How Our Sense of Touch is a Lot Like the Way We Hear

Sliman Bensmaia, PhD, assistant professor of organismal biology and anatomy at the University of Chicago, studies the neural basis of tactile perception, or how our hands convey this information to the brain. In a new study published in the Journal of Neuroscience, he and his colleagues found that the timing and frequency of vibrations produced in the skin when you run your hands along a surface, like searching a wall for a light switch, play an important role in how we use our sense of touch to gather information about the objects and surfaces around us.

The sense of touch has traditionally been thought of in spatial terms, i.e. receptors in the skin are spread out across a grid of sorts, and when you touch something this grid of receptors transmits information about the surface to your brain. In their new study, Bensmaia, two former undergraduates, and a postdoctoral scholar in his lab—Matthew Best, Emily Mackevicius and Hannes Saal—found that the skin is also highly sensitive to vibrations, and that these vibrations produce corresponding oscillations in the afferents, or nerves, that carry information from the receptors to the brain. The precise timing and frequency of these neural responses convey specific messages about texture to the brain, much like the frequency of vibrations on the eardrum conveys information about sound.

Neurons communicate through electrical bits, similar to the digital ones and zeros used by computers. But, Bensmaia said, “One of the big questions in neuroscience is whether it’s just the number of bits that matters, or if the specific sequence of bits in time also plays a role. What we show in this paper is that the sequence of bits in time does matter, and in fact for some of the skin receptors, the timing matters with millisecond precision.”

Researchers have known for years that these afferents respond to skin vibrations, but they studied their responses using so-called sinusoidal waves, which are smooth, repetitive patterns. These perfectly uniform vibrations can be produced in a lab, but the kinds of vibrations produced in the skin by touching surfaces in the real world are messy and erratic.

Secrets of Gentle Touch Revealed

In an article published online this week in the journal Nature, the UCSF team has identified the exact subset of nerve cells responsible for communicating gentle touch to the brains of Drosophila larvae—called class III neurons. They also uncovered a particular protein called NOMPC, which is found abundantly at the spiky ends of the nerves and appears to be critical for sensing gentle touch in flies.

Without this key molecule, the team discovered, flies are insensitive to any amount of eyelash stroking, and if NOMPC is inserted into neurons that cannot sense gentle touch, those neurons gain the ability to do so.

“NOMPC is sufficient to confer sensitivity to gentle touch,” said Yuh Nung Jan, PhD, a professor of physiology, biochemistry and biophysics and a Howard Hughes Medical Institute investigator at UCSF. Jan led the study with his wife Lily Jan, PhD, who is also a UCSF professor and a Howard Hughes Medical Institute investigator.

The work sheds light on a poorly understood yet fundamental sense through which humans experience the world and derive pleasure and comfort.

Why is Touch Still Such a Mystery?

Scientists generally feel that, like those other senses, the sense of touch is governed by peripheral nerve fibers stretching from the spine to nerve endings all over the body. Special molecules in these nerve endings detect the mechanical movement of the skin surrounding them when it is touched, and they respond by opening and allowing ions to rush in. The nerve cell registers this response, and if the signal is strong enough, it will fire, signaling the gentle touch to the brain.

What has been missing from the picture, however, are the details of this process. The new finding is a milestone in that it defines the exact nerves and uncovers the identity of the NOMPC channel, one of the major molecular players involved—at least in flies.

Jan and his colleagues made this discovery through an unusual route. They were looking at the basic physiology of the developing fruit fly, examining how class III neurons develop in larvae. They noticed that when these cells developed in the insects, their nerve endings would always become branches into spiky “dendrites.”

Wanting to know what these neurons are responsible for, they examined them closely and found the protein NOMPC was abundant at the spiky ends. They then examined a fly genetically engineered to have a non-functioning form of NOMPC and showed that it was insensitive to gentle touch. They also showed that they could induce touch sensitivity in neurons that do not normally respond to gentle touch by inserting copies of the NOMPC protein into them.

When she was 9 years old, Camilla would entertain her friends by jumping off her bed and landing directly on her knees. She said she liked to hear the crunching sound they made—just like popcorn.

Another time, Camilla spent an entire school recess period walking around on a broken leg, without so much as a whimper, says neuroscientist India Morrison of the University of Gothenburg in Sweden. The child’s teachers didn’t believe Camilla when she said something was wrong, because she wasn’t sobbing or wailing in pain. Her father thought perhaps her leg needed massaging, but quickly realized the situation was much worse.

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John Rogers of the University of Illinois at Urbana-Champaign and colleagues have designed a flexible circuit that can be worn over the fingertips. It contains layers of gold electrodes just a few hundred nanometres thick, sandwiched between layers of polyimide plastic to form a “nanomembrane”. This is mounted on a finger-shaped tube of silicone rubber, allowing one side of the circuit to be in direct contact with the fingertips. On the other side, sensors can be added to measure pressure, temperature or electrical properties such as resistance.

People wearing the device receive electrotactile stimulation – a tingling sensation caused by a small voltage applied to the skin. The size of the voltage is controlled by the sensor and varies depending on the properties of the object being touched.

Surgical gloves are one potential application. Rogers, who worked with colleagues at Northwestern University in Evanston, Illinois, and Dalian University of Technology in China, says gloves fitted with the nanomembrane could sense the thickness or composition of tissue via its electrical properties. A surgeon could also whittle away at the tissue using a high-frequency alternating current supplied by a battery attached at the wrist and delivered via the nanomembrane itself, says Rogers.

Disney researchers add sense of touch to augmented reality applications 

Technology developed by Disney Research, Pittsburgh, makes it possible to change the feel of real-world surfaces and objects, including touch-screens, walls, furniture, wooden or plastic objects, without requiring users to wear special gloves or use force-feedback devices. Surfaces are not altered with actuators and require little if any instrumentation. 

Instead, Disney researchers employ a newly discovered physical phenomenon called reverse electrovibration to create the illusion of changing textures as the user’s fingers sweep across a surface. A weak electrical signal, which can be applied imperceptibly anywhere on the user’s body, creates an oscillating electrical field around the user’s fingers that is responsible for the tactile feedback.

The technology, called REVEL, could be used to create “please touch” museum displays, add haptic feedback to games, apply texture to projected images on surfaces of any size and shape, provide customized directions on walls for people with visual disabilities and enhance other applications of augmented reality.