gripping, and indirect tapping (using an object to tap on a surface) all gave rise to distinctive vibration signatures. “We can liken this to the different ways that a bell will sound if it is struck by a metal hammer or a rubber mallet,” Visell says.
[Fish fins can sense touch just like fingertips]
“How do those signals reflect what it is that we’re doing and what it is we’re touching? Do parts of the hand nearer to the wrist receive significant information about the shape of the object that we’re touching, what it’s composed of, or how we’re touching it? How are different parts of the hand involved in touch sensing?” Visell says of the fundamental questions that motivated his group to pursue this research. “It is possible that the hand, like the ear, is able to use vibrations produced through contact in order to infer what is being touched, and how the hand is touching it?”
The vibrations generated through touch, and the distribution of vibrations in the hand, depend very closely on the type of action and the object being manipulated. For instance, vibration patterns produced by tapping a single finger were stronger than those made by grasping, gripping, or sliding, but were much more localized in the finger. The patterns of vibration throughout the skin of the hand also varied according to the number of fingers used, the object being manipulated, and the action being performed.
Tapping the index and middle finger alone was sufficient to elicit vibrations that covered most of the surface of the hand. Even the size of the object being grasped—for example, whether a glass was small or large—influenced the vibrations that were felt by the hand.
The findings have a variety of applications, researchers say. They may contribute not only to the foundations of our understanding of touch, but also contribute to fields such as virtual reality by enabling wearable technologies that allow the user to feel if he or she is picking up a feather or a brick while visiting a virtual world.
The work may also enable robots to touch and interact more effectively within changing and uncertain environments, and allow future generations of prosthetic hands to provide their wearers with more natural touch feedback, enabling a greater range of functionality to be restored.
Source: UC Santa Barbara
