Posts tagged sense of touch
Articles and news from the latest research reports.
Amputees discern familiar sensations across prosthetic hand
Even before he lost his right hand to an industrial accident 4 years ago, Igor Spetic had family open his medicine bottles. Cotton balls give him goose bumps.
Now, blindfolded during an experiment, he feels his arm hairs rise when a researcher brushes the back of his prosthetic hand with a cotton ball.
Spetic, of course, can’t feel the ball. But patterns of electric signals are sent by a computer into nerves in his arm and to his brain, which tells him different. “I knew immediately it was cotton,” he said.
That’s one of several types of sensation Spetic, of Madison, Ohio, can feel with the prosthetic system being developed by Case Western Reserve University and the Louis Stokes Cleveland Veterans Affairs Medical Center.
Spetic was excited just to “feel” again, and quickly received an unexpected benefit. The phantom pain he’d suffered, which he’s described as a vice crushing his closed fist, subsided almost completely. A second patient, who had less phantom pain after losing his right hand and much of his forearm in an accident, said his, too, is nearly gone.
Neuroscientists challenge long-held understanding of the sense of touch
Different types of nerves and skin receptors work in concert to produce sensations of touch, University of Chicago neuroscientists argue in a review article published Sept. 22, 2014, in the journal Trends in Neurosciences. Their assertion challenges a long-held principle in the field — that separate groups of nerves and receptors are responsible for distinct components of touch, like texture or shape. They hope to change the way somatosensory neuroscience is taught and how the science of touch is studied.
Sliman Bensmaia, PhD, assistant professor of organismal biology and anatomy at the University of Chicago, and Hannes Saal, PhD, a postdoctoral scholar in Bensmaia’s lab, reviewed more than 100 research studies on the physiological basis of touch published over the past 57 years. They argue that evidence once thought to show that different groups of receptors and nerves, or afferents, were responsible for conveying information about separate components of touch to the brain actually demonstrates that these afferents work together to produce the complex sensation.
"Any time you touch an object, all of these afferents are active together," Bensmaia said. "They each convey information about all aspects of an object, whether it’s the shape, the texture, or its motion across the skin."
Three different types of afferents convey information about touch to the brain: slowly adapting type 1 (SA1), rapidly adapting (RA) and Pacinian (PC). According to the traditional view, SA1 afferents are responsible for communicating information about shape and texture of objects, RA afferents help sense motion and grip control, and PC afferents detect vibrations.
In the past, Bensmaia said, this classification system has been supported by experiments using mechanical devices to elicit one or more of these specific components of touch. For example, responses to texture are often generated using a rotating, cylindrical drum covered with a Braille-like pattern of raised dots. Study subjects would place a finger on the drum as it rotated, and scientists recorded the neural responses.
Such experiments showed that SA1 afferents responded very strongly to this artificial stimulus, and RA and PC afferents did not, thus the association of SA1s with texture. However, in experiments in which subjects moved a finger across sandpaper — the quintessential example of the type of textures we encounter in the real world — SA1 afferents did not respond at all.
Bensmaia also pointed out discrepancies in the predominant thinking about how we discern shape. Perception of shapes has generally been tested using devices with raised or embossed letters to test a subject’s ability to interpret text by touch. These experiments also showed that such inputs produced a strong SA1 response, so they were implicated in perception of shape as well.
In the 1980s, however, researchers developed a device meant to help blind people read by generating vibrating patterns in the shape of letters on an array of pins. While the device was not a commercial success, people were able to use it to detect letter shapes and read, although experiments showed that it activated RA and PC afferents, not the supposedly shape-detecting SA1s.
Bensmaia said such experiments show how devices created to generate artificial stimuli focusing on individual components of the sense of touch can result in misleading findings. Some types of afferents are better than others at detecting texture or shape, for example, but all of them respond in their own way and contribute to the overall sensation.
"To get a good picture of how stimulus information is being conveyed in these afferent populations, you have to look at a diverse set of stimuli that spans the range of what you might feel in everyday tactile experience," he said.
Instead of thinking of individual groups of afferents working separately to process different components of the sense of touch, Bensmaia said we should think of all of them working in concert, much like individual musicians in a band to create its overall sound. Each musician contributes in his or her own way. Emphasizing one instrument or removing another can change the character of a song, but no single sound is responsible for the entire performance.
Adopting this new way of thinking will have far-reaching implications for both the study of the sense of touch and the design of future research, Bensmaia said.
"I think it’s going to change neuroscience textbooks, and by extension it’s going to change the way somatosensory neuroscience is taught. It’s really the starting point for everything."
'Seeing' through Virtual Touch Is Believing
A University of Cincinnati experiment aimed at this diverse and growing population could spark development of advanced tools to help all the aging baby boomers, injured veterans, diabetics and white-cane-wielding pedestrians navigate the blurred edges of everyday life.
These tools could be based on a device called the Enactive Torch, which looks like a combination between a TV remote and Captain Kirk’s weapon of choice. But it can do much greater things than change channels or stun aliens.
Luis Favela, a graduate student in philosophy and psychology, has found the torch enables the visually impaired to judge their ability to comfortably pass through narrow passages, like an open door or busy sidewalk, as good as if they were actually seeing such pathways themselves.
The handheld torch uses infra-red sensors to “see” objects in front of it. When the torch detects an object, it emits a vibration – similar to a cellphone alert – through an attached wristband. The gentle buzz increases in intensity as the torch nears the object, letting the user make judgments about where to move based on a virtual touch.
"Results of this experiment point in the direction of different kinds of tools or sensory augmentation devices that could help people who have visual impairment or other sorts of perceptual deficiencies. This could start a research program that could help people like that," Favela says.
Favela presented his research “Augmenting the Sensory Judgment Abilities of the Visually Impaired” at the American Psychological Association’s (APA) annual convention, held Aug. 7-10 in Washington, D.C. More than 11,000 psychology professionals, scholars and students from around the world annually attend APA’s convention.
A Growing Population in Need
Favela studies how people perceive their environment and how those perceptions inform their judgments. For this experiment, he was inspired by what he knew about the surging population of visually impaired Americans.
The Centers for Disease Control and Prevention (CDC) predicts that more than 6 million Americans age 40 and older will be affected by blindness or low vision by 2030 – double the number from 2004 – due to diabetes or other chronic diseases and the rapidly aging population. The CDC also notes that vision loss is among the top 10 causes of disability in the U.S., and vision impairment is one of the most prevalent disabilities in children.
"In my research I’ve found that there’s an emotional stigma that people who are visually impaired experience, particularly children," Favela says. "When you’re a kid in elementary school, you want to blend in and be part of the group. It’s hard to do that when you’re carrying this big, white cane."
Substituting Sight with Touch
In Favela’s experiment, 27 undergraduate students with normal or corrected-to-normal vision and no prior experience with mobility assistance devices were asked to make perceptual judgments about their ability to pass through an opening a few feet in front of them without needing to shift their normal posture. Favela tested participants’ judgments in three ways: using only their vision, using a cane while blindfolded and using the Enactive Torch while blindfolded. The idea was to compare judgments made with vision against those made by touch.
The results of the experiment were surprising. Favela figured vision-based judgments would be the most accurate because vision tends to be most people’s dominant perceptual modality. However, he found the three types of judgments were equally accurate.
"When you compare the participants’ judgments with vision, cane and Enactive Torch, there was not a significant difference, meaning that they made the same judgments," Favela says. "The three modalities are functionally equivalent. People can carry out actions just about to the same degree whether they’re using their vision or their sense of touch. I was really surprised."
Favela plans additional experiments requiring more complicated judgments, such as the ability to step over an obstacle or to climb stairs. With further study and improvements to the Enactive Torch, Favela says similar tools that augment touch-based perception could have a significant impact on the lives of the visually impaired.
"If the future version of the Enactive Torch is smaller and more compact, kids who use it wouldn’t stand out from the crowd, they might feel like they blend in more," he says, noting people can quickly adapt to using the torch. "That bodes well, say, for someone in the Marines who was injured by a roadside bomb. They could be devastated. But hope’s not lost. They will learn how to navigate the world pretty quickly."
(Source: uc.edu)
Research illuminates ‘touchy’ subject
By solving a long standing scientific mystery, the common saying “you just hit a nerve” might need to be updated to “you just hit a Merkel cell,” jokes Jianguo Gu, PhD, a pain researcher at the University of Cincinnati (UC).
That’s because Gu and his research colleagues have proved that Merkel cells— which contact many sensory nerve endings in the skin—are the initial sites for sensing touch.
"Scientists have spent over a century trying to understand the function of this specialized skin cell and now we are the first to know … we’ve proved the Merkel cell to be a primary point of tactile detection," Gu, principal investigator and a professor in UC’s department of anesthesiology, says of their research study published in the April 15 edition of Cell, a leading scientific journal.
Of all the five senses, touch, Gu says, has been the least understood by science—especially in relation to the Merkel cell, discovered by Friedrich Sigmund Merkel in 1875.
"It’s been a great debate because for over two centuries nobody really knew what function this cell had," Gu says, adding that while some scientists—including him—suspected that the Merkel cell was related to touch because of the high abundance of these cells in the ridges of fingertips, the lips and other touch sensitive spots throughout the body; others dismissed the cell as not related to sensing touch at all.
To prove their hypothesis that Merkel cells were indeed the very foundation of touch, Gu’s team—which included UC postgraduate fellow Ryo Ikeda, PhD—studied Merkel cells in rat whisker hair follicles , because the hair follicles are functionally similar to human fingertips and have high abundance of Merkel cells. What they found was that the cells immediately fired up in response to gentle touch of whiskers.
"There was a marked response in Merkel cells; the recording trace ‘spiked’. With non-Merkel cells you don’t get anything," says Ikeda.
What they also found, and of equal importance, both say, was that gentle touch makes Merkel cells to fire “action potentials” and this mechano-electrical transduction was through a receptor/ion channel called the Piezo2.
"The implications here are profound," Gu says, pointing to the clinical applications of treating and preventing disease states that affect touch such as diabetes and fibromyalgia and pathological conditions such as peripheral neuropathy. Abnormal touch sensation, he says, can also be a side effect of many medical treatments such as with chemotherapy.
The discovery also has relevance to those who are blind and rely on touch to navigate a sighted world.
"This is a paradigm shift in the entire field," Gu says, pointing to touch as also indispensable for environmental exploration, tactile discrimination and other tasks in life such as modern social interaction.
"Think of the cellphone. You can hardly fit into social life without good touch sensation."
(Source: eurekalert.org)
Scientists Provide New Grasp of Soft Touch
A study led by scientists at The Scripps Research Institute (TSRI) has helped solve a long-standing mystery about the sense of touch.
The “gentle touch” sensations that convey the stroke of a finger, the fine texture of something grasped and the light pressure of a breeze on the skin are brought to us by nerves that often terminate against special skin cells called Merkel cells. These skin cells’ role in touch sensation has long been debated in the scientific community. The new study, however, suggests a dual-sensor system involving the Merkel cell and an associated nerve end in touch sensation.
“In this long debate over the role of Merkel cells, it appears that both camps were right,” said the study’s senior author Ardem Patapoutian, a Howard Hughes Medical Institute (HHMI) Investigator and professor at TSRI’s Dorris Neuroscience Center and Department of Molecular & Cellular Neuroscience. “The nerve ends respond to touch, but so do the adjacent Merkel cells.”
The report appears in an Advance Online Publication of Nature on April 6, 2014.
In addition to elucidating the mammalian sense of touch, whose mechanisms until recently have been obscure, the findings could have relevance for certain pain syndromes in which touch sensations trigger pain—even the light pressure of a shirt on the skin or a breeze against the skin.
“Touch and pain are very closely related,” said Patapoutian, “and thus the characterization of these mechanisms of touch should help us to understand pain better too.”
Opening the Flow
The discovery comes four years after the Patapoutian laboratory identified a protein called Piezo2 as a mechanically activated “ion channel” protein with a likely role in touch sensation.
Ion channels are embedded in the outer membranes of various cell types and nerve fibers throughout the body. Piezo2 ion channels have been thought to respond to the stretching of the nerve membrane where they are embedded—a stretching caused by something that presses against the skin, for example.
When activated in this way, the ion channels open to allow an inflow of sodium or other positively charged ions. Such a surge of electrical charge into a nerve can initiate a signal that travels up the nerve and to the brain via a relay of neurons along the spine.
In the earlier study, Patapoutian’s team found evidence that Piezo2 proteins are made within touch-sensing neurons, including gentle-touch neurons that extend their nerves into the skin and against the mysterious Merkel cells.
In the new study, Patapoutian and his colleagues set out to learn more.
In Pursuit of Answers
The team began by creating a line of mice in which the activity of the Piezo2 gene also causes the production of a fluorescing protein called GFP. Guided by these fluorescent beacons as well as other markers, they found a high concentration of Piezo2 in Merkel cells in the skin of the mice.
“You can easily miss Piezo2 expression in the skin, because it’s not highly expressed there, aside from the tiny population of Merkel cells,” said first author Seung-Hyun Woo, a postdoctoral fellow in the Patapoutian laboratory.
Next the researchers sought proof of Piezo2’s role in Merkel cells, essentially by subtracting the protein from those cells and observing the result. To do this—a particularly challenging feat—they created a new line of mice in which the Piezo2 gene is specifically “knocked out” of all skin cells, including Merkel cells, but left intact everywhere else where it is ordinarily produced.
Piezo2 skin-knockout mice and their Merkel cells appeared normal. The mice also responded normally on most standard tests of touch and pain sensitivity. But on the so-called von Frey test, in which thin, bendable fibers are pressed against the mice’s paws with varying force, the effect of the loss of Piezo2 became apparent. “The mice whose Merkel cells lacked Piezo2 didn’t respond to the gentler forces as much as the control mice did,” said Woo.
Examining this change in responsiveness in more detail, Woo and her colleagues isolated Merkel cells from the two groups of mice. They found that those Merkel cells lacking Piezo2 failed to show the usual current flows when gently pushed with a probe.
Collaborating researchers in the laboratory of Cheryl L. Stucky at the Medical College of Wisconsin showed that gentle touch-sensing nerves known as slowly adapting (SA) Aβ fibers generally responded with a lower frequency of signaling in the mice lacking Piezo2 in Merkel cells. Another collaborating laboratory, led by Ellen A. Lumpkin at Columbia University, showed that Merkel cell-associated nerves also responded less durably to test stimuli on skin in these same mice.
“It all shows that the Merkel cells play an important role in touch sensing and that they need Piezo2 to do so,” Woo said.
The findings were bolstered by a separate study from Lumpkin’s laboratory—of which Patapoutian is a co-author—that is reported in the same issue of Nature. In that study, mice engineered to lack Merkel cells exhibited touch-sensing deficits very similar to those described in the Patapoutian group’s study.
(Image: iStockphoto)
Scientists Identify Key Cells in Touch Sensation
In a study published online today in the journal Nature, a team of Columbia University Medical Center researchers led by Ellen Lumpkin, PhD, associate professor of somatosensory biology, solve an age-old mystery of touch: how cells just beneath the skin surface enable us to feel fine details and textures.
Touch is the last frontier of sensory neuroscience. The cells and molecules that initiate vision—rod and cone cells and light-sensitive receptors—have been known since the early 20th century, and the senses of smell, taste, and hearing are increasingly understood. But almost nothing is known about the cells and molecules responsible for initiating our sense of touch.
This study is the first to use optogenetics—a new method that uses light as a signaling system to turn neurons on and off on demand—on skin cells to determine how they function and communicate.
The team showed that skin cells called Merkel cells can sense touch and that they work virtually hand in glove with the skin’s neurons to create what we perceive as fine details and textures.
“These experiments are the first direct proof that Merkel cells can encode touch into neural signals that transmit information to the brain about the objects in the world around us,” Dr. Lumpkin said.
The findings not only describe a key advance in our understanding of touch sensation, but may stimulate research into loss of sensitive-touch perception.
Several conditions—including diabetes and some cancer chemotherapy treatments, as well as normal aging—are known to reduce sensitive touch. Merkel cells begin to disappear in one’s early 20s, at the same time that tactile acuity starts to decline. “No one has tested whether the loss of Merkel cells causes loss of function with aging—it could be a coincidence—but it’s a question we’re interested in pursuing,” Dr. Lumpkin said.
In the future, these findings could inform the design of new “smart” prosthetics that restore touch sensation to limb amputees, as well as introduce new targets for treating skin diseases such as chronic itch.
The study was published in conjunction with a second study by the team done in collaboration with the Scripps Research Institute. The companion study identifies a touch-activated molecule in skin cells, a gene called Piezo2, whose discovery has the potential to significantly advance the field of touch perception.
“The new findings should open up the field of skin biology and reveal how sensations are initiated,” Dr. Lumpkin said. Other types of skin cells may also play a role in sensations of touch, as well as less pleasurable skin sensations, such as itch. The same optogenetics techniques that Dr. Lumpkin’s team applied to Merkel cells can now be applied to other skin cells to answer these questions.
“It’s an exciting time in our field because there are still big questions to answer, and the tools of modern neuroscience give us a way to tackle them,” she said.