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One Singular Sensation

Katie L. Burke

TOUCH: The Science of Hand, Heart, and Mind. David J. Linden. x + 262 pp. Viking, 2015. $28.95.

Considering how much depends on the sense of touch, it seems surprising that more scientists haven’t taken up the subject in books for general audiences. Although some recent books have visited the science of touch within the context of broader cultural explorations (for example, Matthew Fulkerson’s The First Sense: A Philosophical Study of Human Touch, published in 2013, and Constance Classen’s 2012 text The Deepest Sense: A Cultural History of Touch), neuroscientist David J. Linden’s book Touch stands apart. Linden elevates the conversation about touch, providing an unusually detailed and original perspective. He visits all the major tactile sensations in turn—pressure and object perception, proprioception, caress, sexual pleasure, temperature sensation, pain, itch, tickling, and touch illusions—unpacking what we know about how these messages are carried from source to brain. As researchers’ knowledge about the human senses is advancing at an unprecedented rate, Linden serves as a lucid, witty guide who avoids oversimplifying the facts while exploring what is known about our tactile sensations as well as the mysteries that remain.

Linden shows that our touch circuitry powerfully affects surprising aspects of our lives, often without our realizing it. Touching a client in a gentle, nonthreatening way can garner doctors higher health care ratings and wait staff higher tips. National Basketball Association teams that exchange more high fives, fist bumps, shoulder bumps, and other celebrations through touch score more. Indeed, this sense is essential to our social bonding and to our 2015-09NightstandBurkeF1.jpgClick to Enlarge Imagedevelopment. Linden points out in his first chapter that children can develop normally without a sense of sight, sound, or smell. Yet without touch they grow up to be emotionally and socially dysfunctional, as evidenced most distressingly by the Romanian children raised in severely understaffed orphanages during the 1980s. The symptoms resulting from early touch deprivation can only be reversed with a program of therapy incorporating regular touching. Even daily, hour-long sessions of manipulating children’s limbs were found to make a difference. In another instance, Linden describes how a doctor’s practical solution to a lack of incubators at an underfunded hospital led to a startling discovery. He instructed his patients to provide “kangaroo care,” meaning a mother would hold her premature infant against her body to keep the baby warm. This simple change reduced infant mortality rates while also decreasing the babies’ stress levels and improving their sleep and cognition—benefits that remained at age 10.

Linden describes research showing that humans, like vampire bats, rats, and gelada baboons, associate warm touch with safety because of our mother’s touch. It turns out that other social mammals show a need for touch similar to that of humans. For many animals, grooming satisfies this need. Sometimes the bonding that results will yield more than neurological benefits. A vampire bat, for example, can save the trouble of a nightly foraging trip by helping out a groupmate. Grooming another bat significantly increases the chances that the receiver will generously offer up some of its stomach contents in exchange for the favor.

Beyond neural circuitry and psychology, Linden explores the significance of sensory language as well. The words touch, hurt, and itch—among many other tactile words—evoke strong emotional responses. Linden shows that the way the sensory system and the brain interact reveals why emotion and tactile sensation are physiologically linked: Emotional pain triggers some of the same neural pathways as physical pain. While some sensations overlap in strange ways, many important types of touch have their own pathway. For example, pain, a caress, proprioception, and itchiness are all carried on their own neural pathways (although they may overlap sometimes too, as may be the case for itchy and painful sensations). These somewhat independent pathways can result in intriguing disorders. For example, people with impaired pain or proprioception sensors can still feel a caress. The reverse is true as well. Of course, these types of disorders can have social and emotional repercussions too. And all these pathways are not completely separate. Some overlap, and the brain hubs they activate communicate with one another, all of which is under sensory and emotional modulation. Throughout the book, Linden emphasizes that such complexity means “we need to explore not only blended touch sensations but also the effects of touch and nontouch senses combined.”

Genetic disorders that affect the sense of touch can be fascinating as well as disturbing. Linden notes that while a life without physical pain sounds ideal, those who are unable to feel pain because of a genetic mutation in the gene SCN9A often do not live beyond the teenage years. Bacterial infection presents a constant hazard, as ordinary cuts and abrasions go unnoticed. Even fatal injuries cannot be felt, as Linden illustrates with a heartbreaking story of a fearless child who, wanting to impress a group of friends, jumps off a building. Despite picking himself up and walking away from the scene of the fall, he died later the same day from internal bleeding that he could not feel. Another genetic mutation in the same gene can result in pain-sensing neurons “like machine guns with a hair-trigger.” Consequently, mild stimuli can initiate bouts of agonizing pain. However, people with this genetic disorder can have long, full lives, even when the periodic pain goes untreated.

Genes alone don’t determine our sensory experience. Linden also talks about how the sense of touch changes over one’s lifetime. For example, as we age, the density of mechanosensors declines. Feet and toes in a 20-year-old are four times as sensitive as those of an 80-year-old. This reduced sensation may be one reason the elderly have problems with falling and stability, and neuroscientists have not yet figured out why or how this decline happens.

Linden has a lovely way of intertwining spiritual or cultural beliefs with the mysteries that science is unlocking. An especially striking instance involves the Navajo, who believed that a life force, the Spirit Winds, emanated from the fingertips, evidenced by the whorled, windlike patterns of fingerprints. Linden uses this idea as a springboard to discuss how unusual our fingerprints are among mammals, perhaps a reason the Navajo identified it as something special to the human experience. Not all primates have fingerprints—only chimpanzees, gorillas, and humans. Koalas have them, but their close relatives do not. Fishers have them too, but no other members of the weasel family. Why this smattering of mammals have fingerprints remains a mystery, although there is some suggestion that the trait could help with grasping wet, slippery objects.

At times Linden’s scientific background becomes more evident in his writing, which can get pretty technical. Some passages tend toward textbook-style explanations or transitions. However, if one stays with him, he also has a capacity for clever analogies and examples. For instance, he compares Pacinian corpuscles—nerve cells that detect vibration—to seismographs, which similarly make trade-offs between precision and scale. As an example of Pacinian corpuscles at work, Linden describes the importance of “road feel” in car reviews.

Linden’s talent for clever analogy emerges again in his description of two hypotheses about how itchiness and pain might be related: “Is the relationship between itch and other touch sensations like that between a saxophone and a piano? Or is it like the relationship between bebop jazz played on the piano and classical music of the Romantic period played on the piano? They, too, are clearly distinguishable because of their musical structure and context, but they come into being on the same sound-producing device. In the past, this type of question would have been left to philosophers. Today, biology can add to the discussion.”

In “true nerdly fashion,” as Linden himself puts it, the bestselling author covers every common touch sensation, from the “recipe for orgasm” to the reason why hot pepper keeps mammalian vermin out of your bird feeder without deterring birds. Yet even as he catalogs these sensations, he attends to the common purpose underlying them. “All touch sensation (or sensation of any kind) is ultimately in the service of action,” he notes. “Our touch circuits are not built to be faithful reporters of the outside world but are constructed to make inferences about the tactile world based upon expectations—expectations derived from both the historical experience of our human ancestors and from our own individual experiences.” While the sense of touch may not objectively represent the world around us, Linden shows that it is fundamental to our health and well-being as well as vital to our understanding of the human condition.

Katie L. Burke is an associate editor of American Scientist. She received her PhD in biology from the University of Virginia in 2011. She blogs about ecology at the Understory.

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